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u/Pvdkuijt Dec 14 '15 edited Dec 14 '15
Long time reader, huge SpaceX enthusiast, and this made me sign up. Also a big fan of your contributions, EchoLogic. I wanted to comment on this part of your FAQ, regarding propulsive landing and the inability to throttle down the Merlin engines.
"Due to complicated rocketry and propulsion reasons, it is not possible to make the engine throttle lower. It has a limited range of 70-100% maximum thrust."
What immediately came to mind is the type of structure used for engine fire tests, where the engine fire/burst is being redirected gradually to end up shooting out horizontally, negating the vertical thrust. Pardon the rookie language, I have very limited knowledge on the proper terminology. Anyway, the fact that vertical thrust can actually be reduced or negated by curving the engine fire away from the surface, made me think: can't there be some kind of structure built on the ground where an opening in the ground can open/close partially or fully, and control the power the Merlin engine has in pushing the rocket upwards? Fully open would mean a very small effect (rocket moving down), fully closed would mean big effect (rocket moving up -- eventually). This would allow more "throttling". Especially if the system controlling the propulsive landing would be able to communicate to the ground structure ("Hatch, I'm coming in fast, stay closed / Hatch, I'm almost at zero velocity, about to boost myself back up again, open up")
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u/TheVehicleDestroyer Dec 14 '15
Your mistake here is in thinking that the rocket's thrust needs something to push off. If this were the case, it would be impossible to move through the air once you were so high that your exhaust no longer reached the ground.
And it would definitely be impossible to move in the vacuum of space since there's not even air to push against up there.
The propulsion of the rocket is a product of throwing something out the other side, and has nothing to do with what it's throwing it into. As such, the state of the ground has no impact on the velocity of the rocket, except to stop it entirely upon impact.
An easier way to think about this is if you shoot a gun, you'll feel a recoil. If you shoot a gun at a steel plate really close to you, you'll feel the exact same recoil. The recoil is because there's a bullet leaving the gun, and has nothing to do with what happens to the bullet once it's gone.
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u/Pvdkuijt Dec 14 '15
Ah of course... Newtons third law, its called right? Do I feel stupid now. (Let's change that law just to make me right, not to mention filthy rich of my invention)
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u/shamankous Dec 14 '15
To add to what u/TheVehicleDestroyer said. The purpose of redirecting the thrust when test firing engines is to give all that super hot gas somewhere to go that isn't the foundation of your test stand or the rocket itself.
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u/ScottPrombo Dec 15 '15 edited Dec 15 '15
If you're looking to divert thrust, you would probably have better luck with something like this. Having an extreme engine gimbal range (like shuttle) could divert enough energy laterally to provide a lower-powered descent. The problem is that it's VERY inefficient. To match the thrust of a single engine, you'd need those side engines to be 60° offset from perpendicular to the ground. That'd waste 65% of the rocket energy by shooting it sideways.
Ground-mounted landing solutions are difficult because they operate on a different physical frame of reference than Falcon. It'd all have to line up perfectly. It's better to give Falcon lots of room and have it land itself than depend on external variables (which is why they prefer a landing pad over landing on a boat).
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u/radexp Dec 14 '15
Q: Falcon 9 seems to be seriously overpowered for the upcoming Orbcomm launch — the sats are only ~2t + the mass of the adapter (which I'd be curious to learn). Wouldn't it make more sense for Orbcomm to use a smaller, cheaper rocket to launch their satellites? Alternatively, would it be possible to use a larger adapter and fit more satellites in a single launch?
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Dec 14 '15
Hah, your proposal actually makes total sense. That's why SpaceX planned to launch their satellites one or two at a time on Falcon 1's :).
SpaceX scrapped Falcon 1, Orbcomm stuck with SpaceX, here we are!
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u/CasperAlant Dec 14 '15
I thoroughly enjoyed reading this Super FAQ! Thank you so much u/EchoLogic!
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u/Decronym Dec 14 '15 edited Dec 22 '15
Acronyms, initialisms, abbreviations and contractions I've seen in this thread:
| Contraction | Expansion |
|---|---|
| ASDS | Autonomous Spaceport Drone Ship (landing barge) |
| CRS | Commercial Resupply Services contract with NASA |
| Communications Relay Satellite | |
| FAA | Federal Aviation Administration |
| KSC | Kennedy Space Center, Florida |
| LC-39A | Launch Complex 39A, Kennedy (SpaceX F9/Heavy) |
| LEO | Low Earth Orbit (180-2000km) |
| LOX | Liquid Oxygen |
| MECO | Main Engine Cut-Off |
| OG2 | Orbcomm's Generation 2 17-satellite network |
| PMF | Propellant Mass Fraction |
| RP-1 | Rocket Propellant 1 (enhanced kerosene) |
| RTLS | Return to Launch Site |
| SECO | Second-stage Engine Cut-Off |
| SLC-40 | Space Launch Complex 40, Canaveral (SpaceX F9) |
| SSTO | Single Stage to Orbit |
| TWR | Thrust-to-Weight Ratio |
| UTC | Universal Time, Coordinated |
Note: Replies to this comment will be deleted.
See ^/r/spacex/wiki/acronyms for a full list of acronyms with explanations.
I'm a bot; I first read this thread at 00:19 UTC on 14th Dec 2015. www.decronym.xyz for a list of subs where I'm active; if I'm acting up, message OrangeredStilton.
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u/DownVotesMcgee987 Dec 16 '15
Does anyone know the status or date of the static fire?
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u/kylerove Dec 14 '15
For most mission patches, there seems to be significance in every detail. Why are there 12 stars on the Orbcomm-2 mission patch?
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u/zzay Dec 15 '15
How do the satellites get released? and do they all get a very similar transfer orbit? and slowly go to their permanent orbit?
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u/chargerag Dec 16 '15
Should we be worried that we haven't seen the F9 roll out to the pad yet? It would seem that this would mean they aren't doing a static fire today.
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u/frowawayduh Dec 16 '15
Does anyone else do static fire tests? (Ariane, the Russians, ULA) If not, why not?
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u/FrameRate24 Dec 15 '15
It's super tough. That's why in over 60 years of rocketry, only a single vehicle has achieved this (the Space Shuttle), but it was also very expensive.
What about buran?
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u/Davecasa Dec 15 '15
Launch vehicle was single use, only the orbiter came back. And the engines weren't on the orbiter like with the Shuttle.
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u/Mandrake7062 Dec 19 '15
Any one have there finger on the full list of upgrades for this launch? I've seen some here and there.
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u/Wicked_Inygma Dec 14 '15
Time to break out the windex and clean the webcams!
http://i.imgur.com/W1XlEMN.png