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View Full Version : Gradual Reentry and the "Skipping off the atmosphere like a stone" issue



Drbuzz0
2007-Jun-21, 01:27 AM
Quick question for those who might know.

As was demonstrated by Columbia so tragically, reentering the atmosphere and dealing with the heat generated by the deceleration of a spacecraft can be the most dangerous part of a mission. On the shuttle and other spacecraft, the heat shielding is a major consideration.

If a spacecraft could enter at a very shallow trajectory, it would be able to gradually dissipate energy and could decelerate more at higher altitudes, where there is less air resistance. This could dramatically reduce heating.

The problem is that hitting the atmosphere at such a high velocity on a shallow angle has been likened to a stone skipping off the surface of water. IE: The spacecraft could "skip" back into space.


Is there any way around this? Perhaps by using control surfaces or a design with aerodynamics designed to counteract this? Could it even be achieved with reasonably small rocket thrusters?

I would imagine it could be counteracted with enough thrusters, but that would basically defeat the purpose, as large amounts of propellant and complicated systems would be needed to do so.

01101001
2007-Jun-21, 02:45 AM
Topic Shuttle too shallow reentry (http://www.bautforum.com/showthread.php?t=20060) addresses some aspects. From there, skipping is a reentry option: Aerospaceweb.org: Atmosphere & Spacecraft Re-entry (http://www.aerospaceweb.org/question/spacecraft/q0218.shtml):


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.

djellison
2007-Jun-21, 12:12 PM
It's something that you can pull off in the 'Orbiter' sim quite easily - I managed to make re-entry last about two full orbits. It's not easy, it's not practical, it's not even realistic, but it's interesting :)

Doug

Nicolas
2007-Jun-21, 12:29 PM
Didn't Apollo use some kind of skipping re-entry with one skip?

djellison
2007-Jun-21, 12:40 PM
Not really skipping - I think they used the lifting body characteristics to pull up a little to manage the heating profile. The design could manage one skip according to the Spec. Perhaps the profiled entry counts as that? Orion has a skip in its design spec to open up more landing options in an emergency.

Doug

TinFoilHat
2007-Jun-21, 12:52 PM
Unless I am mistaken, making the reentry take longer would actually make the heating issue worse, not better. The amount of heat you have to deal with is related more to the time spent exposed to the heating environment and less to the speed or density of the air. To minimize heating you actually want to decelerate as quickly as your crew and spacecraft can manage. The Apollo capsules had an easier time with reentry partly because they could handle much higher deceleration than the shuttle, and therefore were exposed to the heating environment for less time.

I doubt you'd be able to skip off the atmosphere while decelerating from earth orbit anyway. The Apollo capsule would skip on lunar return because it was coming it at a speed closer to escape velocity. It would skim the atmosphere and lose some speed, but still be traveling fast enough to escape into a short suborbital hop. When you're returning from LEO you've already lost too much energy for that when you hit the atmosphere for the first time.

Nicolas
2007-Jun-21, 01:16 PM
Well, as long as we're talking about atmospheric braking, you slow down your spacecraft into the air, hence looking at it from the other side, the air slows down the craft. The faster you slow down, the more energy you give to the air in the same amount of time, hence the larger the temperature peak will be.

Now you have a long descend with low peak temperature on one extreme, and fast descend with high peak temperature on the other extreme. Somewhere in between is an optimum for your specific craft.

Of course more things come into play, such as blunt body characteristics that have variable transfer of heat from the air to the craft accoridng o the velocity of the craft relative to the air.

It's too complex for me just to say what would be the optimum.

Larry Jacks
2007-Jun-21, 01:36 PM
The old Soviet Zond vehicle was designed to use a double-dip reentry profile. According to this article (http://www.astronautix.com/craft/soyz7kl1.htm):

1970 October 20 - Zond 8 - Program: Lunar L1. Launch Site: Baikonur. Launch Vehicle: Proton 8K82K / 11S824. Mass: 5 390 kg (11 880 lb). Perigee: 200 km (120 mi). Apogee: 400 000 km (240 000 mi). Inclination: 51.60 deg. Period: 15 562.22 min. Duration: 5.17 days.

Final circumlunar flight; successfully recovered October 26, 1970. The announced objectives were investigations of the moon and circumlunar space and testing of onboard systems. The spacecraft obtained photographs of the earth on October 21 from a distance of 64,480 km. The spacecraft transmitted flight images of the earth for three days. Zond 8 flew past the moon on October 24, 1970, at a distance of 1,110.4 km and obtained both black and white and colour photographs of the lunar surface. Scientific measurements were also obtained during the flight. The spacecraft used a new variant of the double-dip re-entry, coming in over the north pole, bouncing off the atmosphere, being tracked by Soviet radar stations as it soared south over the Soviet Union, then making a final precision re-entry followed by splashdown at the recovery point in the Indian Ocean.

1970 October 28 - Zond 8 recovered, demonstrates Mishin's L3 ocean landing trajectory. - Program: Lunar L1, Lunar L3.
Zond 8 is recovered only 15 minutes after splashdown by the vessel Taman. Of five Zonds recovered, this was the only one to fly over the north pole. The remainder re-entered over the south pole. The reason for this was the need to fly over tracking stations on Soviet territory in order to get trajectory updates that allowed a precise landing after the second plunge into the atmosphere. This was the reason Mishin now wants a water landing for the L3. The dilemma is that after a first dip into the atmosphere over the North Pole, tracking for a precision landing is possible, but then the spacecraft cannot land on Soviet territory. Re-entering first over the South Pole means that no trajectory updates are available, but then the spacecraft can land only imprecisely somewhere on Soviet territory.

15 L1's were completed, of which only five ever returned to earth. With this successful final recovery, the programme is cancelled. The main cause of the project's failure was the unreliability of the UR-500K rocket.

Drbuzz0
2007-Jun-24, 03:43 AM
When a spacecraft, such as the shuttle or a capsul enters the atmosphere, it is going at nearly the speed it was orbiting at. It has to dicipate all that kenetic energy and the potential energy (as gravital acceleration).

If you hit the atmosphere at mach 20+ or so, it's obviously going to cause a lot of resistance and thus heat. However, the atmosphere thickens rather gradually. Even in low earth orbit, there are enough gas molecules to cause small amounts of drag on satellites.

So it seems to me, that if you could enter the atmosphere gradually enough, you would not experience such heating. come in at a low angle. Decrease over the course of a couple orbits, in the very upper reaches of the atmosphere. Then glide down slowley, such that by the time you are at 300,000 feet or so, you have already dicipated a lot of the energy...

Or am I missing something?

Nicolas
2007-Jun-24, 12:04 PM
Well, the problem appears to be that the combination of orbital mechanics and our atmosphere makes that you skip off the atmosphere for too low angles, i.e. you get pushed back up. Hence too small angles don't work. Too large angles obviously smash you and burn you to pieces, so they also don't work.

But I'm not too familiar with the subject.

NEOWatcher
2007-Jun-25, 02:26 PM
So it seems to me, that if you could enter the atmosphere gradually enough, you would not experience such heating. come in at a low angle. Decrease over the course of a couple orbits, in the very upper reaches of the atmosphere. Then glide down slowley, such that by the time you are at 300,000 feet or so, you have already dicipated a lot of the energy...

Or am I missing something?
That will only work to slow you to a certain point below the orbit. As soon as your velocity is not fast enough to maintain a higher than atmosphere orbit, the effect is the same. Only a few hundred mph...

cjl
2007-Jun-28, 04:05 PM
The thing about coming in extremely shallowly (as you can discover in sims such as Orbiter) is that what usually happens is simply that you bleed off just enough speed in the high atmosphere to lose your orbital trajectory, and then dive steeply in the rest of the way. If you bounce off completely, you usually will enter very steeply on the next go around.

joema
2007-Jun-28, 06:00 PM
....if you could enter the atmosphere gradually enough, you would not experience such heating. come in at a low angle. Decrease over the course of a couple orbits, in the very upper reaches of the atmosphere. Then glide down slowley, such that by the time you are at 300,000 feet or so, you have already dicipated a lot of the energy...
Yes that would theoretically work, but would require a very high hypersonic lift-to-drag ratio. I don't think such a vehicle shape is possible.

The problem is once you touch the upper atmosphere, drag slows you down, which in turn causes you to descend deeper into the atmosphere, yet you're still moving at Mach 18+.

You'd need to develop sufficient aerodynamic lift at very high hypersonic speeds and extreme altitudes to have a slow decent rate.

E.g, "entry interface" where first aerodynamic effects are noticed, is about 400,000 feet, and speed is about Mach 20. If you want to stretch this for about one orbit, descending only by 100,000 feet while you decelerate, we can calculate the approximate lift-to-drag ratio required:

Approximate orbital circumference at 350,000 ft (106,680 m): 25,094 miles (40,384 km). Orbital velocity is about 18,000 mph (8,046 m/s).

So while traversing one orbit (40,384 km) you want to descend from 400k ft to 300k ft, a drop of only 100,000 ft (30,480 m). This would require a hypersonic lift-to-drag ratio of 40,384,000 m / 30,480 m or 1,324-to-1. The highest performance sailplanes only have a subsonic lift-to-drag ratio of about 60-to-1.

Hypersonic lift-to-drag ratio is typically much worse than at subsonic speed. I think the space shuttle's hypersonic L/D ratio is about 2-to-1, but am not sure. This explains why it's totally impossible.

However if you had a vehicle with a hypersonic lift-to-drag ratio about 20 times better than the best current sailplane, in theory it would be possible.

Nicolas
2007-Jun-28, 06:23 PM
However if you had a vehicle with a hypersonic lift-to-drag ratio about 20 times better than the best current sailplane,

And dodo eggs for breakfast...

Noclevername
2007-Jun-28, 10:52 PM
And dodo eggs for breakfast...

And while I'm wishing, I want a pony, Santa!

Drbuzz0
2007-Jun-29, 06:07 AM
That will only work to slow you to a certain point below the orbit. As soon as your velocity is not fast enough to maintain a higher than atmosphere orbit, the effect is the same. Only a few hundred mph...

Yes, eventually your velocity won't be enough to keep you in orbit, but what about transitioning to aerodynamic gliding?

cjl
2007-Jun-29, 06:15 AM
That's what Joema just described: it would take a lift to drag ration of 1,324 to 1. It simply isn't possible.

Nicolas
2007-Jul-01, 10:49 PM
That is one thousand three hundred twenty four to one, right?

Will not happen soon, and I think it's quite safe to even completely drop "soon" here :).

cjl
2007-Jul-02, 03:02 AM
Yes, that's what it is :)

(I sometimes forget about the differences in writing numbers in other areas)

Dave J
2007-Jul-02, 03:57 AM
I remember that after Columbia, the reviewed the reentry trajectory planned for the shuttles. The finding was that the existing flightpath was the most "benign". A "longer" reentry was somewhat possible, but there was a "heatsink" issue of the extended heating event.
Maybe someone can elaborate?

djellison
2007-Jul-02, 07:07 AM
(I'll pick random numbers here) By heatsink I think they meant that having the vehicle's TPS taken up to say, 2000 degrees for 10 minutes is fine, but having it go to 1500 degrees for 20 minutes, the heat might begin to soak through to the airframe under the TPS.

Doug