Re-entry heating mystery

[Loren Pechtel]

Situation #1:  The craft has a heat shield.  I have learned the hard way that you must not have your periapsis too high or you'll burn off your heat shield without slowing down enough.

Situation #2:  The craft does not have a heat shield.  I'm thinking of my current tourist bus.  If I set the periapsis at 50km it comes through the fire intact.  If I set it any lower the engine explodes.

These can even combine--I've had a situation where I ran out of fuel with the periapsis too high.  After multiple tries I found the solution was to go in nose first.  Things got hot but nothing blew up, on the last skip into space I turned tail and took it on the heat shield.

[Ohm is Futile]

...and the question is?

Anyways, I'm going to guess you might want tips for re-entry. Well, beyond having heat shields and maybe something capable of holding retrograde, you may want your return vehicle's CoM to be closer to the heat shield than the other end of the craft. That prevents your vehicle from flipping around during re-entry, mostly.

Also, not unlike real life, there's a sweet spot for re-entry. As you have noticed, stay up high too long and you burn off the ablator of your heat shields and skip off the atmosphere back into space. Bad. Go in too fast and too sharp an angle and you might as well be landing on the sun. The trick is to drop to an altitude low enough that your craft will slow down at an appreciable rate as heat builds up over time, but not so fast that you crash on the ground before you can open your chutes or instantly explode upon reaching a low altitude (usually about 50km). Of course, there is a point at which no amount of skill can save you without a heat shield and some ablator.

Be mindful of which parts are exposed to the air flow as they have different heat tolerances which can be viewed in the VAB. Be prepared to burn off some bits in bad situation, sometimes saving part of the ship is better than nothing.

[Reusables]

It's because heatshields are so aerodynamic that the craft don't want to bleed off enough speed with high Pe.

So it ends up with longer reentry, where ablator runs out and heat conducts to the shielded parts causing overheating.

 

Besides, parachutes have great heat tolerance - you can, basically, use (radial) parachutes as heatshield. I think it's why the craft sometimes can reenter nose-first. (Probably the craft is more draggy in this way, or there's heat-vulnerable parts at the back)

[Loren Pechtel]

51 minutes ago, Abastro said:

It's because heatshields are so aerodynamic that the craft don't want to bleed off enough speed with high Pe.

So it ends up with longer reentry, where ablator runs out and heat conducts to the shielded parts causing overheating.

 

Besides, parachutes have great heat tolerance - you can, basically, use (radial) parachutes as heatshield. I think it's why the craft sometimes can reenter nose-first. (Probably the craft is more draggy in this way, or there's heat-vulnerable parts at the back)

It doesn't seem to take longer with a shield than with an engine sticking out the back.  Going in nose first definitely takes longer, though, if you are trying to bleed off orbital speed before taking the plunge.

I think what's going on is the ablator is burning off at temps that many components can survive.

[Reusables]

2 minutes ago, Loren Pechtel said:

I think what's going on is the ablator is burning off at temps that many components can survive.

You're right there. Ablator will burn off in relatively low temperature, so longer reentry definitely means it runs out.

But the craft won't explode without any ablator remaining in this case - Is this what you mean?

[Loren Pechtel]

1 hour ago, Ohm is Futile said:

...and the question is?

Anyways, I'm going to guess you might want tips for re-entry. Well, beyond having heat shields and maybe something capable of holding retrograde, you may want your return vehicle's CoM to be closer to the heat shield than the other end of the craft. That prevents your vehicle from flipping around during re-entry, mostly.

Also, not unlike real life, there's a sweet spot for re-entry. As you have noticed, stay up high too long and you burn off the ablator of your heat shields and skip off the atmosphere back into space. Bad. Go in too fast and too sharp an angle and you might as well be landing on the sun. The trick is to drop to an altitude low enough that your craft will slow down at an appreciable rate as heat builds up over time, but not so fast that you crash on the ground before you can open your chutes or instantly explode upon reaching a low altitude (usually about 50km). Of course, there is a point at which no amount of skill can save you without a heat shield and some ablator.

Be mindful of which parts are exposed to the air flow as they have different heat tolerances which can be viewed in the VAB. Be prepared to burn off some bits in bad situation, sometimes saving part of the ship is better than nothing.

I'm trying to understand why the best strategy with a heat shield is go fairly deep, but if you don't have a shield the best strategy is to stay high for as long as possible.  A periapsis of 50 will burn you heat shield away in a few passes through the fire, yet it's a safe altitude to aerobrake anything reasonable, even my mission from Eeloo that was still on an escape orbit after dipping into the fire (albeit now slow enough that burning it's remaining fuel got capture.)

[Lelitu]

56 minutes ago, Loren Pechtel said:

I'm trying to understand why the best strategy with a heat shield is go fairly deep, but if you don't have a shield the best strategy is to stay high for as long as possible.  A periapsis of 50 will burn you heat shield away in a few passes through the fire, yet it's a safe altitude to aerobrake anything reasonable, even my mission from Eeloo that was still on an escape orbit after dipping into the fire (albeit now slow enough that burning it's remaining fuel got capture.)

This is all to do with limitations in the KSP heating model.
A part takes exactly no damage until it exceeds maximum temperature. Once maximum temp is exceeded by any amount, it immediately explodes.
Ablative heat shields burn off ablator above a critical temperature (~1500K) to stop heat conducting to the rest of the ship, and have a very high max temperature (3300K vs ~1500K or less). 

A shallow re-entry has quite low drag, but very high heating, as the heat flux scales with the cube of velocity, and air density but drag only with velocity squared and air density, so max temperature rises much faster than drag. Total heat load is heat flux times time, So a shallow reentry gets a relatively low temperature for a very long time, resulting in very high heat loads. KSP doesn't model part overheating very well, so this doesn't have the same effects it would in real life (roasted astronauts and computers, probably exploded pressure tanks too).

A steep re-entry reaches denser air still travelling very fast, so it's exposed to *very* high temperatures, but not for long, since the high drag slows you down fast. This results in much higher G loads,and a much smaller total heat load. The lower total heat load means that much less heat soaks into the ship, and the interior stays relatively comfortable, despite the plasma filled hell just outside. In KSP, the big advantage is that a heatshield's mass is dependent on how much ablator it has, so the steeper the re-entry the less ablator you need to get through it alive. If you know you're going to come in steep, and only need ~30 ablator, you can cut back to ~50(to leave a margin) and save some mass through the whole rocket.

The steep re-entry is a bit like opening the oven door, a blast of extremely hot air, but not enough to seriously overheat you.
The shallow re-entry is more like climbing in to the oven (at minimum power)

Edited May 26, 2017 by Lelitu

[bewing]

The ablative heatshields have such low drag that they simply won't slow you down much unless your Pe is pretty deep in the atmosphere. For most missions in Kerbin's SOI you don't need a heatshield at all, so the best strategy is not to use one. Players overuse heatshields IMO. If you don't have one, then you need to build your ships for high temp resistantance, and high (or variable) aerodynamic drag.

If you are coming in at interplanetary speeds, then ablative heatshields are certainly going to be necessary for your first pass through the fire, to get captured into the proper SOI. Maybe two passes. But then you dump them, and do the final reentry using (once again) heat resistant parts and aerodynamics.

 

[Lelitu]

1 hour ago, bewing said:

The ablative heatshields have such low drag that they simply won't slow you down much unless your Pe is pretty deep in the atmosphere. For most missions in Kerbin's SOI you don't need a heatshield at all, so the best strategy is not to use one. Players overuse heatshields IMO. If you don't have one, then you need to build your ships for high temp resistantance, and high (or variable) aerodynamic drag.

If you are coming in at interplanetary speeds, then ablative heatshields are certainly going to be necessary for your first pass through the fire, to get captured into the proper SOI. Maybe two passes. But then you dump them, and do the final reentry using (once again) heat resistant parts and aerodynamics.

 

It has almost nothing to do with the heatshield's drag levels, and everything to do with highest temperature experienced. 

[Kerbal101]

I learned the hard way that heating is a product of vehicle re-entry speed VS tolerances of components.
And vehicle re-entry speed is a product of: vehicle energy (total orbit, less is better), vehicle weight (less is better, drop fuel/parts, G influenced) and vehicle air resistance (high and low are bad);
and tolerances of components is a product of: max K of heat-exposed components (higher is better, rotation helps) and atmosphere density (low - heats more than cools, high density - very high friction, medium density - good)
Eitherway drogue chutes, airbreaks and retrograde burn to the rescue.

[Spricigo]

11 minutes ago, Kerbal101 said:

I learned the hard way that heating is a product of vehicle re-entry speed VS tolerances of components.
And vehicle re-entry speed is a product of: vehicle energy (total orbit, less is better), vehicle weight (less is better, drop fuel/parts, G influenced) and vehicle air resistance (high and low are bad);
and tolerances of components is a product of: max K of heat-exposed components (higher is better, rotation helps) and atmosphere density (low - heats more than cools, high density - very high friction, medium density - good)
Eitherway drogue chutes, airbreaks and retrograde burn to the rescue.

While I don't completely agree on how you outlined, that are the factor to consider. Depending on those factor one will heat/cool quickly and long enough to explode/survive.

How to avoid overheating in practice will varies from case to case. Ablator can dish a lot of heat quickly for a short amount of time,  good for a short time of intense heating. For a longer time of moderate heating other options need to be considered.

[Kerbal101]

@Spricigo Yes, heatshields are a good safeguard. Still, they are just one (and very good) component in the formula. I tend actually to avoid long "moderate heating" in the upper atmospheres, because ionospheric plasma heats more than cools.

[Spricigo]

15 minutes ago, Kerbal101 said:

@Spricigo Yes, heatshields are a good safeguard. Still, they are just one (and very good) component in the formula. I tend actually to avoid long "moderate heating" in the upper atmospheres, because ionospheric plasma heats more than cools.

That is the point, the heatshield will not be effective in all circumstances (which seems . Give enough time (or give a lot more heat than it can support) and it will explode. To avoid that is necessary to eliminate the causes of heat (slow down or leave the atmosphere) everything else is just buying time.

Ablator in particular are more efficient at higher temperatures, so a 'slow burn' waste it with lesser effect (similar to using a vacuum optimized engine at sea level). But any other form of heat dissipation is also more efficient at higher temperatures.

[Loren Pechtel]

15 hours ago, Abastro said:

You're right there. Ablator will burn off in relatively low temperature, so longer reentry definitely means it runs out.

But the craft won't explode without any ablator remaining in this case - Is this what you mean?

Disagree.  Capsule + service bay + heat shield.  You need the shield or the service bay is going to blow, but with a high periapsis the shield will burn and the bay blow anyway.  However, if you aerobrake nose first until you're ready to fall out of orbit you can get it through the fire intact.

[Lelitu]

1 hour ago, Loren Pechtel said:

Disagree.  Capsule + service bay + heat shield.  You need the shield or the service bay is going to blow, but with a high periapsis the shield will burn and the bay blow anyway.  However, if you aerobrake nose first until you're ready to fall out of orbit you can get it through the fire intact.

1.25m service bay has hardly any mass, only 100Kg, pod has 800Kg mass. Assuming they're made of basically the same materials, that's 8 times as much heat before the pod overheats and explodes.

[Reusables]

3 hours ago, Loren Pechtel said:

Disagree.  Capsule + service bay + heat shield.  You need the shield or the service bay is going to blow, but with a high periapsis the shield will burn and the bay blow anyway.  However, if you aerobrake nose first until you're ready to fall out of orbit you can get it through the fire intact.

So the heatshield got blown up?

Interesting, it has the highest heat tolerance of 3300K..

[Loren Pechtel]

52 minutes ago, Abastro said:

So the heatshield got blown up?

Interesting, it has the highest heat tolerance of 3300K..

You mean the shield will still work when all the ablator is burned off?  I never actually ran it out of ablator, just saw it was going to run out long before I was down and reloaded.

[Reusables]

6 minutes ago, Loren Pechtel said:

You mean the shield will still work when all the ablator is burned off?  I never actually ran it out of ablator, just saw it was going to run out long before I was down and reloaded.

Yeah. Actually I think the ablator is purely cosmetic (with bunch of mass)

It has freaking 3300K of heat tolerance, which is more than enough for most cases! Also it will still shield the reentry heat.

What ablator does is just preventing conduction of a tiny bit of heat (from the heatshield to the adjacent part).

Provided you don't put any heat-vulnerable part(near 1000K) right next to the heatshield, it won't burn up.