Shallow reentries and heat shields

[agentgonzo]

Do spacecraft need heat shields?

I've just done a test flight in the XR2 using the standard docked at ISS scenario. Deorbit burn from the ISS at ~350km to get perigee to 75km. From there, I kept the AoA at 15º and used the bank to keep vertical speed as small as possible and thus keep reentry as smooth as possible. Doing this, the temperature of the hull never got above 160ºC and the dynamic pressure rose very gradually. It stayed about 2kPa through most of the entry and only rose up to 6kPa for landing. The whole procedure only took a little over 4 hours, so about one and a half orbits.

At the maximum temperature of 160ºC and no significant dynamic pressure, spacecraft could do away with TPSs and just have simple hull made out of the same material as the rest of the vessel. The only thing that I can think of is that the longer reentry would be harder to plan (but then reentries can't be simple to plan anyway) and that the systems would need to be cooled somehow, as there was about 3 hours of hull temps at about 100-150ºC where radiators wouldn't do a lot of good.

Obviously spaceflight is different to this and whilst the XR2 isn't representative, it does show that this technique could be done.

Obviously, for lunar return or high earth orbit returns this may not work or would just take longer. I presume that this has been thought about by the space agencies, but why is it not done?

 

[myles]

as there was about 3 hours of hull temps at about 100-150ºC where radiators wouldn't do a lot of good.

...

Obviously, for lunar return or high earth orbit returns this may not work or would just take longer. I presume that this has been thought about by the space agencies, but why is it not done?

This.

Plus it's always good to have for protection in case something happens or you end up needing to do a steeper trajectory. I don't think the space agencies would be so uncautious as to remove the heat shields, especially after Colombia.

 

[EtherDragon]

I take it that was on Orbiter2006? Dr. Schweiger has a much more accurate atmospheric model planned for O2009.

I would be interested to know how a shallow reentry would work, though.

 

[agentgonzo]

I take it that was on Orbiter2006? Dr. Schweiger has a much more accurate atmospheric model planned for O2009.

Nope, 2009 RC1.

 

[Urwumpe]

The XR2 temperatures are not based on real supersonic airflow and aerodynamic heat flux. Just like the heat shield material is fictional.

You can't use the temperatures as reference for any calculation - not even partially. Also, the lift of the XR2 is not based on real hypersonic flight characteristics, if you have existing lift functions as reference, such a controlled reentry would be impossible to achieve.

 

[agentgonzo]

Plus it's always good to have for protection in case something happens or you end up needing to do a steeper trajectory. I don't think the space agencies would be so uncautious as to remove the heat shields, especially after Colombia.

Though if you're doing a shallow reentry, then the Columbia problem (large hole in heat shield) wouldn't have been so bad:

1. The wing would be stronger as it could be made from stronger materials than the TPS, so the damage would have been reduced
2. Columbia disintegrated when hot plasma of reentry entered this hole. At a shallow reentry where temps are lot, there would be no hot plasma to cause problems.

---------- Post added at 11:32 PM ---------- Previous post was at 11:29 PM ----------

The XR2 temperatures are not based on real supersonic airflow and aerodynamic heat flux. Just like the heat shield material is fictional.

I know that they're not perfectly modelled on real life, but they surely give an indication of what would happen?

You can't use the temperatures as reference for any calculation - not even partially

Why not?

Also, the lift of the XR2 is not based on real hypersonic flight characteristics, if you have existing lift functions as reference, such a controlled reentry would be impossible to achieve.

Again, why? The shuttle has a very large cross-range ability due to the lift generated by the wings. Why would a similar reentry profile with a much shallower AoA and longer reentry time be impossible to achieve?

---------- Post added at 11:51 PM ---------- Previous post was at 11:32 PM ----------

Basically, my line of thought was as follows:

• The shuttle can generate significant lift during reentry from it's 40-degree AOA and uses S-turns so that this lift doesn't bounce it out of the atmosphere.
• If you lower the AOA then you will (up to a certain point) get more lift from the shuttle.
• If you do a shallower reentry, the air will be thinner, you will decelerate more slowly and will experience less heating on the shuttle as a result.
• As you decelerate, you will drop down in altitude, but as you drop you will experience thicker air, resulting in more lift which will slow your descent - you can use this to have a very gradual and slow descent through the atmosphere.
• As you do this slow descent, you will bleed off all your speed gradually, and thus heat the shuttle less.

All of this line of thought results in a longer reentry but lower temperatures. I assume that this has been thought about and rejected - my question is why?

 

[Urwumpe]

I know that they're not perfectly modelled on real life, but they surely give an indication of what would happen?

No. The actual peak heating depends a lot on the geometric shape of the vessel and is not just some trivial relation of attitude or so. Most heating will happen ALWAYS at the points that produce the bow shocks. The less blunt a surface is, the higher the temperature (since you have many shocks spreading out over a small surface, the contact of such a shockwave with the surface produces much higher density at high airspeed.) And often, you have the initial shocks also recontacting at surfaces downstream (for example shuttle nose shock on wing nose). This is then extremely painful, since the first shock waves already come with higher density and the following shocks around the surface will make things worse.

You can't even extrapolate from the XRs or DGIV - both are terribly unrealistic, despite appearing realistic - they obey fictional physics, but still have rules, which you have to follow and on which you can rely. Pseudophysics, if I may say so.

Also, the downstream surface of the XRs and DGIV are for example always at lowest temperature, despite these in reality being higher than the surfaces regions outside all shockwaves.

Again, why? The shuttle has a very large cross-range ability due to the lift generated by the wings. Why would a similar reentry profile with a much shallower AoA and longer reentry time be impossible to achieve?

The shuttle does not reach "just" 160°C. Even at the maximum lift glide profile, it will get in excess of 1200°C, because it produces less lift at high speeds, than at subsonic flight. You have to drop deeper into the atmosphere before you have any control of the flight, beyond Newton.

And doing aerobraking in many dozen shallow dives into the atmosphere does also not work: Once you have 88 minutes orbit period, you will reenter in one passage - if you like it or not.

 

[agentgonzo]

The actual peak heating depends a lot on the geometric shape of the vessel and ... will happen ALWAYS at the points that produce the bow shocks.

The shuttle ... Even at the maximum lift glide profile, it will get in excess of 1200°C, because it produces less lift at high speeds, than at subsonic flight. You have to drop deeper into the atmosphere before you have any control of the flight, beyond Newton.

I think that's what I was after. So (in layman's terms) no matter which way you point the shuttle, for the first part of its reentry it will produce no aerodynamic lift and will enter as a capsule. Only when it has slowed significantly and enterred thicker air will it produce lift - which will have occurred after it has already passed through a lot of air and been heated?

If so, at what point in the shuttle's reentry does it start producing aerodynamic lift?

 

[Urwumpe]

It even produces lift in orbit. Satellite aerodynamics are different to subsonic aerodynamics, but similar in effect.

BUT: The aerodynamic forces will not be enough to compensate weight force until you have reached less than 80km altitude.

The shuttle reaches a hypersonic L/D of 1.6 This might sound a lot since this means 1.6 times more lift than drag, but include the gravity:

If you compensate a weight of 1N, you will produce 1/1.6 N drag as well, and altogether have the whole (1N+1/1.6N) * velocity as power, which you will get by aerodynamic friction. At 8000 m/s at this point initially, this will mean a needed aerodynamic heat flux of 13000 W. For a weight of 1N.

You can reduce now the heating by having a larger surface and blunter corners, but guess what: this does not go well with producing a L/D of 1.6. And when you slow down, the weight force will grow with the drop of speed and the heat flux will only slowly drop.

 

[RisingFury]

Well, you're not the first to come up with that idea:

[ame="http://www.orbithangar.com/searchid.php?ID=3731"]Very soft atmospheric reentry[/ame]

Oh my!!! What a shocker!!!


Knowing every trick in orbiter and getting from KSC to AA in 20 minutes does NOT make you a rocket scientist. There are thousands of people who work for various space agencies and aerospace engeneering companies and frankly it's an insult to their combined intelligence to suggest that they didn't think of "soft reentries".

 

[T.Neo]

Knowing every trick in orbiter and getting from KSC to AA in 20 minutes does NOT make you a rocket scientist.

Uh, yeah. Nor does working for a aerospace engineering company.

Although I agree that space agencies/aerospace engineering companies MUST have discussed "soft reentries" at some point (just look at the hundreds of failed and sometimes outrageous proposals that can be found on the 'net), I find the thinking that only people with qualifications and jobs "within the system" are some sort of super-human deities totally flawed.

 

[Urwumpe]

Still, the difference between them and you is: They have listened to at least 8 semesters of lectures and passed some many exams about the topic, and maybe worked for years in the business and gathered experience in solving spaceflight problems.

Even if 30% of the stuff you get to hear in lectures is only for annoying students and you will forget 60% of the rest before your studies are over, you will have learned at least something there, which you usually won't learn in Orbiter.

 

[Kozak]

The problem is simplier: the rest temperature of the flow depends only on initial temperature and mach number. Whatever is the trajectory you choose - you will experience similar temperature conditions. And heat dissipation both due to emission and due to ablation would only be significant starting from at least 500K (the second - if you have an ablative shield). Unless your crew is fireproof - this needs to be isolated from the interior.
In reality the amount of heat dissipated by radiation is proportional to T^4. I believe that's just not how it works in XR-series.

One can try staying in very long "dive" if he has high enough hypersonic L/D ratio. But such a vehicle won't be able to land on subsonic speeds - "planes" that fly at mach 8 already look as needles.

 

[T.Neo]

Still, the difference between them and you is: They have listened to at least 8 semesters of lectures and passed some many exams about the topic, and maybe worked for years in the business and gathered experience in solving spaceflight problems.

Even if 30% of the stuff you get to hear in lectures is only for annoying students and you will forget 60% of the rest before your studies are over, you will have learned at least something there, which you usually won't learn in Orbiter.

Granted, that may be true- but that still doesn't mean they'll be able to do their job.

I myself have seen people who while they posess qualifications (perhaps however not of the calibre you are describing), posess very little real knowledge of how to do their jobs, and have a command of the English language that is so bad, that it would get them banned off of this forum.

 

[Urwumpe]

Granted, that may be true- but that still doesn't mean they'll be able to do their job.

Do you want to bet on that? Unless you can do the job better, please remain silent and learn.

 

[Sky Captain]

I can`t remember where but I know I read somewhere about proposed spaceplane that would lack a shuttle style heat shield but would be made out of high temperature resistant metal alloy and only the cockpit would be insulated to protect humans and electronics from heat while the rest of the structure would be allowed to heat up during reentry.

 

[Urwumpe]

That was Dynasoar.

 

[T.Neo]

Do you want to bet on that? Unless you can do the job better, please remain silent and learn.

I never suggested I could do the job better, I was discussing their competence, on an internet forum.

If you believe one shouldn't be able to do that because they do not posess a fancy piece of paper, please revaluate your reality.

 

[Wraith]

So in other words, at orbital speeds in the upper atmosphere the hypersonic airflow is strong enough to heat the vessel, but is still weak to produce enough lifting force, is that correct?

What about deorbiting a newspaper at a shallow angle? (Remember that?) Will it just burn then?

 

[Andy44]

The OP asked a simple, harmless question, guys. Come on, this is Orbiter forum, not RocketSnobs.org, let's be nice. You don't learn if you never ask.

Dynasoar was a "hot body" and it also had expendable coolant fluid on board (water?) which would absorb the heat and be vented during the re-entry, carrying the heat away with it.