Starship heat shield

[OTV Booster]

The emissive (ablative?) paint is flaking off a bit on S24:

https://twitter.com/cnunezimages/status/1632856594672222208

I'm pretty sure this is just a paint and has nothing to do with heat shielding

Why would they only paint next to the heat shield were heat can still be high?

Paint can be a heat shield, using two different methods:

1.  The emissivity of stainless steel is 0.35.  A black paint, the special kinds, can get to 0.99, same as the tiles.  This nearly triples the watts being emitted from a hot section of steel.
2.  Ablative


Either of those count towards managing the heat of reentry, and thus count as a heat shield

A possible alternative: Elon is big on visuals.

[chopsticks]

Otherwise there's zero reason to paint at all.  "best part is no part".

I think I'm going to barf if I hear this one more time.

You realize that they painted B4's engine bells, too right?

Pray tell, what was "the necessary part" in this case?

[JamesH65]

I suspect the conversation went as follows:

Elon: Hmm, those jagged edges of the tiles make it look unfinished.
Someone else: We don't want to make and attach half tiles just to make it look nice!
Elon: Just paint on a straight black edge, it will look fine from a distance.

[wannamoonbase]

Heat shields are tough, time consuming and expensive. 

SpaceX is going to have to improve this whole process a lot, but first, gotta fly!

[dondar]

I think the paint is just for "pretty".

John

is this really a paint? looks to be a goo they used as the base to clue tiles on. (it is quite thick as you can see in the damaged spots.The form indicates initial design specs.

[xvel]

I think the paint is just for "pretty".

John

is this really a paint? looks to be a goo they used as the base to clue tiles on. (it is quite thick as you can see in the damaged spots.The form indicates initial design specs.

I think you are quite wrong, most of the tiles are not glued but mechanically attached and those that are glued are glued directly with something like high temperature rtv sillicone, and this was painted after the tiles were attached

https://twitter.com/NicAnsuini/status/1533077216036274179

photo before painting

[uhrflieger]

3 years ago I was able to follow the extensive discussion about the absolute avoidance of straight joints. 
https://forum.nasaspaceflight.com/index.php?topic=50748.msg2073366#msg2073366
https://twitter.com/elonmusk/status/1107379727302451200
Now I've been wondering for quite some time about the practical implementation of the problem where the creative tile shapes discussed here have not been used.  Instead, dead-straight joints were arranged in the nose cone area.  Sorry I haven't gone through all the >170 pages here yet, can someone please give me a link if this has already been discussed and justified here?

[Crispy]

Now I've been wondering for quite some time about the practical implementation of the problem where the creative tile shapes discussed here have not been used.  Instead, dead-straight joints were arranged in the nose cone area.  Sorry I haven't gone through all the >170 pages here yet, can someone please give me a link if this has already been discussed and justified here?

The nose cap tiles (and other "non-standard" areas with aligned joints like the flap hinges) are glued instead of bracketed, and have a filler between them to keep the hot gas out.

[TheRadicalModerate]

Do we have a consensus guess for the maximum entry speed that Starship must handle?  Do you think it'll be higher or lower than 11km/s?

Am I correct in assuming that, from a thermal standpoint, it doesn't matter much whether the entry is on Earth or Mars?  (Constraining the amount of permissible negative lift needed to keep the vehicle in the atmosphere on Mars is another story...)

[whitelancer64]

Do we have a consensus guess for the maximum entry speed that Starship must handle?  Do you think it'll be higher or lower than 11km/s?

Am I correct in assuming that, from a thermal standpoint, it doesn't matter much whether the entry is on Earth or Mars?  (Constraining the amount of permissible negative lift needed to keep the vehicle in the atmosphere on Mars is another story...)

Higher than 11 km/s by definition. Minimum velocity on a free return trajectory from Mars is 11.4 km/s but is typically around 12-13ish km/s (NASA limited studies for human missions to a maximum reentry velocity of 16 km/s).

The exact velocity is heavily dependent on the Hohmann transfer window and the dV of the initial orbit injection burn.

Heat loads are significantly higher on Earth than on Mars, due to the higher velocity of entry. Mars entry velocity is typically around 4.5-7.5ish km/s for the various probes NASA has sent.

[TheRadicalModerate]

Heat loads are significantly higher on Earth than on Mars, due to the higher velocity of entry. Mars entry velocity is typically around 4.5-7.5ish km/s for the various probes NASA has sent.

I was looking for an apples-to-apples, entry-speed-normalized answer to that question.  I'm assuming that heat loads for an 11.4km/s entry speed (not v∞) on Earth and Mars should be roughly the same.  (If you know the entry speed, you can figure out the v∞, a vice versa.)  The Mars entry trajectory will stabilize lower in the atmosphere, but the differences in composition should have only a small effect, correct?

Good point about needing to be able to cover the free return case.

I'm fooling with this because I want to be able to constrain arrival v∞ in both directions.  That seems to be more of a gating item on time of flight than the departure v∞.  However, if you're flying a with a full load of prop, you can keep some propulsive braking prop.  But that of course limits your departure speed in a different way...

[steveleach]

Heat loads are significantly higher on Earth than on Mars, due to the higher velocity of entry. Mars entry velocity is typically around 4.5-7.5ish km/s for the various probes NASA has sent.

I was looking for an apples-to-apples, entry-speed-normalized answer to that question.  I'm assuming that heat loads for an 11.4km/s entry speed (not v∞) on Earth and Mars should be roughly the same.  (If you know the entry speed, you can figure out the v∞, a vice versa.)  The Mars entry trajectory will stabilize lower in the atmosphere, but the differences in composition should have only a small effect, correct?

Good point about needing to be able to cover the free return case.

I'm fooling with this because I want to be able to constrain arrival v∞ in both directions.  That seems to be more of a gating item on time of flight than the departure v∞.  However, if you're flying a with a full load of prop, you can keep some propulsive braking prop.  But that of course limits your departure speed in a different way...

If entry speed is your limiting factor then wouldn't it be best to include a braking burn almost by definition?

Take whatever dv you have available above that which gives the max entry speed, and divide it across acceleration and braking.

[sebk]

Heat loads are significantly higher on Earth than on Mars, due to the higher velocity of entry. Mars entry velocity is typically around 4.5-7.5ish km/s for the various probes NASA has sent.

I was looking for an apples-to-apples, entry-speed-normalized answer to that question.  I'm assuming that heat loads for an 11.4km/s entry speed (not v∞) on Earth and Mars should be roughly the same.  (If you know the entry speed, you can figure out the v∞, a vice versa.)  The Mars entry trajectory will stabilize lower in the atmosphere, but the differences in composition should have only a small effect, correct?

Not really.

Mars has different atmospheric composition which, among other things, means:

* A higher Mach number (for the same velocity). Your 12.5km/s Mach 46 Earth atmosphere entry is Mach 60 in CO2 atmosphere.
* Very different radiative environment in and behind the bow shock.

The net result is that even Mars ballistic entries are considered worse than Earth ones at the same speed.

But all of this is minor.

The major part is that at Mars you must do negative lifting entry at anything even similar to Earth's LEO entry, not to mention interplanetary entry. And the thing is at high Mach numbers your L:D ratio is severely limited. This dictates high g-loads. And high g-loads mean ~proportionally higher heating rates.

IOW At Earth you could limit your heating rate by skidding on top of the atmosphere for quite a while. Not so much on Mars where you must accept high g-load and accompanying heating rate.

[livingjw]

Heat loads are significantly higher on Earth than on Mars, due to the higher velocity of entry. Mars entry velocity is typically around 4.5-7.5ish km/s for the various probes NASA has sent.

I was looking for an apples-to-apples, entry-speed-normalized answer to that question.  I'm assuming that heat loads for an 11.4km/s entry speed (not v∞) on Earth and Mars should be roughly the same.  (If you know the entry speed, you can figure out the v∞, a vice versa.)  The Mars entry trajectory will stabilize lower in the atmosphere, but the differences in composition should have only a small effect, correct?

Not really.

Mars has different atmospheric composition which, among other things, means:

* A higher Mach number (for the same velocity). Your 12.5km/s Mach 46 Earth atmosphere entry is Mach 60 in CO2 atmosphere.
* Very different radiative environment in and behind the bow shock.

The net result is that even Mars ballistic entries are considered worse than Earth ones at the same speed.

But all of this is minor.

The major part is that at Mars you must do negative lifting entry at anything even similar to Earth's LEO entry, not to mention interplanetary entry. And the thing is at high Mach numbers your L:D ratio is severely limited. This dictates high g-loads. And high g-loads mean ~proportionally higher heating rates.

IOW At Earth you could limit your heating rate by skidding on top of the atmosphere for quite a while. Not so much on Mars where you must accept high g-load and accompanying heating rate.

Max L/D changes hardly at all after Mach ~4.  Max L/D of ~1 should be achievable throughout Mars entry.

John

[sebk]

Yes, Starship according to various reports has L:D from 0.5 to 0.8.

At reportedly nominal 60° AoA I'd expect L:D ~0.5

Still, at Earth, due to lower curvature you could skim the atmosphere for quite a bit without even trying to produce lift. You could fly for about 2800km (with half of that in a densest zone providing ~90% of braking) with zero lift and stay with g-loads below 1. So you could start your reentry at over 12km/s and without any lift and without exceeding 1g slow down enough to capture.

If you tried the same exercise at Mars, at 12km/s you'd just reduce your velocity to 11.75km/s or so and wouldn't be appreciably closer to capture.

[InterestedEngineer]

Answers the questions I had about how SpaceX might monitor Mars aerocapture/entry

https://www.jpl.nasa.gov/images/pia22317-marco-being-tested-in-sunlight

[eriblo]

I did some simplified sims a while ago - as stated above entries at Mars are generally gentler because they are slower. At the same entry velocity the peak temperature and g-forces are higher due to the smaller radius and gravity.

A 6 km/s entry at Mars has about the same peak temperature as an 8 km/s entry at Earth. If you have a 5 g₀ limit the max entry speed at Mars is 12 km/s and at Earth it is 15 km/s.

I expect an fully reusable tile bases heat shield to be good for LEO and multi-pass cis-lunar as well as slow Mars transfer and Mars Orbit. For fast Mars transfers and Earth return I think they will need an ablative heat shield.

[InterestedEngineer]

I did some simplified sims a while ago - as stated above entries at Mars are generally gentler because they are slower. At the same entry velocity the peak temperature and g-forces are higher due to the smaller radius and gravity.

A 6 km/s entry at Mars has about the same peak temperature as an 8 km/s entry at Earth. If you have a 5 g₀ limit the max entry speed at Mars is 12 km/s and at Earth it is 15 km/s.

I expect an fully reusable tile bases heat shield to be good for LEO and multi-pass cis-lunar as well as slow Mars transfer and Mars Orbit. For fast Mars transfers and Earth return I think they will need an ablative heat shield.

How did you you simulate maximum heating and what was the criteria for "max entry speed"?

[TheRadicalModerate]

Yes, Starship according to various reports has L:D from 0.5 to 0.8.

At reportedly nominal 60° AoA I'd expect L:D ~0.5

Still, at Earth, due to lower curvature you could skim the atmosphere for quite a bit without even trying to produce lift. You could fly for about 2800km (with half of that in a densest zone providing ~90% of braking) with zero lift and stay with g-loads below 1. So you could start your reentry at over 12km/s and without any lift and without exceeding 1g slow down enough to capture.

If you tried the same exercise at Mars, at 12km/s you'd just reduce your velocity to 11.75km/s or so and wouldn't be appreciably closer to capture.

Ultimately, all I'm trying to get is a maximum value for arrival v∞ for both Mars and Earth.

There will be a maximum allowable entry speed:

vEntryMax = min(vMaxThermal, vMaxAccel)

where:

1) vMaxThermal is the speed that results in the maximum allowable heat pulse at some magic altitude.  The magic altitude will be different for Earth and Mars, but my assumption is that vMaxThermal at that magic altitude will be the same.  I don't know how to compute this. 

I'm also not sure that this normalization of the problem to a single entry speed at some planet-dependent magic altitude is valid.  I see that, just above, eriblo doesn't think this is true.  Why?

2) vMaxAccel is the maximum allowable speed that keeps the payload and/or crew from being crushed from lift acceleration while maintaining Starship at the magic altitude:

maxLiftAccel = vMaxAccel²/rMagic - g
where
rMagic = magicAltitude + rBody
and
g = μ/rMagic²

So the constraint is:

vMaxAccel <= sqrt(rMagic*maxLiftAccel + μ/rMagic)

I'm guessing that maxLiftAccel will be about 3 gees (29.4m/s²).

Once we've computed vEntryMax, we can set the hyperbolic periapse to rMagic and solve for the arrival v∞:

vInfinity = sqrt(vEntryMax² - 2μ/rMagic)

Clarification:  I expect that vMaxThermal will be the limiting condition for Earth, but vMaxAccel will limit Mars entry.

[sebk]

I think for thermal you have 2 primary constraints, not 1:

1. Heat pulse, i.e. the one you mentioned. This is the amount of energy which gets to your ship.
2. Peak heating. This is the maximum heating rate. Among other things this dictates skin temperature.

Ablative heat shields are primarily concerned with the first (1). But non-ablative ones are concerned with both (1 & 2), and it's easier to adjust them for 1 if a need arises (generally you improve the insulation layer). But adjusting for a higher peak heating (2) essentially means coming up with a different material which is not an easy change, and often means redoing your heatshield from scratch. And If you ramp peak heating just a tad too much we're simply out of known materials at all.

The thing about (2) is that it depends not just on velocity. It depends on deceleration as well. If you replace 2g braking with 4g braking you essentially doubled your peak heating.

At the same time you have likely reduced(!) heat pulse! This is counterintuitive, because kinetic energy to lose is the same regardless of how quickly you drop it. But the thing is, at certain point the more rarefied the ambient atmosphere the larger fraction of heat gets to the vehicle (and less is taken away by the ambient air). So when you pass entry interface heating initially grows much faster than deceleration. Just look at Shuttle reentry heating and deceleration profiles. Or some (SpaceX?) materials about Starship reentry. You're dropping speed slowly while heating is already high.

So if you have an ablative heatshield, you want to punch through the too thin air fast into high deceleration region. Total heat pulse is less that way, so you need less material to ablate. And you don't care that much about peak heating (within limits), as the temperature is primarily set by your ablative material's phase changes.

But if you have non-ablative heatshield, you prefer skidding on top of the atmosphere. The heat pulse is larger (often significantly), but you keep peak heating in check by keeping the whole kinetic energy drop rate in check. But if you have to generate more (negative) lift to hold onto a small low planet, you need to increase drag according to L:D ratio of your vehicle, which means higher deceleration which means higher kinetic energy drop rate and higher heating.