Reentry Options

It seems that slowing down you would lose altitude and still have to deal with atmospheric friction, you would just come down faster it seems to me....

https://space.stackexchange.com/questions/8026/do-you-need-a-heat-shield-to-enter-the-atmosphere-from-non-orbital-speeds

That idea will "never get off the ground", as it were.

They are improving the heat resistance features of leading surfaces at high Mach number all the time, and some of the new composites coming down the pipe now will make the old stuff seem even more primitive.

Skip or Boost Glide reentry techniques have been studied and may be used in the future. In theory, this technique can extend reentry time for heat load reduction. I think they found the irregularities of the high level atmosphere caused this approach to be abandoned. I think the abandoned Dyna-Soar project was intended to use this approach.

Yes that project now is a real Dyna-Soar. LOL

Now you'll tell me you wish to buy a Chevrolet. But only to see the U.S.A. in your new Chevrolet.

Nah! My next truck, I will go back to Dodge, much smoother ride, all the creature comforts, and a drive train that I cannot tear up.

I used to think that too. Then I saw a calculation of what that would involve. There is no possible retro rocket that could put a dent in the 17,000 mph speed of a spacecraft. If you built one, it would be enormous like a booster rocket, and impossible to put into orbit. In order to put anything in orbit, most of the thrust of the rocket is used to get it going horizontally really really fast, as it must to be in orbit at all. Not much of the energy is needed to raise it up into space. So to slow it down again, you need a similar amount of thrust.

Interesting. I guess if we assumed a spacecraft mass, we could calculate how much force would be needed to slow it to some speed. If we stopped it to zero, it would fall straight down. If we assume its at about 200 miles up, then it would free fall for, oh, maybe 190 miles before drag began to seriously slow it. Rough calcs have it taking 4 minutes and reaching 5500 mph at 10 miles altitude. Sound right? Still pretty fast. Wonder if drag chutes could be used...

Thinking of Apollo and Saturn 5, most of the energy was used to lift the launch vehicle, which was left behind! We ended up with a relatively small payload in orbit. Let's see; kinetic energy = 1/2 mv². So knowing the mass of the spacecraft and it's speed, we can figure how much energy must be lost to slow to a given speed. Sound right so far? Knowing that, some rocket scientist could calculate how big a retro would be needed.

Now, I suspect that NASA is way ahead of me on this. But I still want the numbers and will press on.

Using the conservation of energy principle, the energy needed for the retro-rockets should be about equal to the energy needed to bring it up to space, less the friction losses in each directions.

This means that your retro rocket could be about 50 to 75% of the initial rocket (just a guess).

The present strategy of using the atmosphere to dissipate this energy seems to be the most economical at the moment.

What would be nice is if we could some how recuperate part of this energy and transfer it to the next rocket we want to bring up. We simply need a big pulley and a long cable attached to a sky hook to operate like an elevator with its counterweight.

How about we skip that, and just design a craft that is highly supersonic up to 60,000 or 70,000 ft altitude, with a "calf" ship that is rocket boosted, of smaller mass.

Also design a re-entry vehicle that could channel some or all the hot gases (this would be an engineering breakthrough that does not seem remotely possible at present), into a thrust turbine engine (thus it is air-breathing, even at high speed, high altitude)?

The generated thrust would brake speed to survivable levels before the "icing" melts.

Just throwing a really wild idea out there. The exhaust of this proposed engine would have to have a velocity of at least 3-10 times the velocity of the craft falling into the atmosphere, and have a comparable amount of mass flow to the gases flowing around the craft as in penetrates the atmosphere.

Yea wow, just how many of these large explosive devices would be needed for the average mission?

I have no idea whether this is true or not but I once read that the space shuttle has to turn itself 90 degrees to the atmosphere and use its wings to "knife" it's way into the atmosphere.

The reason stated was: if it tries to re-enter in normal flat flight mode it will just bounce off the atmosphere.

Does anyone know if it's actually true?

Can't find anything at all about it on a quick search.

In any re-entry, attitude of the craft must remain within a tightly controlled envelope, or she will truly "skip" out and miss the LZ. If attitude control is bad enough, it simply acquires too much frictional heat and ablates everything, including the crew.

Therefore, attitudes must be maintained with an eye toward the safety envelope at all times during this mission critical step. Any fidget of the controls, or misfire is a bad day for all concerned.

Definitely a need for cool heads in that situation.

with the amount of lift produced even in upper atmosphere at those speeds with 40 ° nose up attitude, yes, she would be "skipping" back upward. By using high-bank S turns, while maintaining nose up attitude 40, the energy is lost as would be with any aircraft making steep bank turns, rather than increasing altitude, thus maintaining the braking action of the atmosphere, limiting heat up (the extra energy going into producing the turn), and keeping the shuttle on her targeted flight path to the LZ.

Thanks SE, that was awesome.

Thanks SSCpal, that is a lot more complex than the simple sentence I quoted but it does convey the difficulties extremely well.

A hell of a ride, no question about that.

Big picture, we've been using aerobraking since the dawn of spaceflight and, to my knowledge, never had a failure. The media says things like "THE ASTRONAUTS ARE ENGULFED IN A BALL OF FIRE!" Makes it sound really risky. But none of our failures have been associated with aerobraking. Seems like the easiest way to shed energy on reentry. Until we get the space elevator, I think we'll keep using it.

Actually, the space shuttle Columbia did indeed fail on aerodynamical braking.

Still, retro rocket braking is totally infeasible, and aerodynamic braking is almost free, if you can stand the heat!

Columbia was compromised by a hole in a wing leading edge, which had been caused by a chunk of insulation foam that hit it during takeoff.

Yes, what you say is correct. However, this compromise did not become a fatal flaw until the "controlled" aerodynamic braking of reentry.