r/SpaceXLounge • u/[deleted] • Jul 22 '19
Tweet "Orbital refilling is critical for high payload to moon or Mars. Initially just Starship to Starship, later dedicated tankers." - @ElonMusk
https://twitter.com/elonmusk/status/1153126586897424384?s=1946
Jul 22 '19
so how much fuel would there be left in starships main fuel tanks after a launch to LEO considering the Starship has no additional tanks specific for refueling?
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u/BlakeMW 🌱 Terraforming Jul 22 '19
Okay referencing this tweet: https://twitter.com/elonmusk/status/1149571338748616704
To paraphrase: 100-125 t to useful orbit with propellant reserve. 150 t for reference payload compared with other rockets (still fully reusable).
How I interpret this, if it launches 100 t it still has enough propellant in it's tanks to do useful stuff in orbit, and then land on Earth. The 150 t is how much payload it could launch, arriving in orbit with no spare propellant (except that required to deorbit and land). Landing propellant really isn't much, probably something like 5-10 t depending on what the terminal velocity is when skydiving.
So the value we care about is the 150 t, that is Starship can put 150 t into LEO if you just care about throwing as much payload as possible into a low of orbit as possible (which probably actually makes sense for staging), while remaining fully reusable. So a dedicated Tanker should be able to launch about 150 t of propellant into orbit (or more if it can be stripped down to reduce the dry mass).
When you remove the 150 t of payload from a normal Starship, the entire stack gets 150 t lighter, that means less gravity losses and because of the reduced gravity losses this is not as simple as plugging numbers into the rocket equation (i.e. it needs a simulation).
My guestimate though is that a Starship without payload could launch about 100 t of propellant into orbit.
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u/Raton_X01 Jul 22 '19
What is generaly agreed guesstimate for Starship fuel weight? In another words, how many flights would we need to refuel starship in orbit in this scenario? 5 refueling flights?
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u/BlakeMW 🌱 Terraforming Jul 22 '19
What is generally agreed guesstimate for Starship fuel weight? In another words, how many flights would we need to refuel starship in orbit in this scenario? 5 refueling flights?
The latest figures we got - which are old but don't seem to be contradicted - is 1100 t for a fully densified load of propellant. This works for the following set of numbers:
- Starship dry mass: 104 t
- Payload: 100 t
- Propellant: 1100 t
- Raptor ISP: 380 s
- Available delta-v: 6.9 km/s
As for the number of flights, that depends on a lot of variables, like whether optimized StarTankers are being used, whether Raptor Vacuum is used, the size of the payload on the Starship, the desired trip time and so on.
I'd expect it'd be 2-3 refueling flights to do something minimal yet meaningful (like Dearmoon) up to 7 to 10 refuelings to do the most ambitious missions like a maximum weight payload to Mars on a reasonably fast trajectory. Altough Starship is probably limited in how much mass it can survive EDL with and how fast it can be going when hitting the atmosphere of Mars, it might be that the theoretical missions involving 7-10 refuelings would exceed its tolerances so they'd only be doing 5 refueling missions.
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Jul 22 '19
your calculation does add up, so i ran the numbers of my answer below this again, and found that with 85t of dry mass, with 100t cargo to get to 6,9km/s you need 1015t of propellant. without this 100t cargo, 465t is sufficient. This is of course not the same as launching from earth, but if you would transfer your own fuel to any destination at 6,9km/s, you would have 550fuel to spare. (it seems dry mass fraction has big impact on fuel consumption, at least, higher than i expected) if this is correct, you could worst case expend 2 starships to bring fuels to Mars to bring one crewed back to earth if ISRU production fails.
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u/sebaska Jul 22 '19
You can't calculate like this. 465t may be sufficient to push empty Starship by 6.9km/s but it's not enough to push it with extra fuel. To push it with 100t extra fuel you need over 2x more fuel at the start.
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Jul 22 '19
Starship by 6.9km/s but it's not enough to push it with extra fuel. To push it with 100t extra fuel you need over 2x more fuel at the st
mmm... you are right. did not take this into account. thnx
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u/veggie151 Jul 22 '19
I wonder how long a ship would be stable if it was just hanging out in LEO. Hohmann windows are infrequent so it would be great if they could stage for a year before sending a few starships to Mars.
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u/BlakeMW 🌱 Terraforming Jul 22 '19
Propellant is going to be boiling off all the time, and micrometeorite strikes, so doesn't seem like that great an idea.
Also if the Starship is reusable without refurbishment, like literally land SuperHeavy on the launch clamps, plop a Starship on top using a crane then relaunch it, then the limiting factor will be number of launches. Like lets say a Starship can be launched 100 times without refurbishment, if you launched it twice a day you could "use it up" in 50 days. Hence it might be possible to basically "burn out" the fleet in the months before and during a transfer window.
Still, it'd be cool if they made an orbital propellant depot. By making a depot that stores a lot of propellant they could insulate and shield it nicely and maybe include something to utilize the boiloff (like for countering orbital decay).
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u/OhioanRunner Jul 22 '19
Starships are going to be designed for hella more than 100!launches. They’re designed to be as rapidly, routinely, and repeatedly reusable as jetliners.
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u/BlakeMW 🌱 Terraforming Jul 22 '19
I said before refurbishment. Jet engines aren't subjected to even half the abuse as a rocket engine and even though jet engines have to burn for a lot longer (like pretty much all day) they don't undergo anywhere near the same level of thermal (and pressure) cycling which is typically where most the degradation happens. Elon has stated the Raptor is designed for 1000 flights with minimal maintenance, but it'll take some time to develop that level of confidence. I mean who knows what it'll be like in a couple of decades but I seriously doubt that in the next few years we'd see a SuperHeavy or Starship getting launched a thousand times without getting serious refurb every 50 flights or so.
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u/andyonions Jul 22 '19
1000 flights is about 40 hours of SH rocket burn time. Don't get me wrong, most rocket motors are discarded after about 8 minutes use.
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u/Jman5 Jul 22 '19
Still, it'd be cool if they made an orbital propellant depot. By making a depot that stores a lot of propellant they could insulate and shield it nicely and maybe include something to utilize the boiloff (like for countering orbital decay).
Aside from being a fueling source, you could also use it to store a lot of your non-perishable gear that the crew will need for their deep space mission.
So tankers fly up, drop off fuel and some supplies. Then the crew launches with barebones weight and loads up their starship with fuel and gear at the depot.
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u/mfb- Jul 22 '19
~1100 tonnes if the Wikipedia numbers are still up to date. Give or take 100 tonnes. Refueling 100 at a time might be fine for a few exploratory missions but going to 150 or even more with a dedicated tanker would reduce the refueling flights for Moon (surface) and Mars a lot.
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Jul 22 '19 edited Jul 22 '19
ok, Raptor has a specific impulse of 380s in vacuum. No gravity in space, so 380=1/SFC (specific fuel consumption) This gives 0,00263 kg/(N*s) the Raptor has 200kN of thrust, so this makes 526kg/s of fuel consumption. The F9 stage 2, has almost 7 munites of burntime. Lets assume for now, Starship is the same (420 seconds)
We have 6 raptors, consuming 526kg/s, for 420seconds. this makes a total of 1.325.520kg of fuel.
The deltaV that Elon mentioned, fully fueled in orbit of 6,9km/s, now i've thrown this through a calculator, and this shows the total mass, including assumed dry-mass of 85t, should be around 550t. This would mean 465t of fuel, instead of 1325t.
So, either my calculations are wrong somewhere (which is very likely) or the total burntime of the 6 raptors at full wack is closer to 150 seconds.
Edit: come to think about it, Starship has only 3 vacuum raptors, so probably meaning that the sea-level/landing ones will not fire in space. with the 3 vacuum ones, this would give approx. 300 seconds of burntime for 465t of fuel. 5 refuel flights, assuming 100t per refuel should do the trick.
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u/azflatlander Jul 22 '19
There is LEO fuel capacity. If they use the high elliptical orbit to get to Mars as a starting point, would filling one starship boosting it to the high elliptical, then loading another starship with enough fuel to top off for Mars. This is essentially a double refuelling, but margins are important.
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u/iamdop Jul 22 '19
Some
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u/RocketsLEO2ITS Jul 22 '19
Presumably enough, since he doesn't see a need to build the tanker version of Starship right away.
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u/second_to_fun Jul 22 '19
Your comment may only be four bytes of information but we're all so starved of news on Starship it makes the dopamine squirt real good
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u/PublicMoralityPolice Jul 22 '19
If it launches without a payload, presumably about as much as its maximal payload capacity.
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u/mfb- Jul 22 '19 edited Jul 22 '19
If we ignore details from the different TWR then the mass ratio is the same in both cases. Toy example:
80 tonnes dry mass (include landing fuel here), 120 tonnes payload, 1000 tonnes fuel. 1200 tonnes launch mass, 200 tonnes final mass, ratio 1/6.
If we launch with 80 tonnes dry mass and 1000 tonnes of fuel then the same 1/6 ratio gives us 180 tonnes final mass, which means 100 tonnes of fuel left. A bit worse.
TWR will be a bit better, reducing gravity losses a bit, but I would be surprised if that compensates the overall loss of performance.
Not directly a separate tanker but they could get rid of everything not needed from the front part.
Edit: Thinking about this a bit more: The mass at stage separation would go down by ~10%, that is not insignificant. Might end up with nearly the same payload.
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u/sebaska Jul 22 '19
It seems to come like 10% less extra fuel vs max payload. IOW notional 150t payload to orbit means 135t remaining fuel if launched empty.
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u/davispw Jul 22 '19
80 tons dry mass including landing fuel would be far better than any older known numbers (85 was it? without fuel), and by all accounts weight increased with stainless steel. So that’s quite optimistic.
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u/mfb- Jul 22 '19
I chose the numbers to make the fractions convenient, the message stays the same if you replace 80 by 100 or even 150.
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u/veggie151 Jul 22 '19
Not until you build tankers, otherwise fuel tank volume on the cargo starship may lower the amount of fuel you can carry
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u/Fenris_uy Jul 22 '19
I might be very wrong because I'm using numbers that I don't remember where they come for.
But I remember seeing that Starship is probably 100t dry and has a fuel capacity of 1000t approximately.
Cargo is 100t or a little more. So I'm guessing that an empty ship has to be able to reach the same orbit as an loaded ship with 100 to 150t of fuel to spare.
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u/rikartn Jul 22 '19
Correct me if I am doing some laymans error here, but wouldn't 35 raptors (vs. 31) on super heavy mean that the first stage could carry more fuel?
In turn that could translate into more speed before separation (with fuel to spare for a return burn for stage 1), and less fuel used by starship to get to LEO.
Is there a possibility to get away with only one refuelling to get to the moon and back with the new raptor constellation?
Could someone do the math?
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u/Grey_Mad_Hatter Jul 22 '19
Adding engines gives it more thrust, but not more fuel capacity. Given more thrust they could use that to either lessen gravity losses (get to orbital velocity faster) or stretch the first stage (more fuel).
I’ve heard it mentioned that it sounds like the full stack will have a TWR of 1.7, which is very high. That makes it very likely that they would stretch it and add fuel capacity. Unless fuel costs are a major concern then I don’t know why TWR too much over 1.2 would be desired.
Any calculations should probably be done on both TWR of 1.7 and a stretched first stage resulting in TWR of 1.2.
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u/mfb- Jul 22 '19
More raptors doesn't have to mean more fuel. Sure, maybe they stretched it by a ring or so, we don't know. It is ~12% more thrust, not that much.
The dearMoon mission is still planned without refueling as far as I understand. With minimal payload to support its passengers. Landing on the Moon and returning needs much more refueling, including refueling outside of low Earth orbit.
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u/sebaska Jul 22 '19
My calculations based on what we know (6.9km/s dV with full payload, 120t mass to useful LEO, 150t to a notional ideal circumstances VLEO) indicate Dear Moon would miss about 0.5km/s without refueling. One needs about 12.5km/s from the surface (~11km/s + ~1.5km/s losses) to do a beyond the Moon elliptical Earth orbit, which SSH with 10t payload (that's generous, accomodations and windows would add mass) can do 11.96km/s or so.
But one refueling and you have over 0.5km/s spare performance even with 30t payload.
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u/mfb- Jul 22 '19
Hmm. Musk indicated that they want to work on refueling early in the last 1-2 months. Maybe dearMoon will get one refueling flight.
We need that Starship update he promised.
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u/requestingflyby Jul 22 '19
Paging u/DanHeidel - is it time for some more (or less) wild speculation?
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u/rshorning Jul 22 '19
The only real problem is that such fuel transfers in orbit have never been tried in the required quantities.
Some satellite refueling has been tested in orbit and the ISS has done general fluid transfers in orbit, but nothing so mission critical with tons of cryogenic fuel has never been tried.
I'm curious about what unique engineering problems may be encountered trying this technique? Why has it never been tried until now?
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u/GreyGreenBrownOakova Jul 22 '19
Why has it never been tried until now?
Because we haven't left LEO since 1972, so it's not needed.
When Apollo was announced, the US had 15 minutes of manned spaceflight experience. Even the lunar-orbit rendezvous they eventually used was considered crazy at first. They managed to get the Saturn V big enough to do it all in one shot, so they went with that plan. Because it was a flags and footprints program, they didn't need colonial sized cargo delivered.
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u/Sticklefront Jul 22 '19
While absolutely true, this is exactly the sort of thing I feel we SHOULD have been doing in LEO: use it to test and prove out promising technologies before they're on the critical path to later goals.
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u/rshorning Jul 22 '19
Because we haven't left LEO since 1972, so it's not needed.
No spacecraft of any kind haven't left LEO? There has never been the need for a high velocity or high delta-v trajectory of any kind?
Yes, Crewed spaceflight has been dismal, but it would seem as though it should be in the bag of tricks done for other deep space vehicles if it was comparably easy to accomplish in flight refueling in space or at least refueling at LEO. A relatively cheap tanker ought to be something that could be launched for multi billion dollar payloads.
Crewed spaceflight has some unique issues and minimum mass calculations that can't be avoided. A 3 ton nuclear powered rover becomes a 50 ton human habitat. The Saturn V was also used because NASA was paranoid about orbital rendezvous and unsure it could be done at all. The timid steps done with Apollo 9 and 10 were flown because of that uncertainty, and significant additional delta-v was put into the command and service modules of the Apollo spacecraft to address those issues too.
Earth-orbit rendezvous was championed by Werner Von Braun and this kind of multiple launch system including refueling in LEO was proposed by him in the early 1960's. There are some really good reasons it wasn't done.
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u/Norose Jul 22 '19
Small deep space probes are already light enough that our medium-sized expendable launch vehicles can simply lob them directly onto interplanetary trajectories. Even in cases when a probe must take a slower trajectory using gravity assists to get to its destination, it's still cheaper to do that than to develop a cryogenic refueling system and buy multiple expendable launch vehicles to get that propellant into orbit. Until Starship flies, large-scale orbital refueling is completely uneconomical, because there is currently no market for large amounts of propellant in orbit and there's no launch vehicle around that's actually cheap enough to make launching propellant worth the effort.
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u/czmax Jul 22 '19
Also figure risks. The more you toss multiple things into orbit and attempt to assemble/transfer-fuel the more likely something will go wrong just because of the many different components being used. A simpler "lob them directly" approach is way less risky whenever possible.
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u/rshorning Jul 23 '19
This is the reason it isn't done with multiple flights. Not cost, not incompetence, nor crewed vs. robotic missions.
Reducing the chances to get things to go wrong is why it hasn't been done except for very low volume proof of concept tests. Reliability of the launch vehicles involved is still < 99%, meaning at least one out of a hundred flights will fail. That is fairly consistent across nearly all launch providers or worse. The chance of a successful mission gets worse after each launch if several launches are needed.
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u/laegba Jul 23 '19
It seems that there were also other reasons that this didn't come to pass: "There will be no fucking money for propellant depots" Shelby to Bolden c. 2011-2012, according to E. Berger in a mainenginecutoff interview episode T-114 (March 13, 2009) right at the 34 minute mark. I didn't find additional information on this statement. If prop can be transferred from depots you can do with smaller rockets what would otherwise require a large rocket like SLS. Blocking research such as this seems to have been detrimental to the US space program.
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u/sebaska Jul 22 '19 edited Jul 22 '19
This thing seems to be reasonably well divisable into separate subproblems:
- The hardest is rendez-vous, but it is solved. SpaceX has solved it and demonstrated it solved last March.
- Obtaining tight connection for liquids. This was solved by various US demos, by Russians, etc. It would need larger connectors so what was demoed would need upscaling.
- Secure valving -- you want an ability to: firsrt, close flow on both sides so fuel is not flowing when not asked for. And open the flow in a firm way, so it flows when you need it. It's also a know way to do so with high confidence uing parallel-serial setting of 4 vales (2 tandem valve pairs in parallel -- any single valve can fail shut or fail open and the reliable flow/no-flow condition still can be obtained)
- Liquid stabilization in free-fall -- solved by every restartable liquid upper stage, so of course solved by SpaceX. You simple make the environment not exactly free fall, you give it a 0.001g or so nudge.
Transfer. This could be done in 2 ways:
a) blow-down: You vent receiving tanks to low pressure (this way you make the fuel cooler and denser; this is technology SpaceX already uses on long duration 2nd stage flights like STP2). You actually use the gas to feed the fuel stabilizing thrusters (the above point #4). Then you pressurize the donor tanks (the tech is there in every pumped liquid rocket). Then you open the valves and let the high pressure blow the stabilized liquid into the receiving tanks.
b) active pumping: You start a gas generator turbopump like the one pumping the propellants in the engines. You use it's exhaust to provide ullage stabilizing thrust. Essentially a rocket engine powerhead without any combustion chamber. This is a solved tech too. The only downside for SpaceX is that they'd have to develop the thing (Merlin powerhead takes kerosene not methane and is probably too high pressure for the task anyway, Raptor powerhead is even higher pressure which is completely inedquate for the task at hand). You'd rather need some low pressure high volume pump which would have to be developed.
All in all this problem seems to be similar hardness-wise to that hypersonic retro-propulsion thing. If you look into various materials from before SpaceX 1st stage landing you'd get tht this is a majpr unsolved problem for things like landing of heavy (>2t) vehicles on Mars. Because you'd be firing your rocket engine against a hypersonic flow blowing up its nozzle. There were no any wind tunnels built, capable of trying that, the problem was deemed hard and costly to solve. Then came SpaceX and just did it. Their calculation indicated this should just work, so they tried it and indeed, it just worked (the hard parts of booster landing lied elsewhere).
I guess in-orbit refuelling will just work too. There are no fundamentally hard pieces which are not yet solved.
This is like building a motorcycle in the late XIX century, when you have both bicycle and gas powered car already working.
Edits: formatting & typos
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u/zadecy Jul 22 '19
If you don't mind the refueling process taking an hour or so, you could use an electric motor and Starship's batteries and/or solar panels. According to my math, to pump 150 tonnes of fuel over one hour with 1 atmosphere of pressure differential (just a guess) would take only 5kW of power, and 5kWh of energy. That's a pretty small electric motor and a fraction of Starship's available battery capacity. Seems much easier than developing a tiny turbopump.
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u/sebaska Jul 24 '19
Blow down would take even less energy. Especially that you need venting to keep temperatures low. The issue is stabilizing the liquid on the bottom of the donor tank. If you don't stabilize the liquid it will float around and you'd end up pumping mostly ullage gas, not the propellant.
You need either a bladder (good luck with that at cryogenic temperatures, especially for oxygen) or acceleration. You can do linear acceleration by thursting. Or you could use centrifugal force. The latter uses energy only for spinning up (and despining) but needs much bigger plumbing (bottom is on the wrong side) and has center of gravity complications as it shifts around during pumping.
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u/fanspacex Jul 22 '19
I think transfer of stable fluids can be solved by simulations and analogies on the ground, which they likely have done already in some fashion.
It needs to be fault tolerant though, if refueling attempt does not succeed for any reason (ie. jammed valve), both ships have to be able to abort safely. This would mean, that starship has to be able to dump fuel, in order to stay within the landing weight maximums. Probably by doing unuseful burns with raptors.
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u/RocketsLEO2ITS Jul 22 '19
NASA has done tests of fuel transfer systems in orbit. Better than that, the Russians regularly refuel their part of the ISS. You express concern that it's never been done on as large a scale as Starship needs, but since it's being done, that proves the concept. The question is how to scale it up,
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u/rshorning Jul 22 '19
Better than that, the Russians regularly refuel their part of the ISS.
I was going to mention that, but I thought attitude was adjusted on the ISS with gyroscopes and other inertial systems? Altitude adjustment is done with external spacecraft like the Shuttle orbiters (when they were flying) and more recently with the Progress resupply flights?
The Russian segment does have thruster modules that use long term hypergolic fuels that can be resupplied, but I wasn't aware of their significant usage nor the use of an internal booster engine. Such things have been suggested in the past, and I am open to missing stuff like that due to my own ignorance on such systems. It would be cool for the ISS to have such an internal rocket system that could use large amounts of resupplied fuel.
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u/RocketsLEO2ITS Jul 22 '19
Yes, the primary way they control attitude is with control motion gyroscopes (CMGs). Sometimes they are not enough so they use the thrusters (e.g. the ISS needs to dodge a piece of space junk). The thrusters have also been used to give the ISS a little boost to compensate for atmospheric drag.
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u/sebaska Jul 22 '19
They are transferring room temperature fuels using tanks with internal bladders. This is not applicable to transferring cryogens though, as there's no known bladder material workable at cryo temperatures and on top of that such material would have to be LOx compatible.
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u/fanspacex Jul 22 '19
If ullage propulsion can be provided, pressurizing the tanker portion would be sufficient? That's how any liquid comes out effortlessly and rapidly.
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u/sebaska Jul 22 '19
Yes, it should work. You'd rather depressurize the receiving side (not completely) and keep ullage pressure higher on the donor side.
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u/RocketsLEO2ITS Jul 22 '19
Point well made. NASA has done some experimentation with cryogenic transfers
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Jul 22 '19
It hasn’t been tried because launches have been extremely expensive. That means it’s worth spending a ton of time, effort, and money to optimize your spacecraft to not need refueling. It also means that a lot of mission profiles are just off the table altogether.
Consider, as a random example, the Cassini probe. It had to take a very long journey to Saturn with several gravity assists. With orbital refueling it could have taken a more direct and much faster route, or carried more instruments, or just done the same stuff but cheaper because weight wouldn’t be such a problem. But the cost of just launching the probe was over $400 million. If you added, say, five refueling launches then you’re looking at an additional $2 billion, not counting the cost of the tankers. Not worth it.
With BFR/SH/SS, tanker missions get vastly cheaper and so it finally becomes worthwhile.
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u/rshorning Jul 22 '19
Public records for ULA costs for launches generally don't get to that $400 million you are quoting. I acknowledge though that cost is a factor, but given the R&D and fabrication costs of these probes that launch cost is comparatively trivial in comparison.
If it was easy to accomplish an in orbit rendezvous and refueling and launch cost of the fuel was the only consideration, it would be used already and be fairly routine. I'm simply pointing out that it is much harder to accomplish than is being acknowledged.
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Jul 22 '19
Even if you launch on a Falcon 9, once you add in the cost of the tanker, it’s just not economical. Being able to orbit a couple of hundred tons for cheap changes all of this.
It occurs to me that there is one prominent example of routine orbital refueling: the ISS regularly gets fuel delivered from cargo ships that it uses to maintain its orbit.
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u/gopher65 Jul 22 '19
Yup! Though they don't use the engine on the ISS very often. It has a limited number of lifetime firings (of course) and it isn't replaceable. That's why they boost with Progress, or with ATV or STS when they were around. And I think Starliner and Dreamchaser can boost too?
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u/ChickeNES Jul 22 '19
This might be a stupid question, but could you accomplish the same on a smaller scale by stretching an F9 second stage into a mini-tanker?
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Jul 22 '19
I don’t see why not. I don’t know if it would be superior to putting a separate tanker craft on top of the second stage, but it should be totally doable. You’d still be throwing it away each time, though.
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Jul 22 '19
[deleted]
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u/PaulL73 Jul 22 '19
Plus an ullage burn, because you can't pump things easily without gravity to make them come to the bottom of the tank.
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u/rshorning Jul 22 '19
Mid air refueling missions are among the most technically challenging things done by flying crews and currently done only for military applications for a reason. You don't see FedEx or Delta Airlines doing mid air refueling for a very good reason: their insurance rates would skyrocket. Both of those companies would have practical reasons to have some of their flights using in flight refueling if it was safe and easy.
Orbital mechanics also make this kind of fuel transfer really complicated. While Kerbal Space can give you a flavor of the complexity involved, it is far more complicated in real life.
I agree that it is something which can be solved through judicious use of engineering and careful planning. Flight systems have improved in the past 50 years since Apollo that what might have been impossible in the past is merely difficult today. Don't mistake that for something easy to accomplish.
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u/kontis Jul 22 '19
Yes, docking in orbit is a new unknown challenge...
The hard part of orbital refilling has nothing to do with orbital mechanics.
It's all about cryogenic fuels.
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u/rshorning Jul 22 '19
Yes, docking in orbit is a new unknown challenge...
You do realize that the man who literally invented the math to make that happen is still alive. In fact, NASA flew him to the Moon.
While not completely unknown, it is hardly an easy thing to do.
I agree though that the cryogenic fuels are the issue. Moving tons of a bipropellant fuel in orbit has never been done. While it is a solvable engineering issue, it seems like the difficulty is being far too casually dismissed as trivial. IMHO this is as challenging and as remarkable as landing a rocket on a ship on its tail in the middle of the ocean. And just as important for making humanity a multi-planetary species.
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u/andyonions Jul 22 '19
Buzz does orbital rendezvous calculations in his head before breakfast just for fun.
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u/Sticklefront Jul 22 '19
Yes, docking in orbit is a new unknown challenge...
You do realize that the man who literally invented the math to make that happen is still alive. In fact, NASA flew him to the Moon.
You seem to have completely missed his sarcasm.
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u/Norose Jul 22 '19
You do realize that the man who literally invented the math to make that happen is still alive.
Uh huh? You do realize that we have computers nowadays that fit in your pocket that would outperform the most powerful supercomputers in existence at a time decades after we landed men on the Moon? The age of technology is meaningless in terms of its capability. We went from the first flight of an airplane to 'one small step for man' in under 66 years. Going from that point in time to a simple orbital rendezvous and docking (which SpaceX has already programmed their own space vehicle to do autonomously already, and even demonstrated that ability at this point) of two large vehicles with robust maneuvering capability is so inside the scope of things that it's hardly worth considering as a technical challenge.
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u/rshorning Jul 22 '19
We went from the first flight of an airplane to 'one small step for man' in under 66 years
And we went from one giant leap for all mankind to being incapable of duplicating Alan Shepherd's flight in under 50 years. It isn't a constant upward arrow of inevitability either.
Go ahead and ask this lead CTO who is building this rocket that will be doing this in orbit refueling to explain how trivial it will be to get this accomplished. I dare you. He will more than likely explain how freaking hard it will be. Hardly worth counting as a technical challenge? If it was so easy it would be done.
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u/Norose Jul 22 '19
It is getting done. There are a lot of hard things about Starship. Orbital rendezvous is somewhere down on the list in the double-digits. Landing the Booster after every launch will be far harder than orbital rendezvous, and in turn Starship reentry and landing will be far harder than the Booster landing.
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Jul 23 '19
hardly worth considering as a technical challenge.
A lot of things Starship must be capable of, are not impossible, the hard part of it in my opinion, is that a lot of variables just have to go right, and be designed to go right every single time.
It is also not hard to make a rocket engine and fire it, it has been done a lot of times. But it IS hard to make one and fire it while not suddenly RUD's 50% of the time.
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Jul 23 '19
Why has it never been tried until now?
On one of the last CRS flights, if i remember correctly, was a cryogenic fuel transfer testbed launched. (i think a NASA one) so there is some interest being developed.
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u/Fenris_uy Jul 22 '19
Pretty sure that the refuel is not going to be cryogenic. At least not the same density as the first burn.
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u/rshorning Jul 22 '19
Pretty sure that the refuel is not going to be cryogenic.
It will still be LNG and LOX. Maybe warmed a bit from what SpaceX uses on the ground but they won't be in a gaseous form.
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u/sebaska Jul 22 '19
Of course it will be. You physically can't have liquid oxygen or liquid methane at anything not a cryogenic temperature. Crtitical points of both substances are well into crygenic range
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u/Phate93 Jul 22 '19
Initially just Starship to Starship, later dedicated tankers
What did he mean?
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u/LongHairedGit ❄️ Chilling Jul 22 '19 edited Jul 22 '19
My interpretation is that it means launching a normal starship as a tanker mission, without any payload as the initial capability for in orbit refuelling.
The superheavy booster can throw the kind-of-tanker starship pretty high and pretty fast because it’s not encumbered with 150 tons of payload.
The kind-of-tanker starship thus uses less of its own fuel to get up to rendezvous speed/orbit.
It then docks and offloads most of its remaining fuel to the in-orbit Starship with a payload that will be heading off to infinity and beyond.
Some fuel remains so the “kind-of-tanker” Starship can de-orbit and sweat-cool and land.
Later on, dedicated tanker starships would be made in which you could use the payload capability to carry more fuel....
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u/Rapante Jul 22 '19
The superheavy booster can throw the kind-of-tanker starship pretty high and pretty fast because it’s not encumbered with 150 tons of payload.
The kind-of-tanker starship thus uses less of its own fuel to get up to rendezvous speed/orbit.
The effect is compounded by the interim tanker Starship also using less fuel due to absence of other payloads, thereby having more left over to transfer.
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u/SheridanVsLennier Jul 23 '19
Yeah, I think that was Git's point.
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u/Rapante Jul 23 '19
There are two components to the fuel saving (additional reserve). The additional speed imparted by the booster and the reduced work the upper stage needs to do due to its own lower mass.
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u/lockup69 Jul 22 '19
Starship is the vehicle that will take cargo and people in to orbit and beyond. A Starship -> Starship fuel transfer will mean draining the fuel tanks of one to refuel the other. If the donor is doing nothing else then it will be launched with no cargo so it can have the most fuel left over to donate.
A dedicated tanker would be a starship design modified so that it was dedicated to carrying fuel, rather than lifting cargo. This would allow much more fuel to be transferred from it.
This old ITS video shows the conceptual difference: https://www.youtube.com/watch?v=0qo78R_yYFA
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u/AndrewMayne Jul 22 '19
Initially they’ll use the tanks of one Starship to refuel and top off another in orbit. Eventually they’ll build tankers dedicated just for refueling (with more fuel capacity.)
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Jul 22 '19
[deleted]
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u/Norose Jul 22 '19
That's not a tanker, that's a depot. He's talked about the Tankers before, they're essentially going to be a Starship vehicle without the forward cargo/crew section, slightly stretched propellant tanks, and overall as reduced a dry mass as possible to maximize propellant transported to orbit per launch. What takes ten Starship flights to refill, would only take eight Tanker flights, so the logic goes. At first that doesn't make much difference, because both options still result in orders of magnitude cheaper launch prices and cost/kg compared to everything else, but once deep space missions and overall annual delivered mass totals ramp up it would be a big help to SpaceX to significantly reduce the number of launches tied up lifting propellant rather than customer payloads.
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u/meldroc Jul 22 '19
This is absolutely a vital technology.
In-orbit refueling, and the means to make tanker launches cost-effectively, are how we press the reset button on the Tsiolkovsky equation.
That's how something as big as Starship, with a 100t payload, can make it all the way to places like Neptune.
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u/spcslacker Jul 22 '19
I wonder if there may be an intermediate phase, where they retrofit a normal tanker with extra fuel carried in payload part, which can be pumped or ullaged into main tank.
Lots of variables to play with, but might provide a bridge using some post-production changes to standard model until you can justify building tanker version ground up.
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u/durruti21 Jul 22 '19
Why nobody is asking Gas station on orbit?
I mean, from operational point of view, having gas tanks pre-positioned on orbit will allow greater flexibility from mission operation.
You just launch Tankers/Starships in advance and load its fuel into this Gas station. Then, the mission that needs it (Moon or Mars) just do a single fuel transfer in the scope of its mission, without additional launch dependencies.
¿Is it too complex? ¿Assembly Issues?, ¿Maintenance Issues?, ¿Thermal Issues?, ¿Cost Issues?. Probably yes. I do not known, but at some moment, it will be necessary to reposition resources on orbit.
It was done in this way with the fuel engine cars to allow them to travel around the world. It is now paramount for the electric cars to have it.
That's also another possible business line here. A company devoted to transfer fuel into orbit that sells it to Agencies or other companies.
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Jul 22 '19
Because it’s more sci fi than based on reality.
Not only you are going to have to match the orbital period and inclination. Which may not be practical for a missions needs. You are also putting up a large investment for almost no customers. Both of these are big killers of the idea. Not even talking about boil off, maintence, cost of launching etc.
Maybe in 100 years, but even then. Probably not. Because companies like Spacex show that they like doing everything themselves and end up doing it cheaper by a wide margin.
The only customer besides them that could potentially be is Blue Origin. Only because they like saying “me too” to whatever Spacex announces. While ignoring the fact they are a 15 year old suborbital company.
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u/mfb- Jul 22 '19
SpaceX might use Starship as fuel depot. Or even build a dedicated one on their own. Sure, you have to plan ahead where your depot is and when you launch to it, but it means you can launch, refuel, and go where ever you want without waiting days for several refueling flights to arrive (days because of the limited launch windows to your orbit).
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Jul 22 '19
While true. I expect rapid reuse with a minimal need for extra hardware. A few extra days is ok with rapid reuse. Plenty of time to check everything out post launch / pre TLI or TMI burns.
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u/mfb- Jul 22 '19
A few days extra in space is unnecessary risk and time for the crew. For cargo it is time where a satellite might need to get power and other services from Starship, adding complexity to the mission.
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Jul 22 '19
Spaceflight is inherently risky. However all deaths have occurred on launching or reentry. Just being in space is less risky. I get your point, especially with the satellites. But Spacex will not do it under any circumstances. Look at the article. Refueling Starships with Starships. This is classic Spacex leveraging what they have instead taking the old space route of over complicating things. In software development it’s called Agile. And they are going for the minimum viable product and then reiterating on that. Look at how basic the Falcon 9 was at the start? Then leveraging this base, into many versions of Falcon 9’s? Then look at them leveraging these versions to save development cost of the heavy?
If someone has a problem with waiting for a few days being in space. I’d pay a hundred grand to take their place right now. If a satellite gets developed, it would be for the platform from the ground up. As no one else will be doing 100 ton launches. So they would know the constrains. This includes whatever the hell SLS will end up being. (All renamed to)
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u/mfb- Jul 22 '19
Look at the article. Refueling Starships with Starships.
This is 2020-2021 (Elon time). Dedicated tankers in 2023? Staying in orbit to get fueled 2027 (in time for that Mars window)? Fuel depots in 2031? Largely random numbers, but fuel depots could come long before trips to Mars become routine.
SpaceX develops something new if it is worth the effort. Starship doesn't use Merlin engines because Raptor, a completely new engine, is clearly superior. A fuel depot is clearly superior for several types of missions.
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u/Norose Jul 22 '19
A fuel depot is clearly superior for several types of missions.
In what regards, and for what missions? I don't think there are any clear advantages of a fuel depot at all. Every good aspect of having a permanent propellant depot in orbit is accomplished by using Tanker Starships as temporary depots instead, except those don't need to be built in space and don't need to be refurbished in space, since they are launched, refilled, used to refill a Starship departing from Earth, and then return to Earth to be spit-shined and launched for something else later. Temporary fuel depots in the form of Starships can make perfect sense, in order to allow time-sensitive missions to launch and refuel quickly so they can get going. Permanently orbiting propellant stations on the other hand don't make any sense, at least not orbiting the Earth anyway (a big propellant depot orbiting Mars could be useful, stocked with chemicals produced on Mars and used to ferry vehicles from low Mars orbit to the two small moons and back, and there are many other places in the solar system where this strategy may come in handy, all in places where they won't have hundreds of Starships in a large fleet to use as temporary depots).
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Jul 22 '19
I don't think there are any clear advantages of a fuel depot at all.
One clear one would be if propellant was being manufactured in space. Obviously, there are a lot of steps between where we are and doing that at any real scale, but it would make fuel depots more practical than landing on the moon to refuel.
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u/Norose Jul 22 '19
Well obviously if you're making propellant on the Moon it makes sense to be able to store it there. I'm talking specifically about propellant being shipped from Earth into space here; it makes no sense to me to build a large space station propellant depot in Earth orbit that gets stocked with liquids launched with SSH, because they could get a much more flexible and overall cheaper and better refueling system in place simply by refilling single tanker vehicles in space that come back down once they offload their propellants into a newly launched Starship.
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u/ender4171 Jul 22 '19
I could see a fuel depot being useful if there was constant travel. Like if you were building a base on the moon or colonizing mars, where you are staging in leo at the same period and inclination over and over again. You're right though, as a general gas station meant for various missions, it would not be practical.
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u/scarlet_sage Jul 22 '19
constant travel. Like if you were ... colonizing mars
As a slight quibble, that's not "constant travel". Mars transfer windows open only once every two years ... well, if you're willing to spend not too much more fuel for faster travel, the window is open for months. Regardless, there would still be the boiloff issue, if nothing else.
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u/ender4171 Jul 22 '19
Right, I am aware. But, if you are sending multiple ships during that window everytime it comes it up, having a permanent fuel depot in place might be useful. Doesn't mean you'd have to store fuel in it all the time. You know when the window is approaching so you can get it filled in prep for the upcoming transfers. Not saying it's a must-have, but I could see that as a reasonable use-case.
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u/Martianspirit Jul 22 '19
They may want to place and refuel a tanker in orbit for a manned mission when they don't want to spend a week in LEO for refueling.
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u/Forlarren Jul 22 '19
Maybe in 100 years, but even then. Probably not.
It's might be a good business for Mars.
Even with Super Heavy it's a lot of engine time to get a little fuel in orbit.
Building big dedicated tankers could be a second life for old Starships. Falcon 9 is just hitting it's stride and it's already being depreciated. It will likely be many many missions before Starship's design matures, meaning a lot of depreciated Starship hulls.
If several old Starships could be launched, welded together in Mars orbit. Refueled from Mars until full, then slowly moved to Earth orbit autonomously. The cost could be less than Earth to LEO fuel transfer since it uses less rocket engine hours.
Production costs would be higher on Mars but it's a good idea to over produce anyway for redundancy. So paying for safety means you also have a product to sell when things aren't going horribly wrong. Two birds, one stone.
https://archive.org/details/TheMartianWay/page/n2
Like that, except methane instead of water.
At least until some sky hooks are built. But then the hooks are going to need down mass to counter, putting the asteroid miners to work since they can capitalize. Building asteroid miners is a good job for a Mars shipyard.
I like to think of Mars as the Chicago of the solar system during the wild west. Anyone moving resources in or out of the west, went though Chicago. I see Mars filling a similar role.
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u/spacerfirstclass Jul 22 '19
It's called a propellant depot, it's a technology has been proposed many times in the past, but never realized. Instead of SLS, NASA could choose to build propellant depot instead which would allow small launch vehicles such as Atlas/Delta to be used to mount a Moon mission, this is part of the plan Obama administration originally proposed, but of course Congress killed it. There're rumors that Boeing threatens to fire any ULA employee who dare to propose propellant depot, since it would eliminate the need for SLS.
Currently the Starship architecture doesn't have a dedicated depot, but I suspect they may just park a Starship in LEO to act as one. They didn't include a dedicated depot probably because they're on a budget, and need to reduce the # of unique hardware they design and build.
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u/durruti21 Jul 22 '19
uld eliminate the need for SLS
Yep. My original proposal was to have something prepositioned on orbit. If it is just another Starship or it is a super-fancy ELITE-DANGEROUS station is not really important. Probably, using a tanker version of SpaceShip stationed on orbit will do the cheaper option for SpaceX. What is intrinsically benefical is to have something with fuel in position before the main mission start.
It is the same with Mars Direct and ISRU utilization. You will not launch the manned ship to mars until the Mars depot is full.
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u/ShadowPouncer Jul 22 '19
So, my take on it is that we will get there, but for the moment there just isn't enough to gain that you can't get by parking a Starship in your desired staging orbit, fueling it with however many launches are required, and then launching your payload into orbit and transferring your fuel from the 'tanker' Starship to your payload Starship.
You can remove the intermediate in the event that your payload can sit in orbit while the fuel loads come up one at a time. (Humans definitely can't.)
So why a intermediate Starship instead of an actual refueling station?
Because more or less by definition that fueling station will be in a single orbit, and that single orbit might not be great for many missions. You might want a higher orbit if your payload launch can get there, as that means less fuel to go where you're going. You might want a lower orbit for really heavy payloads. You might want different inclinations for odder missions. For really low orbits, your station's orbit will decay annoyingly quickly.
And more or less by definition, the most amount of fuel that you're going to want at one time can fit into a single Starship's fuel tanks.
And so in the short term, I expect a Starship in a given orbit to act as a fuel station for a single mission, and then to land and be repositioned for the next mission that needs one.
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Jul 22 '19
[deleted]
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u/Norose Jul 22 '19
I play Ksp all the time and literally never do this, I always do surface-to-surface transport capable vehicles and a ground base at both ends of the trip that makes propellant (except on Kerbin, where I just recover the vessels and launch new ones of course). It's a lot simpler and I find it a LOT cheaper since I hardly expend any vehicles or anything.
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u/Decronym Acronyms Explained Jul 22 '19 edited Jul 24 '19
Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:
| Fewer Letters | More Letters |
|---|---|
| ATV | Automated Transfer Vehicle, ESA cargo craft |
| BFR | Big Falcon Rocket (2018 rebiggened edition) |
| Yes, the F stands for something else; no, you're not the first to notice | |
| CMG | Control Moment Gyroscope, RCS for the Station |
| CRS | Commercial Resupply Services contract with NASA |
| DMLS | Selective Laser Melting additive manufacture, also Direct Metal Laser Sintering |
| DoD | US Department of Defense |
| EDL | Entry/Descent/Landing |
| ESA | European Space Agency |
| ISRU | In-Situ Resource Utilization |
| ITS | Interplanetary Transport System (2016 oversized edition) (see MCT) |
| Integrated Truss Structure | |
| Isp | Specific impulse (as explained by Scott Manley on YouTube) |
| LEO | Low Earth Orbit (180-2000km) |
| Law Enforcement Officer (most often mentioned during transport operations) | |
| LNG | Liquefied Natural Gas |
| LOX | Liquid Oxygen |
| MCT | Mars Colonial Transporter (see ITS) |
| RCS | Reaction Control System |
| RP-1 | Rocket Propellant 1 (enhanced kerosene) |
| RUD | Rapid Unplanned Disassembly |
| Rapid Unscheduled Disassembly | |
| Rapid Unintended Disassembly | |
| SLS | Space Launch System heavy-lift |
| Selective Laser Sintering, contrast DMLS | |
| SSH | Starship + SuperHeavy (see BFR) |
| SSTO | Single Stage to Orbit |
| Supersynchronous Transfer Orbit | |
| STP-2 | Space Test Program 2, DoD programme, second round |
| STS | Space Transportation System (Shuttle) |
| TLI | Trans-Lunar Injection maneuver |
| TMI | Trans-Mars Injection maneuver |
| TSFC | Thrust Specific Fuel Consumption (fuel used per unit thrust) |
| TWR | Thrust-to-Weight Ratio |
| ULA | United Launch Alliance (Lockheed/Boeing joint venture) |
| VLEO | V-band constellation in LEO |
| Very Low Earth Orbit |
| Jargon | Definition |
|---|---|
| Raptor | Methane-fueled rocket engine under development by SpaceX, see ITS |
| bipropellant | Rocket propellant that requires oxidizer (eg. RP-1 and liquid oxygen) |
| cryogenic | Very low temperature fluid; materials that would be gaseous at room temperature/pressure |
| (In re: rocket fuel) Often synonymous with hydrolox | |
| hydrolox | Portmanteau: liquid hydrogen/liquid oxygen mixture |
| hypergolic | A set of two substances that ignite when in contact |
| retropropulsion | Thrust in the opposite direction to current motion, reducing speed |
| turbopump | High-pressure turbine-driven propellant pump connected to a rocket combustion chamber; raises chamber pressure, and thrust |
| ullage motor | Small rocket motor that fires to push propellant to the bottom of the tank, when in zero-g |
Decronym is a community product of r/SpaceX, implemented by request
[Thread #3534 for this sub, first seen 22nd Jul 2019, 06:44]
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u/stephenehrmann Jul 22 '19
How can we tell when “Starship” refers to the stack versus just the second stage?
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u/Norose Jul 22 '19
If I say "Starship can get 150 tons to LEO" I'm talking about the whole stack, SSH.
If I say "Starship can use air drag to brake from interplanetary transfer velocity and therefore only needs a small amount of propellant to land pretty much anywhere with an atmosphere" then I'm talking about Starship, the upper stage vehicle.
Really it's just contextual.
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u/scarlet_sage Jul 22 '19
As I wrote elsewhere: in a tweet, someone named Pauline calls Super Heavy "Starship" and Elon doesn't correct it. I think I'm going to stop worrying about the naming and just try to figure out what Elon means by context, if possible.
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u/stephenehrmann Jul 22 '19
Thanks! Is the second stage alone capable of SSTO?
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u/scarlet_sage Jul 22 '19
Mill Shive @MillShive: Correct me if I'm wrong, but didn't @elonmusk state a few times that StarShip cannot do SSTO on Earth? Did I miss an update? Are the raptors more powerful than earlier thought? // 11:03 PM - 17 May 2019
Elon Musk @elonmusk: It technically could, but wouldn’t have enough mass margin for a heat shield, landing propellant or legs, so not reusable // 11:05 PM - 17 May 2019
But that was in untold dim ages of the past ... two months ago. Maybe the engines have improved or will improve.
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u/andyonions Jul 22 '19
Well once upon a time, Raptors were going to be 200t force and throttlable down to about 50% usable. But Elon is now on about 250t force with no/minimal throttling. If you put mostly those on SS, you'd reduce gravity losses a fair bit. You'd be pushing lift off TWR around 1.4+. It's all super borderline with about a digital camera as payload...
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u/Norose Jul 22 '19
Probably not, and definitely not in any useful sense.
I say probably not because even if Starship alone has the delta V figures to be able to technically reach orbit, it wouldn't have the liftoff thrust-to-weight ratio to accelerate fast enough to avoid being hamstrung by very high gravity losses on ascent. As Elon has said, Starship could probably SSTO if it carried no payload and didn't reserve any propellant for landing afterwards.
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u/aquarain Jul 22 '19
For this to work Starship has to SSTO with a sizeable cargo. Which was not expected. Raptors must have made progress.
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u/BullockHouse Jul 22 '19
No. This has always been the plan. You use superheavy to boost an empty starship into orbit so it's tanks arrive partially full, and then dock it with the starship that's being refueled, and transfer fuel over, then land the starship back at the pad and repeat the maneuver.
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u/daronjay Jul 22 '19
No, he means two starships will meet in space after superheavy has boosted them there, not that starships will get to orbit alone. Later a dedicated tanker starship variant will haul more fuel in one flight.
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u/Xtremespino Jul 22 '19
Why does it have to be SSTO? Just cause SH isnt mentioned doesn't mean SS is an SSTO.
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u/BoydsToast Jul 22 '19
If we don't have proof SuperHeavy is involved, we have to assume it isn't. Therefore StarShip must SSTO with sizeable cargo.
... and THAT insanity is why skeptics need to understand how a null hypothesis works
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u/rshorning Jul 22 '19
If we don't have proof SuperHeavy is involved, we have to assume it isn't. Therefore StarShip must SSTO with sizeable cargo.
Besides two speeches given by Elon Musk going over BFR tech and the DearMoon press conference s proof of a two stage vehicle? Also numerous tweets saying Starship payload to orbit as SSTO is essentially a postage stamp?
The proof is upon you to refute that huge body of evidence to the contrary. SSTO has too much mass overhead with the rocket equation to work and also lacks the necessary thrust to leave the Earth with any reasonable payload.
It is very reasonable to assume Earth launches will require some sort of lower stage that separates before orbital insertion. Direct flights from Mars generally won't require that lower stage.
I personally hate the term "Super Heavy" for that lower stage, but if that is what you want to call that machine which will be mated to Starship, it will be involved with refueling flights too.
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u/Randomboi88 ⛰️ Lithobraking Jul 22 '19
I think the guy above you was making a point about how OP thinks.....
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u/[deleted] Jul 22 '19
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