# Why couldn't the space shuttle slow down in space?

• zuz
In summary, the space shuttle was unable to slow down in space and eliminate the dangerous temps during reentry. This led to the fiery re-entry. The tyranny of the rocket equation was a problem because the Shuttle needed to dissipate a lot of energy quickly. However, using aerobreaking could solve this problem.
zuz
Why couldn't the space shuttle slow down in space and eliminate the dangerous temps during reentry?

zuz said:
Why couldn't the space shuttle slow down in space and eliminate the dangerous temps during reentry?
and how does that eliminate the fiery re-entry ?
Coming in at almost any speed will produce friction heat

I suppose that they could, theoretically. But there's no practical way to carry the fuel that would require up to orbit. Dealing with reentry heat is a much easier problem.

As said before, the Shuttle didn't carry enough fuel to do that. Indeed, the fuel it would take to do this would weigh substantially more than the Shuttle itself.

pinball1970 and hmmm27
OK Thanks.

Nik_2213
You'd have to bring up an equal amount of fuel to slow the shuttle as you need to launch it into orbit. Except all that extra fuel would need even MORE fuel at launch to get it into orbit. So you'd actually need somewhere around 10x the normal fuel to get all that mass into orbit.

The tyranny of the rocket equation...

m4r35n357
Using the braking of the atmosphere is really good value (weight for weight, compared with fuel). You need to dump all the orbital energy and, at the same time, you can't allow the inside of the vehicle to get too hot. So there are time constraints. The the process needs to be fairly quick and to be over in time for the cooling effect of the lower atmosphere to remove the heat of the skin outwards rather than letting it roast the occupants. Ablation of the whit hot outer surfaces of the protective tiles removes a lot of heat from the skin.
Gliding down, more slowly would still result in the same amount of energy dissipation but would result in more heating of the occupants.

Klystron
sophiecentaur said:
Using the braking of the atmosphere is really good value (weight for weight, compared with fuel).

Indeed. Landing on the Moon required the Lunar Module to carry roughly 18,000 lbs (8,200 kg) of fuel in its descent stage. You could replace all of this fuel with an aerodynamic fairing (not sure if you need a heat shield since the initial descent speed isn't that high) if the Moon had a substantial atmosphere. This fairing would have a fraction of the mass of all that fuel.

A one-off entry is easy enough to arrange, but there are also 'skip' approaches, where craft dips into the atmosphere, dissipates some velocity, then bobs out again to cool off. Rinse and repeat...
IIRC, skewing 'skips' also allow for significant 'orbital plane' changes with scant fuel use...

Downside, your heat-shield & structure must tolerate such 'short cycling', must not 'soak' cumulative heat and, yes, must dissipate one dip's heat before starting the next...

FWIW, I don't think any crewed craft have used this (Per '2010' ?), only some space probes doing Mars' atmospheric braking to save fuel...

Nik_2213 said:
FWIW, I don't think any crewed craft have used this (Per '2010' ?), only some space probes doing Mars' atmospheric braking to save fuel...
Who'd want to test pilot such a re-entry system? Several 'déja vue' situations in a row and each one just as likely to involve frying.

FactChecker
Also keep in mind that the earlier you try to decelerate to avoid the heat, the longer gravity pulls you and thus the more fuel you need to constantly stay below the velocity needed to avoid the heat.

Stephenk53 said:
Also keep in mind that the earlier you try to decelerate to avoid the heat, the longer gravity pulls you and thus the more fuel you need to constantly stay below the velocity needed to avoid the heat.
You don't solve any problems if you just use fuel to try to avoid the heating. That way is a straight trade off of fuel against heating penalty as far as I can see.
You have to accept the heat and dissipate it as effectively as possible without using fuel. You must either bite the bullet and play fireballs or use multiple mini-entries and dissipate the heat / energy in stages at the highest points in a series of eccentric orbits. I guess that the control available using present software can look after the need for an optimum.

Nik_2213
sophiecentaur said:
dissipate it as effectively as possible without using fuel.
Perhaps not "without using fuel" but with some intelligent refrigeration system. The temperature difference between the surface of a 'hot' craft and the effective temperature of surrounding space is actually quite large so the performance coefficient could be pretty good.

Nik_2213 said:
Downside, your heat-shield & structure must tolerate such 'short cycling', must not 'soak' cumulative heat and, yes, must dissipate one dip's heat before starting the next...

FWIW, I don't think any crewed craft have used this (Per '2010' ?), only some space probes doing Mars' atmospheric braking to save fuel...

Aerobreaking for orbiters (for the few cases it has been employed) only dip into the top of the atmosphere to reduce mechanical and thermal loads. As far as I know aero-capture (i.e. dropping from hyperbolic to elliptical speeds via aerodynamic drag alone - a more intense form of aero-breaking) has not been performed in practice yet.

The skip-entry maneuver I know about was a design idea to extend range of suborbital aerospace vehicles and thus, not per-see for aero-capture (with hyperbolic speed).

I believe the Apollo direct-entry from moon orbit (the fastest manned reentry we have done so far) is a good example of a practical lifting entry that made it possible to bleed of speed much higher in the atmosphere thus significantly limiting the g-loads and thermal load compared to a non-lifting (ballistic) reentry. I seem to recall a figure around 9G minimum peak load for ballistic reentries, compared to much lower for a lifting reentry. However, the Apollo reentries were not skip-entries as such either since the capsules stayed in the atmosphere for the whole reentry.

Regarding "soaking up" heat in the heat shield I do not think that is an option, skip-entries or otherwise. Some (older) designs use the conceptually simple ablative heat shield where the material of the heat shield itself would carry the heat away. Due to the power levels involved, non-ablative heat shields are more or less forced to have a design that allow it to radiate away all incoming power without any significant amount of storage. Or put in other words, storing heat only makes it worse to deal with. The space shuttle TPS, for instance, had tiles that could be white hot on one side and almost room temperature on the other:

Filip Larsen said:
I seem to recall a figure around 9G minimum peak load for ballistic reentries, compared to much lower for a lifting reentry.

Over 11 g's for mercury capsules if I recall correctly.

sophiecentaur
sophiecentaur said:
Perhaps not "without using fuel" but with some intelligent refrigeration system. The temperature difference between the surface of a 'hot' craft and the effective temperature of surrounding space is actually quite large so the performance coefficient could be pretty good.
The front is really hot, the side is still very hot, and the back is a vacuum where you only have radiation. Dumping the heat into stuff inside the spacecraft is the only reasonable option for things that need cooling.
Filip Larsen said:
As far as I know aero-capture (i.e. dropping from hyperbolic to elliptical speeds via aerodynamic drag alone - a more intense form of aero-breaking) has not been performed in practice yet.
Zond 6 and 7 made a full orbit after slowing in the atmosphere.

Filip Larsen and Nik_2213
mfb said:
Zond 6 and 7 made a full orbit after slowing in the atmosphere.

Didn't knew about those probes, thanks.

You mention they did a full orbit after aero-breaking, so I assume the probes did a "simple" ballistic aero-capture followed by reentry at next perigee? The wikipedia article mentions they employed skip-entry, but as I understand skip-entry that is a maneuver for sub-orbital speeds where lift is used to turn the trajectory upwards to extend the range.

mfb said:
The front is really hot, the side is still very hot, and the back is a vacuum where you only have radiation.
I wouldn't argue with that but refrigeration systems do not rely on external convection to shed heat in principle. I did use the term "intelligent refrigeration" and that would need to involve a very hot heat sink. Obvs a liquid based refrigeration cycle wouldn't work.
I was thinking in terms of something 'electrical' like a high temperature Peltier system. In fact, if the orbit were elliptical, the cooling would be better during the lower speed portion of the orbit and then the whole surface of the craft could radiate heat. The energy generated during the slowing process would need to be stored and released during the (much longer) part of the orbit when the friction is not there.
There is a problem with the energy (fuel) needed to change orbit and that would be significant.
It would be interesting to know the ratio of heat generated from friction and the amount of heat loss during the frictionless phase. That would be the clincher, I think. Helping the process along with some refrigeration wouldn't perhaps make too much difference.

It's really a matter of fuel. The shuttle had tiny little tanks and big thirsty engines (the most powerful throttleable rocket engines on Earth ouat). At best it could fire about 8 seconds of thrust with the main engines, assuming they could get the things lit in space.
But if you had the fuel, yes, you could fire your engines until your speed was 0, and fall straight down, negating the need for a heat shield.
But we're talking about a lotta fuel.I have actually tested this very scenario in my simulator and although the shuttle had the thrust to get from ground to 25k with it's onboard fuel (empty shuttle), it does not have the thrust to overcome 17k worth of orbital velocity. The best I could ever get it to do was drop to maybe 9 or 10k.

Fun stuff with flight simulators

Here is proof that the shuttle actually flies better than people think.
Everyone always says it flies like a brick, but I have flown a brick and it does not fly this well.
Really. The shuttle flies just fine as long as you fly her right.

Ralph Rotten said:
Everyone always says it flies like a brick

The shuttle had (by design) a pretty low L/D around 1 (between 1 and 2 orders of magnitude lower than normal airplanes) which combined with the fact that it is unpowered gave it a fairly steep approach that for most regular pilots would likely be more comparable to controlled falling than level flying. I think its fair to say that most regular pilots probably would not be able to successfully land the shuttle manually without some study and training first

The instrumentation, flight control system and approach maneuvers were also specially designed to allow a safe landing despite the steep approach, so in its own way the shuttle flew exactly as it should. Later: found a nice summary of the landing process.

By the way, you can get a feeling for the actual steepness and fast pace of events near landing on this video:

There is also this longer, but low-quality video

Last edited:
256bits and russ_watters
Yep. Shuttle was the result of the lifting body tests combined with the X15.
But her gliding is on par with a 747.
Just keep her at <370KIAS and she flies well enough
What's hard is flying the full approach by hand. I think Story Musgrove was the only one to do it in real life, but he was the Chuck Norris of astronauts. I've done it in the sim, but I've created a smoking hole as many times as I have pulled it off successfully.

Filip Larsen said:
Didn't knew about those probes, thanks.

You mention they did a full orbit after aero-breaking, so I assume the probes did a "simple" ballistic aero-capture followed by reentry at next perigee? The wikipedia article mentions they employed skip-entry, but as I understand skip-entry that is a maneuver for sub-orbital speeds where lift is used to turn the trajectory upwards to extend the range.
I don't find the orbit description any more that I read a while ago but you can check e.g. Beyond Earth. Page 81 describes the Zond 6 re-entry, the first skip reduced the velocity to 7.6 km/s, that is the velocity of a low Earth orbit. With a eccentricity that gives you nearly a full revolution before you enter the atmosphere again. You could call it a very long skip.
sophiecentaur said:
In fact, if the orbit were elliptical
You are not in orbit in the situation considered here.
For aerocapture or atmospheric skips you get maximum heat emission if you let the surface stay hot.

Ralph Rotten said:
Yep. Shuttle was the result of the lifting body tests combined with the X15.
But her gliding is on par with a 747.
Just keep her at <370KIAS and she flies well enough
Well, Google tells me the shuttle has an L/D max (glide ratio) of 4.5 on approach, but a 747 is 15. A Cessna 172 is 9. Sure, the method is the same - pitch for the glide airspeed - but it's a lot steeper.

Is that an empty 747?
I usually fly the 747 full, and the shuttle empty.
I imagine they'd handle the same on a dead stick.

Heh, I once put rockets on a 747 just to see if it would get into orbit.

mfb said:
you get maximum heat emission if you let the surface stay hot.
Yes - I agree but in order to avoid too have a net gain of heat into the core of the craft during the process, you have two choices. You can let the surface get extremely hot and then cooling it down in the atmosphere before the core gets too hot or getting it less hot and losing the heat by another mechanism (based on some sort of refrigeration cycle / heat pumping ) but still avoiding the core getting too hot. Time is a factor because of the limited insulation available.

Well, there is simply no useful mechanism to get rid of heat by anything apart from dumping it into some material (the heat protection itself or some cooling system), evaporating material (the heat protection material, or some cooling fluid) or thermal radiation from the hot surface.

russ_watters said:
Google tells me the shuttle has an L/D max (glide ratio) of 4.5 on approach

In the landing video it appears to be in a stable glide (constant angle, constant speed) with an glide angle of around 18 deg corresponding to glide ratio of 3. If I read the HUD information correctly the speed brakes are retracted at that segment (and the gears don't come out until near the end of the flare) so the glide ratio should be close to maximum for that speed. I am not aware of any shuttle configuration change that could make that number go to 4.5 and to my knowledge all the shuttles were aerodynamically equivalent. So if it really could do a 4.5 glide ratio I guess they on purpose flew the approach away from optimal glide speed, which would make sense as they then had some capacity to increase glide range in case of unexpected down-wind or similar.

Later: found this thesis that gives approximations based on shuttle wind tunnel data. In figure 4 the L/D for some speeds and alphas can be seen ranging up to 5, but if and how that exactly can be applied to the landing phase (with speed around 0.5 Ma) requires some more analysis I think.

Klystron and russ_watters
mfb said:
Well, there is simply no useful mechanism to get rid of heat by anything apart from dumping it into some material (the heat protection itself or some cooling system), evaporating material (the heat protection material, or some cooling fluid) or thermal radiation from the hot surface.
I couldn't object to that statement - it's a general principle that applies everywhere. All I am suggesting is that a heat pumping system (perhaps made of unobtanium, of course) could increase the temperatures of parts of the surface that are cooler than the front and so increase the radiant heat loss. A re-entry that takes a long time has more of a problem with keeping the core cool because there is no atmosphere to provide last-minute fast cooling before the heat has diffused into the core.

The additional radiation would be negligible and you would need much more material that can withstand high temperatures, vacuum, the atmosphere, and much more. Once the heat is in the cooling system you are good, you don't need that much cooling fluid.

Better insulation is pretty much always better than a better cooling system for protection against the effects of outside heat getting in.

mfb said:
Once the heat is in the cooling system you are good, you don't need that much cooling fluid.
Do you mean somewhere to dump the heat that gets through? How would that not imply a large mass would be required?
I understood that a slow re-entry would result in the core getting too hot - that the heat in the shields has to be dissipated, soon after the shields get to max temperature, by the cool atmosphere. I have read that a multiple dipping in the upper atmosphere would cause a dangerous rise in core temperature unless heat could be dissipated during the frictionless portions of the multiple orbits. Any cooling system could only shed heat by radiation (from the back end, I imagine).

sophiecentaur said:
Any cooling system could only shed heat by radiation (from the back end, I imagine).

For the Shuttle, https://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/wsp/index.html was used both at lift-off and at reentry for normal cooling of internal systems (e.g. APU's and hydraulics), and these vent water steam overboard. While it would be strange to use this cooling mechanism in orbit where radiative cooling is a reliable option (the Shuttle used radiators on the inside of the bay doors when in orbit) I don't see any reason why it could not be used. Of course, due to expenditure of water the amount of heat it can remove will be limited by the amount of water that is accessible.

Filip Larsen said:
or the Shuttle, https://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/wsp/index.html was used both at lift-off and at reentry for normal cooling of internal systems (e.g. APU's and hydraulics), and these vent water steam overboard
That's interesting but the amount of heat to be dealt with would be much more if water spray were used for re-entry. Latent Heat of water is pretty good but would it be enough? It's like other ablative systems in that you have to carry it with you all the way. A heat pump feeding a radiator would also require fuel mass. Would . could that be comparable with the mass of water needed for the same effect? I think the sums are beyond me and appropriate heat pump technology is probably not there yet, in any case.
But the present system seems to work OK and it's a real commercial issue so we can rely on the industry to be considering the situation; it's very much in their interests not to waste any payload mass.

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