Reentry techniques

[V8Li]

I've finished another mission with my XR1 today. I was on my way "home" from the Moon and I did 2 aerobrakings to take the Ap from 400k Km to about 10k Km. I was low on fuel and decided to do a third one, in which I got the Ap to a 200Km value (I was targeting 150Km to set a standard orbit that I use before reentry). And during this third aerobrake I got the XR1 to about 70Km and just kept the vertical speed 0 and the airbrakes on. The deceleration was about 4m/s^2, a max dynamic pressure of about 15kPa and the nose cone got to only 200 deg C max temperature. Which leads me to my next question: why on earth do they have to reenter with the Space Shuttle at a 10m/s^2 deceleration (and probably a high descend rate too) which leads to a hight dynamic pressure and surface temperature and can, in turn, lead to a disaster (as it has before).

I've seen some graphs with the reentry profile a while ago and they do stay at about 80-75km for about 30% of the flight, but I mean, we all know the images with flames around the Shuttle and have seen reports on TV which state that the surface heats up to 1000 deg C. Is a "soft" reentry possible in reality?

 

[pete.dakota]

The 'skip' re-entry was considered for STS. But most of the engineers postulated that it was not known how well the TPS would cope with being heated, cooled, and then heated again.

I think, with the Shuttle, it's safer to have a very hot, but very short peak heating phase, rather than a long more mild one.

 

[Piper]

AFAIK, the problem with keeping the shuttle in the upper atmosphere to slow it down is that the heat doesn't dissipate, and just builds up. The current route is one that exposes the shuttle to the least amount of total heat buildup.

 

[legios]

Why can't you use thrusters to basically get your speed way down before you actually enter the thick part of the atmosphere? Would the ability to use lighter materials (and less in-between flight maintenance) make up for the increase in mass you had to take into orbit?

 

[Scarecrow]

Why can't you use thrusters to basically get your speed way down before you actually enter the thick part of the atmosphere? Would the ability to use lighter materials (and less in-between flight maintenance) make up for the increase in mass you had to take into orbit?

Because they don't have an extra 5km/s of Delta-V to play with. The STS has very little excess DV.

 

[Piper]

Why can't you use thrusters to basically get your speed way down before you actually enter the thick part of the atmosphere? Would the ability to use lighter materials (and less in-between flight maintenance) make up for the increase in mass you had to take into orbit?

Because you need the same amount of delta-v (fuel) to slow you down out of an orbit, as you do putting you into orbit. In other words, to slow the shuttle down enough that it doesn't need to re-enter in the traditional way, you'd basically need another external fuel tank, and then of course the additional fuel to put that additional mass into orbit.

 

[legios]

Because you need the same amount of delta-v (fuel) to slow you down out of an orbit, as you do putting you into orbit. In other words, to slow the shuttle down enough that it doesn't need to re-enter in the traditional way, you'd basically need another external fuel tank, and then of course the additional fuel to put that additional mass into orbit.

But since you only have to deal with Newton and not air resistance, wouldn't slowing down take a lot less fuel than getting into orbit?

I know that using the atmosphere is free...it just seems so drastic and unforgiving. You really are "rolling the dice" everytime you have to come home. And the spacecraft must take an enormous performance hit because it has to be over-engineered.

By the way...I am not talking about the STS. I know that it doesn't carry enough fuel. I was wondering why spacecraft in general are not designed to slow down before re-entering. I'd do the math myself, but I'm lazy and have 2 toddlers taking up my time.

 

[Quick_Nick]

But since you only have to deal with Newton and not air resistance, wouldn't slowing down take a lot less fuel than getting into orbit?

I know that using the atmosphere is free...it just seems so drastic and unforgiving. You really are "rolling the dice" everytime you have to come home. And the spacecraft must take an enormous performance hit because it has to be over-engineered.

By the way...I am not talking about the STS. I know that it doesn't carry enough fuel. I was wondering why spacecraft in general are not designed to slow down before re-entering. I'd do the math myself, but I'm lazy and have 2 toddlers taking up my time.

Even without having to fight the air, it requires a very high Dv. And if you did slow yourself to a velocity of 0, you'd start falling as steep as possible(directly down) and still build up speed in the fall. It would still end up being a fast, steep, dangerous reentry. When you have a steeper reentry, you have less time to slow down before reaching the surface and you reach higher density air faster. By the time you reach a low enough altitude to deploy a parachute, you'll likely still be travelling too fast to do so.
Someone please correct me if I'm wrong about how much speed you would still gain in the fall.
P.S. I'm talking about a capsule really. A winged craft would probably not be a good idea in a steep reentry. By the time you reach thick enough air to fly, you have built up too much speed to change direction.

 

[legios]

Even without having to fight the air, it requires a very high Dv. And if you did slow yourself to a velocity of 0, you'd start falling as steep as possible(directly down) and still build up speed in the fall. It would still end up being a fast, steep, dangerous reentry. When you have a steeper reentry, you have less time to slow down before reaching the surface and you reach higher density air faster. By the time you reach a low enough altitude to deploy a parachute, you'll likely still be travelling too fast to do so.
Someone please correct me if I'm wrong about how much speed you would still gain in the fall.
P.S. I'm talking about a capsule really. A winged craft would probably not be a good idea in a steep reentry. By the time you reach thick enough air to fly, you have built up too much speed to change direction.

But you do not have to slow down to 0. Just down to a much slower speed. And instead of slowing down all at once...step down using scramjets then ramjets then turbofans.

Or just slow down to the point where aerodynamic surfaces are effective. And fly down under power.

 

[V8Li]

There are a couple of problems that I can think of when slowing down with the thrusters. First, as already mentioned, is the fuel needed to slow down. Second is that you build up negative vertical speed since you're in orbit and put the Pe way under the Earth's surface. By the time you hit the atmosphere you might have so much VS that the ship will overheat anyway. I set the Pe to 20Km and am in an orbit of 150Km/20Km when I hit the atmosphere, AFAIK they put the Pe at about 60Km. To slow down from 7000m/s+ to 4500m/s for example, from a similar orbit to start with, the Pe should be almost in the center of the planet. Imagine how much vertical speed you build up.

I think that pete.dakota and Piper are right, never thought about the problem this way; it's plausible.

 

[TSPenguin]

I believe you can only accelerate with (sc)ramjets...

 

[DanP]

I believe you can only accelerate with (sc)ramjets...

That sounds true to me, since they need incoming airflow.

 

[Xantcha]

This is fuel- costly but should work fine. Falling 20 km will give about 600 m/s - so no overheating. And atmosphere is dense, so no troubles with gliders too.

And I think Legios was not talking about slowing down with scrams. And even if he was talking about that - why do you think it's impossible? No one is talking about flying tail first - you can deflect exhaust.

 

[Piet Barber]

I've seen some graphs with the reentry profile a while ago and they do stay at about 80-75km for about 30% of the flight, but I mean, we all know the images with flames around the Shuttle and have seen reports on TV which state that the surface heats up to 1000 deg C. Is a "soft" reentry possible in reality?

First, the Shuttle has a lot more mass than your XR-1 does, so it requires a lot more lift to maintain that altitude for so long. But they have so much more mass because of all those heavy tiles. It would have been possible to build the Shuttle lighter with minimal (or at least less) heat shielding and doing a more gentle descent. An old buddy of mine was on the Columbia Accident Investigation Board, (Actually, I knew two of these guys) and we talked about this for a while. As I understand my conversations with him, the original design requirements had a goofy one get stuck in there somehow.

Back in the 1960s somebody in the Air Force said, "Hey we need this thing to be able to launch, do a once-around the globe, take some pictures of the Soviets, and come back to the same base it took off from so we could process the film. Of course, to do a once-around you need to do this huge aerodynamic turn to make up the rotation of the Earth in the time that it took to go around the world. And Space Shuttle tiles as we now know them would fit this requirement very well.

Meeting the militaryʼs perceived needs while also keeping
the cost of missions low posed tremendous technological
hurdles. The Department of Defense wanted the Shuttle to
carry a 40,000-pound payload in a 60-foot-long payload
bay and, on some missions, launch and return to a West
Coast launch site after a single polar orbit. Since the Earthʼs
surface – including the runway on which the Shuttle was to
land – would rotate during that orbit, the Shuttle would need
to maneuver 1,100 miles to the east during re-entry. This
“cross-range” requirement meant the Orbiter required large
delta-shaped wings and a more robust thermal protection
system to shield it from the heat of re-entry.
Source
​

Of course, the Air Force never was that interested in the Shuttle, but the requirement stuck with the design.

For the XR-1, one thing to point out is that it takes so long to do this type of re-entry that usually my computers are overheating. I wonder if this applies to the Shuttle, too.
The solution for this (for the XR-1, at least), is to keep the radiators open for as long as possible while the dynamic pressure is in the 3-5 kPa range.

I guess the next question is could anybody re-enter the Shuttle in Orbiter while keeping the temperature under a particular limit. I have gotten pretty good at doing precision dead-stick re-entries with the DGIV and XR1 without ever getting hotter than 500C. (but unfortunately there are no cool flames to watch in the external view). I don't fly the Space Shuttle on Orbiter, but last I checked there were no temperature sensors, and the AeroBrake MFD only shows relative, not absolute temperatures. I suppose it would be an interesting challenge

For the XR1, one of the tricks to doing this long slow approach is to have the XR1 fly at an AoA just a little nose-lower than maximum L/D; about 6.5 degrees or so. If the XR1 starts to climb, just reduce the nose angle by 0.5 or 1 degree as necessary, and try to get the nose back to the desired 6.5 Degrees of pitch. I have gone around the planet once with this strategy. If I end up being a bit short of my intended destination, I can increase the angle to max L/D which improves the overall performance.

I've also used this on an ascent with the XR1; so if I'm a little ahead of the ISS by the time I've gotten to nearly max the scramjet burn, I just skip along the atmosphere while I wait for the ISS, with a higher velocity ( orbital speed ) to catch up with me. Figuring out when to launch the XR1 to scramjet ascent to match an orbiting object isn't quite something I have figured out yet.

The discussion seems to have gone to doing a vertical descent, by burning off 5 km/s tangent velocity with a retrograde burn; which is not what the original topic was, I think.

 

[V8Li]

The topic was about a "soft" reentry in particular, but about reentry techniques in general, more like possible ones.

Reentry is, as I've already mentioned, my favorite thing in Orbitersim. I even do 1 hour missions, simply get into a circular orbit and deorbit asap. To make a challange out of it, I always empty my main and scram tanks and use the COG shift that comes with the XR1 without the autopilot. Piet Barber, you are right about the weight factor and a heavy 100t Atlantis has to count for something, but a slow reentry can be done anyway, descendin faster in order to get higher dynamic pressure needed to control the descend and deceleration.

From what I've experimented, I consider 2 factors that heat up the surface: descend rate and deceleration. Both produce drag and drag is turned into heat. I use the reentry MFD to tell me my deceleration needed to get above a base. While I descend, dynamic pressure increases so the deceleration increases too (plus the heat from that friction). To that, the heat produced by the friction from the vertical speed is added. At one point, I match the deceleration of the ship with the deceleration required to hit the base (reentry MFD). So I increase the VS to about -20m/s and keep it there since a -20m/s gives me a proper descend rate to keep a constant VS/deceleration ratio. So at that point I'm in a perfect balance where I can keep the exact deceleration and still descend. At that point the temperature stays constant too.
So this leads me to another question: are the Atlantis tiles keep storing heat during the descend and can't be kept to a constant temperature? May be so and we don't know about it. However, I've seen on Discovery something about them, an engineer heat up a tile until it was red-hot and hand it to the reporter while it was still glowing. And, obviously, he was able to hold it in his palm. So the tiles are able to loose temperature fast, right?

I don't know, there are things that don't make sense, at least for me, a NASA outsider . Prior to Atlantis, afaik they were using capsules to reenter, right? Maybe they just considered the technique as "safe" and already tested?

EDIT:

And I think Legios was not talking about slowing down with scrams. And even if he was talking about that - why do you think it's impossible? No one is talking about flying tail first - you can deflect exhaust.

Not 180 deg.

 

[legios]

Not 180 deg.

The way I envisioned it was thus:

1. Slow down to about Mach 7-8, before entering dense part of atmosphere.

2. Engage scramjets. Throttle down scramjets and execute a controlled slow down until ramjet regime.

3. Repeat until you slow down to turbojet speeds.

The whole time you are flying home underpower just like any other aircraft. And you don't have to slam into the atmosphere at Mach 25 and hope everything holds together.

 

[tomek]

Thing to remember is that those TPS are attached to titanium airframe. Titanium is relatively easy to melt - with reentry temperatures at least. TPS do an amazing job at isolating heat and radiating it back, but - first, they can't radiate heat as nearly as quickly as they are heated up by reentry; and second, they still do transmit heat. So it's the question of reaching the ground (where you can safely cool off) quickly enough that most of the heat from outside surface doesn't reach the inner surface (which contacts with metal frame). You can heat the surface by a lot for a short period of time, but the heat will all be on the surface and will quickly radiate. But if you keep heating it for long period of time, heat will gradually get further and further inside the tile and start transferring heat to airframe - then even if you suddenly remove all the heating, you still have heat that's too deep inside, so as it spreads, it will reach the airframe first. Then it's just a question of time before the airframe heats up and softens. It doesn't even have to melt all the way, being heated up once might alter metal properties enough to make it unsafe to use in the future.

 

[Urwumpe]

Thing to remember is that those TPS are attached to titanium airframe. Titanium is relatively easy to melt - with reentry temperatures at least. TPS do an amazing job at isolating heat and radiating it back, but - first, they can't radiate heat as nearly as quickly as they are heated up by reentry; and second, they still do transmit heat. So it's the question of reaching the ground (where you can safely cool off) quickly enough that most of the heat from outside surface doesn't reach the inner surface (which contacts with metal frame). You can heat the surface by a lot for a short period of time, but the heat will all be on the surface and will quickly radiate. But if you keep heating it for long period of time, heat will gradually get further and further inside the tile and start transferring heat to airframe - then even if you suddenly remove all the heating, you still have heat that's too deep inside, so as it spreads, it will reach the airframe first. Then it's just a question of time before the airframe heats up and softens. It doesn't even have to melt all the way, being heated up once might alter metal properties enough to make it unsafe to use in the future.

I hope you mean aluminum. Titanium alone can take most of the reentry heat quite well, you only need special cooling for the really hot areas of nose and wing nose. In fact, Dynasoar was planned to use a hot titanium structure, where the whole structure acts as heat sink during reentry.

Titanium melts at 1941K at standard conditions - that is as hot as the hottest parts of the Shuttle. Aluminum already at 800K and even worse, it gets already soft at 500K. Titanium on the other hand likes to burn at sea-level conditions when heated to 900K and gets soft at 800K...

 

[tblaxland]

And, obviously, he was able to hold it in his palm. So the tiles are able to loose temperature fast, right?

Actually they lose heat really slow, that is why his hand does not get hurt. The temperature of the tile is not what burns your hand, it is the radiation from the tile.

 

[tomek]

I hope you mean aluminum.

Erm... I must have confused it with something else. It would indeed make more sense for me to mean aluminum.

Actually they lose heat really slow, that is why his hand does not get hurt. The temperature of the tile is not what burns your hand, it is the radiation from the tile.

Actually that guy grabbed the tile by the side opposing to the one that was heated up, and that's why he didn't get his hand burned. Because, as I said, the tiles transmit the heat inside themselves very slowly - it takes less time for heat to radiate away than it takes it to reach the other side of the tile.

And no, if you're holding something in your hands, heat isn't transferred by just radiation. At that point most of the heat is transferred by conduction - basically tile's atoms, "vibrating" at higher "frequencies", start banging the atoms of your hand until they start to "vibrate" (heat up) as well.