A question about de-orbiting

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Zarpheous

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As I read through these forums...these questions pop into my head...

Why is it better to return spacecraft with a steep, hot re-entry path? If the craft can control it's speed, couldn't it
approach earth in a shallower descent? It would take longer, but also eliminate all the high G's and high temps. Is there some way physically that this is not possible?
TIA
 
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MeteorWayne

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In reality, the reentry angle for the shuttle is quite shallow; i.e. it's not steep and hot. It it at an angle that the TPS is designed to handle.

Returning from the Moon, as the Apollo spacecraft did is necessarily steeper, but it had a different heat shield system.

And all reentry paths are selected and managed to keep the heat loads and G forces within the capabilities of the craft and the occupants.
 
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dangineer

Guest
That's a pretty good overview, MW. :)

Here's the more detailed explanation. First, to address whether a spacecraft can control its speed on reentry - yes, but they usually don't. It costs considerably less to build a heat shield and let the Earth's atmosphere slow you down than to build a powered flyback system that slows the spacecraft, as you would need to bring along extra fuel to do this.

There are different reentry trajectories for different missions as well. The three basic categories of trajectories engineers use are gliding, skip, and ballistic.

If I recall, the Space Shuttle follows a skip trajectory where it actually bounces of the atmosphere once to bleed of speed before it reenters on a basically gliding trajectory. This trajectory has the lowest peak heating rate but the highest overall heat absorbed (it's in the atmosphere the longest). This is exactly what the ceramic tiles on the Space Shuttle are designed to do, absorb a lot of heat over a relatively long period of time.

Capsule designs generally follow a gliding reentry path (similar to the last part of the shuttle reentry path). During the descent, the capsule is usually inclined at some angle of attack to produce lift. This decreases the total acceleration load as the spacecraft remains in the upper atmosphere for a longer period of time and also decreases the peak heating rate, although not as much as the skip reentry.

Ballistic reentries are usually used for things like ballistic missiles or probes that require the least amount of heat flux. This trajectory is characterized by the the spacecraft (or missile) producing no lift. The spacecraft experiences the highest peak heating rate compared to the other two trajectories, but it also experiences the lowest total heat flux as it is not in the atmosphere as long.

So depending on the mission requirements and the design of the thermal protection system, any of these three trajectories can be chosen.
 
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dangineer

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Just another note, new "third generation" launch vehicles, as they are called, are designed to follow very shallow high lift gliding trajectories, spending a longer time in the atmosphere than even the Space Shuttle. This is because they have fairly sharp leading edges and so succumb to aerodynamic heating far more readily than the Space Shuttle or capsules.

During reentry, these vehicles will spend on the order of hours slowly decclerating through the atmosphere with very little acceleration loads and peak heating rates. The tradeoff comes as a very high total heat flux in the end, so the thermal protection material needs to be extremely durable and robust, sepecially if it is to be reusable.

So these vehicles, which are still in their infancy, may represent a fourth type of reentry - long duration gliding reentry.
 
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Klavdivs

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Which technique does SpaceshipTwo use? Also, does anyone know what are (roughly) the typical angles for each technique for Earth? "Shallow" and "steep" are relative terms.

Just Curious
 
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dangineer

Guest
SpaceshipTwo uses a special kind of gliding reentry called a "feathered" reentry. Basically, the wings move so that the spacecraft stabilizes in a high drag orientation. The wings move back later in the descent to produce less drag and more lift so that it can land like an airplane.

It's imporant to note, however that SS2 is a suborbital vehicle and reenters at a significantly lower velocity than the Space Shuttle or other orbital vehicles.

Gliding reentries are usually on the order of 3-5 degrees (same for skip), although they can be as high as 10. Ballistic reentries are usually higher than 10 degrees.
 
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gordon_flash

Guest
Not mentioned specifically in previous answers is why there are any heat loads at all. Well, every bit of the energy it took to get the spacecraft into Earht orbit must be counteracted by the re-entry and landing technique. The cheapest way, in terms of fuel, is to let the atmosphere do the work. All the heat created during re-entry equates to all the energy used to get the spacecraft into orbit in the first place. Having said that, I'm wondering if that statement is just as true for multi-stage vehicles, winged vehicles, and SSTO vehicles. It would seem so, however (any comments?).

On a side note, according to a reference by Dr. von Braun, it is easier, in terms of fuel consumption, to get a spacecraft to, and land on, Mars than it is to do the same thing to and on the Moon. The reason is that Mars has a significant atmosphere to assist in both getting into an orbit around the planet and entering its atmosphere for a landing. The extra fuel to "escape" Earth is insignificant, because it isn't much more than the fuel used to just get to the vicinity of the moon.
 
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Jeroen94704

Guest
So in principle, it is possible to build a re-entry vehicle without any sort of complicated heat shield: Simply decellerate actively using your engines instead of atmospheric drag. Of course this requires a lot more fuel, but perhaps that's where the orbital fuel depots another poster mentions come in handy.

There's a lot more to it, I'm sure, but it would be an interesting design excercise.

For starters, you can't simply decellerate to 0 in space, or you start falling. In the lower densities of the upper atmosphere, your terminal velocity is going to be so high you will still burn up in the lower, denser atmosphere. So you need to find a velocity profile that allows your wings to generate lift as soon as possible (to stop you from falling) yet at the same time does not melt the vehicle.

Jeroen
 
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James_Hawk_III

Guest
EarthlingX":22p5uzs1 said:
Would orbital fuel depots make SSTO easier ?

I'm not convinced that an SSTO system (Single Stage To Orbit) system would be helped much by fuel depots that are already in orbit. Once in orbit one only uses the smaller thruster-type rocket motors to change delta-vee, so I'm not following the gain. (My assumption is that ordinary SSTO configurations wouldn't be used for long-range missions.)
 
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dangineer

Guest
The idea is to bleed off most of your speed in the upper atmosphere where the aerodynamic heating is very low. If your reentry vehicle has significant lift, then it can stay in the upper atmosphere for a very long time, until the heating decreases to a managable level - somewhere between Mach 4 and 10.

"Having said that, I'm wondering if that statement is just as true for multi-stage vehicles, winged vehicles, and SSTO vehicles. It would seem so, however (any comments?)."

Gordon, I'm not sure what you are talking about here.
 
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EarthlingX

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Thank you all for answers :)
I had in mind something like X-33 (atmosphere ascent and landing) or DC-X (vacuum landing and ascent).
 
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Mighty

Guest
I had read once that the Apollo missions also used the skip technique for heat management. I can't remember how many times they climbed to bleed off heat.

One of the genius attributes of the Apollo capsule, to me, was that the weight was loaded deliberately off-center. That resulted in one side falling through the atmosphere first, and the bottom ended up as a control surface. Then, they simply rotated around the velocity axis to control their flight. Orient the bottom towards the Earth to raise the altitude. Orient it toward space to dive down. Orient it to the sides for some cross-track distance.
 
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lwblack

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All this talk of re-entry schemes reminds me they all are doing one thing. Fighting gravity. Now, if you could negate the effects of gravity, you could simple float back down if you set the ant-grav to be slightly less than the pull of gravity. Ahh, but that technology doesn't exist to date. Whoever discovers how will be the space fairing genius of the future. Image all the applications it would be applied to?
 
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MeteorWayne

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Mighty":2y5fuef2 said:
I had read once that the Apollo missions also used the skip technique for heat management. I can't remember how many times they climbed to bleed off heat.

One of the genius attributes of the Apollo capsule, to me, was that the weight was loaded deliberately off-center. That resulted in one side falling through the atmosphere first, and the bottom ended up as a control surface. Then, they simply rotated around the velocity axis to control their flight. Orient the bottom towards the Earth to raise the altitude. Orient it toward space to dive down. Orient it to the sides for some cross-track distance.

I don't think that's correct. AFAIK, the Apollo lunar capsules used a straight ballistic trajectory. While they might have used mass and alignment to control the descent heat loads, it was not a "skip" trajectory.

If you can find something that shows me wrong, I'd appreciate it

Wayne
 
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dangineer

Guest
http://www.astronautix.com/details/ski16333.htm

The above link seems to imply that the Apollo module followed a skip reentry.

When the Apollo module is inclined (usually at negative 15 to 20 degrees from the velocity vector) it can generate a considerable amount of lift. The stagnation region at the windward side experiences a lot of high pressure. At the same time, the air is accelerated past the windward side and the incline of the conical section allows the flow to seperate and accelerates the flow, causing the pressure to drop. The pressure differential then creates lift.
 
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MeteorWayne

Guest
Interesting, thanx.

It may have been studied, but to my knowledge, that's not the trajectory they actually used.

Time for a little research :)

Wayne
 
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shuttle_guy

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dangineer":1scwiqiv said:
That's a pretty good overview, MW. :)

There are different reentry trajectories for different missions as well. The three basic categories of trajectories engineers use are gliding, skip, and ballistic.

If I recall, the Space Shuttle follows a skip trajectory where it actually bounces of the atmosphere once to bleed of speed before it reenters on a basically gliding trajectory. This trajectory has the lowest peak heating rate but the highest overall heat absorbed (it's in the atmosphere the longest). This is exactly what the ceramic tiles on the Space Shuttle are designed to do, absorb a lot of heat over a relatively long period of time.


The Shuttle Orbiter does not skip during it's entry.
 
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shuttle_guy

Guest
dangineer":2pb5eldj said:
http://www.astronautix.com/details/ski16333.htm

The above link seems to imply that the Apollo module followed a skip reentry.

When the Apollo module is inclined (usually at negative 15 to 20 degrees from the velocity vector) it can generate a considerable amount of lift. The stagnation region at the windward side experiences a lot of high pressure. At the same time, the air is accelerated past the windward side and the incline of the conical section allows the flow to seperate and accelerates the flow, causing the pressure to drop. The pressure differential then creates lift.


That is correct however I wouls not say the technique generates "considerable" lift.

Using this technique the trajectory modification can be adjusted to increase or decrease the range and change the cross range a small amount. Gemini also used this technique.
 
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shuttle_guy

Guest
EarthlingX":2tj0ugnu said:
Would orbital fuel depots make SSTO easier ?


No SSTO is to get to orbit. The propellant storage is in orbit....................
 
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MeteorWayne

Guest
From here:

http://www.aerospaceweb.org/question/sp ... 0218.shtml

"The vast majority of spacecraft returning to Earth follow a ballistic entry trajectory. Here, the vehicle generates very little aerodynamic lift resulting is a lift-to-drag (L/D) ratio less than 1. The craft instead plunges into the atmosphere and falls through it under the influence of gravity and drag. This drag force slows the vehicle so that parachutes can be deployed for a soft touchdown. The landing point is predetermined by conditions when the vehicle first enters the atmosphere, and the pilot has no control over the capsule's trajectory or its landing point once he leaves orbit and begins the ballistic plunge. Since the craft essentially falls vertically through the atmosphere, its downrange distance, or ground track, from the point where it first entered the atmosphere to landing is relatively small. Manned space capsules, like those used during Mercury and Gemini flights, all followed ballistic entry trajectories to a splashdown at sea. Russian Soyuz capsules continue to use ballistic entry paths today and usually touchdown on land in Siberia.

An alternate re-entry approach is the glide trajectory in which the vehicle flies through the atmosphere similarly to an aircraft. The spacecraft enters the atmosphere at a high angle of attack and generates aerodynamic lift. In so doing, the vehicle experiences a lift-to-drag ratio over 4 that allows it to travel further downrange than a ballistic capsule. The chief advantage of this technique is that the pilot has much greater control over the vehicle's trajectory and can, in theory, choose the landing site. A further advantage is that the vehicle typically lands intact on a runway and can possibly be reused again. This technique is exemplified by the Space Shuttle, the only vehicle currently employing a glide entry trajectory
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The attraction of the skip trajectory is that a vehicle can travel much farther downrange than either the ballistic or glide options allow. The primary disadvantage, however, is significantly higher aerodynamic heating since the friction heat absorbed during the skips grows at a higher rate and requires heavier shielding to protect the vehicle. As a result, skip entry has never been used for a manned spacecraft. A related technique known as aerocapture has been applied to unmanned craft, though the method is typically used to slow a vehicle and enter orbit around a planet rather than as a means of re-entry."
 
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shuttle_guy

Guest
gordon_flash":adwllhvq said:
Not mentioned specifically in previous answers is why there are any heat loads at all. Well, every bit of the energy it took to get the spacecraft into Earht orbit must be counteracted by the re-entry and landing technique. The cheapest way, in terms of fuel, is to let the atmosphere do the work. All the heat created during re-entry equates to all the energy used to get the spacecraft into orbit in the first place. Having said that, I'm wondering if that statement is just as true for multi-stage vehicles, winged vehicles, and SSTO vehicles. It would seem so, however (any comments?).

You made a very good observation. For multi stage vehicle much of the energy to get the payload to orbit is used to get the lower stages to their the velocity where their propellant is expended and the stage is released to splash down. thus the energy used to get the payload slowed down dur entry does not equal the engery required to get it to orbit.

For a SSTO with wings "all" of the energy to get it to orbit must be expended to get the vehicle back to earth. I say "all" since for any vehicle some of the launch energy expended in drag through the atmosphere, steering losses etc.
 
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