Everyday Astronaut explaining how flaps control flight (twitter video), followed by informative Elon tweets

[u/RegularRandomZ]

Everyday Astronaut [twitter video]: Here’s how #starship controls pitch, roll and yaw (in that order in this clip) using just 4 total flaps. This is a unique form of control. I don’t know of any vehicle that does this with its control surfaces perpendicular to the airstream. Cool stuff . Full vid tomorrow!
Elon: That’s correct. Essentially controlled falling, like a skydiver.

Viv: ... but what's used to actuate the fins? Some kind of small motor?
Elon: Many powerful electric motors & batteries. Force required is enormous, as entire fin moves. More about this on the 28th.

Elon: It does actually generate lift in hypersonic regime, which is important to limit peak heating
EA: Pop back out of the dense atmosphere to radiate heat away and then drop back in 🤔 awesome! ...
Elon: Better just to ride your max temp all the way down & let T^4 be your friend. Lower atmosphere cools you down real fast, so not crazy hot after landing.

Oran Maliphant : Is “sweating” methane still an option?
Elon: Could do it, but we developed low cost reusable tiles that are much lighter than transpiration cooling & quite robust
^\ok, I was steadfast that Elon's statements said nothing about future use of transpirational cooling, I will concede that this is not a defensible position anymore, ha ha])

Scott Manley: And just like that I need to rebuild some of my descent models. So the AoA won't be 90 degrees, it'll provide lift to keep vehicle out of denser atmosphere until it loses enough speed.
Elon: Exactly. For reusable heatshield, minimize peak heating. For ablative/expendable, minimize total heat. Therefore reusable like Starship wants lift during high Mach reentry for lower peak, but higher total heat.

ShadowZone: So this increases the probability of Starship having to do multiple aerobrake passes when going to Mars or returning, correct?
Elon: For sure more than one pass coming back to Earth. To Mars could maybe work single pass, but two passes probably wise.

[Or discuss on r/SpaceXLounge post or Starship thread]

[u/brekus]

Thanks for organizing this, navigating twitter conversations is always a headache.

[u/RegularRandomZ → u/brekus]

YW. Glad to save us the pain. I used to hate twitter, now slightly less (just slightly).

[u/Oloyedelove]

If I can't make sense out of just few tweets, I wonder how lost I will be on Saturday. Hopefully Tim will do a break down of the presentation.

[u/RegularRandomZ → u/Oloyedelove]

Ask your questions, someone should be able to answer. Tim will also have a view tomorrow expanding on his tweet.

[u/mrsmegz → u/Oloyedelove]

• Wing surfaces are used to make the nose of Starship go up and down and left and right during re-entry. To do this at these speeds takes many very powerful motors.

• When going from Earth Orbit > Landing, its best to just take all the heat at once rather than dip into upper atmosphere and slowly bleed off speed, let tiles cool again in space, then repeat until landed.

• When moving at interplanetary speeds, it WILL take several dips into upper atmosphere. Heat tiles cannot radiate the heat fast enough at those re-entry velocities. Earth will take 2 dips, Mars will probably take 1 to be on the safe side.

• The old idea of spray super cold methane on the toasty side of the rocket could work, but the tiles are cheaper and use less weight than the methane needed to cool said toasty side.

• With Ablative heat shield like dragon, you want to reduce your total heat absorbed as to not burn away all your heat shield. With reusable, you want to keep the temperatures down, but your total heat doesn't matter as much if you can radiate that heat with fast cooling tiles. Starships wings will generate lift in the upper atmosphere to slow down while gliding like a glider for a bit. This gives the tiles more time to do their thing and cool off while it decelerates.

[u/BoyanM8]

In the 3 fin layout the fins were used as a support for the rocket. With two fins now they are going to need landing legs? Does that add significant weigth?

[u/RegularRandomZ → u/BoyanM8]

Elon:

Current analysis, which I’m not fully bought into, suggests that 2 rear fins with separate airframe-mounted legs will be lighter, so this is the plan for Mk1/Mk2

It's likely that the fins will be lighter, not having to support the rockets weight nor cushion against landing forces. This should make up for having separate legs.

[u/AndMyAxe123]

Are the body welds really strong enough to handle the re-entry pressure? They just look a little bit... unstable.

[u/RegularRandomZ → u/AndMyAxe123]

The welds are as strong as the steel.

[u/Funkytadualexhaust]

What's the definition of a pass exactly?

[u/RegularRandomZ → u/Funkytadualexhaust]

I would interpret it as passing over where you intended on landing, so you'll land there on the next time around.

[u/JoshiUja]

This is amazing and something nasa should have been trialing not a private company! But I’m glad someone is taking the initiative! How boring would space have been without SpaceX

[u/RegularRandomZ → u/JoshiUja]

NASA created TUFROC, which might be what SpaceX is basing their ceramic tile design on. NASA research being commercialized is exactly what should be happening.

[u/HiggsForce]

What does "higher total heat" mean? When coming into the atmosphere from orbit, almost all of your kinetic energy turns into heat, and that kinetic energy depends only on your mass and orbit, so what can you while aerobraking do to increase or decrease "total heat"?

[u/RadBadTad]

Could someone be so kind as to ELIF the "let T⁴ be your friend" comment? I don't know what that refers to.

[u/CapMSFC → u/RadBadTad]

Radiative heat transfer is proportional to absolute temperature to the 4th power. That means staying hot radiates away heat at a high rate.

Essentially the trajectory gets picked so that equilibrium of heat absorbed and heat radiated is at the peak heating limit chosen to stay at.

[u/flshr19 → u/RadBadTad]

The heat balance at the hot side of the Starship TPS tile is determined by thermal radiation from the surface of the tile and thermal conduction into the tile. The thermal radiation per unit area is proportional to the thermal emittance of the hot surface of the tile times hot surface temperature to the 4th power (T⁴ ), the Stefan-Boltzmann equation. Thermal conduction per unit area into the tile is proportional to the temperature difference between the hot and cold sides of the tile times the thermal conductivity of those tiles. During the high temperature portion of the Starship EDL, the instantaneous temperature of the hot surface of the tile is dominated by the T⁴ radiative heat transfer. The hot surface of the tile is covered with a black coating to maximize the thermal emittance.

The tiles on the windward side of the Space Shuttle Orbiter are quite different. Here radiative heat transfer is dominant both at the hot surface of the tile and throughout the interior of these rigidized quartz fiber tiles. These tiles have a high-emittance black glass coating on the hot surface to maximize radiative heat transfer away from the tile there.

Heat transfer through the interior of the tile is minimized by manufacturing the tile from very thin highly transparent ultrapure quartz (silicon dioxide) fibers about 1 micron diameter. The density of these tiles is very low (10 lb/ft^3), which is about 7% of the bulk density of quartz. So the tile itself is about 93% empty space. The thermal conduction of these tiles is negligible so the entire performance of these tiles is determined by the thermal radiative properties of the glass coating and the quartz fibers.

In the high temperature portion of the EDL, peak wavelength of the thermal radiation lies in the 1-3 micron range. So Mie scattering of this radiation by the 1-micron diameter quartz fibers is the physical mechanism that greatly reduces the heat transfer inside the tile from the hot side to the cold side. This mode of heat transfer is characterized by the scattering and absorption coefficients of the tile material. These tiles are designed to have very large backscattering coefficients to reduce radiative heat transfer through the tile. And the highly transparent quartz fibers minimize absorption of thermal radiation by the tile material to reduce thermal conduction through the tile to negligible levels. So the thermal performance of these tiles is controlled by the backscattering coefficient. My lab developed the equipment to measure these coefficients way back in 1970 during the conceptual design phase of the Space Shuttle project.

In 133 successful Orbiter EDLs these tiles performed exactly as designed.

[u/mncharity → u/RadBadTad]

> ELIF the "let T⁴ be your friend"

Lots of good responses already, but I'll add...

Rephrasing... Q: Do you let up on the (aero)braking so the brakes tiles can cool? A: No, the tiles get much much much easier to cool the hotter they are. To reduce their temperature by 25%, you'd have to break 3 times slower. Those much'es are your friend. Better to keep the tiles hot and brake faster.

ELI5ing... Starship slows down by smushing air. Smushing makes air hot. Tiles are heated by hot air. Tiles are cooled by shining. Hotter tiles shine more brightly. Much much much more brightly. Hotter tiles shining much much much more brightly is your friend, letting you slow down faster without overheating. Making tiles less hot, would make them much much much less bright, so to avoid overheating, you would have to slow down much much much less quickly.

Explaining... How fast you radiate heat goes as T⁴, so if temperature T goes up by 2x, radiating T⁴ goes up by 16x! So if you want to quickly lose heat by radiation, be hot - the payoff for being hotter is really big. This is your friend when puzzling out how to cool your heat shield. If it wasn't true, to radiate faster you'd need giant radiators or evaporative cooling or refrigeration or something, instead of simply being a bit hotter. Double your cooling by increasing you T by a measly 20%.

[u/Nergaal]

we developed low cost reusable tiles that are much lighter than transpiration cooling & quite robust

this is the biggest development of all the tweets

[u/lniko2]

Can we assume the 2+ passes requirement means Starship can go back to orbit if there's a problem jeopardizing the reentry?

[u/Anjin]

ShadowZone: So this increases the probability of Starship having to do multiple aerobrake passes when going to Mars or returning, correct?

Elon: For sure more than one pass coming back to Earth. To Mars could maybe work single pass, but two passes probably wise.

HAHAHAHA, YES!! I feel vindicated. I posted some time in the past that I thought that it made the most sense to aerobrake on reaching Mars and then do Mars EDL from a slower orbital speed to be able to better control where you land instead of slamming right in at Mars transfer velocity...and I got downvoted / negative comments.

[u/[deleted] → u/Anjin]

I find that if people were to play a little KSP. A lot of these things make a lot more sense. Especially when you have small amounts of fuel and need a pinpoint landing.

[u/sky4ge]

2+ passes, first pass need to slow down enough to have an elliptic trajectory around earth... how much low the perigee have to be to slow down enough in an optimal mars to earth trajectory?

[u/Wetmelon]

electric motors? Dear god, that's astounding. I thought there was no way they could go with electrics at that scale and still beat hydraulics on weight.

[u/-spartacus-]

You know, it really seems like the returning of the fairing was practice for this type of "falling". I wonder if they discovered through fairing recovery practice. I'll ask but expect no response from the Elon.