# Deorbiting q, low speed reentry?

1. Aug 19, 2008

### MonstersFromTheId

Why do spacecraft have to undergo such high temperatures during reentry? Is that basically to save on fuel?

For example - to take the ridiculous extreme, suppose you performed a reentry burn of the same magnitude used to place the craft in orbit in the first place? As in you use something the size of the Saturn V, with it's attending appetite for fuel, to drop the craft's orbital speed from tens of thousands of miles per hour, down to hundreds of miles per hour, before it has the chance to drop into the atmosphere.

Is that possible?
Is it as fuel intensive as I'm making it out to be?

2. Aug 20, 2008

### Staff: Mentor

Yes, it is to save fuel. You are correct that deorbiting at low speed would require a rocket as big as the one used to launch the spacecraft. And to do that, of course, you'd need to increase the size of the launch rocket beyond the realm of what is feasible.

3. Aug 20, 2008

### Janus

Staff Emeritus
To illustrate what Russ said about the size of the launch rocket. To put an object into orbit takes about a 10 to 1 mass ratio (kg of fuel per kg of payload). To use rockets to bring the payload back (BTW this is called a "gravity turn" landing.) takes the same 10 to 1 ratio. Ergo, for every 1 kg of payload, you need to launch 11 kg into orbit (payload + fuel for return), meaning you need 10 times that to get it into orbit, or 110 kg of fuel at launch per kg put into orbit and returned. You've increased your fuel usage by a factor of 11!

4. Aug 20, 2008

### MonstersFromTheId

Gotcha, makes perfect sense.
So I suppose that the Apollo LLM essentially did a "gravity turn" on its way from orbit to the surface, with the difference being that the Moon is only 1/6th as deep a gravity well to have to drop into, thus requiring 1/6th the fuel or so to do the "gravity turn", and -- well lets face it, with NO atmosphere for use in burning off speed, it's use a gravity turn, or forget landing.

There's another thing I remember hearing back in the Apollo days about the critical nature of the reentry angle. The first part makes sense - "too steep and they'll burn up", but the second part sounded more like a reporter, lacking a fine grasp of the subject, and grasping for a little drama, that being - "too shallow an angle, and they'll skip off the atmosphere and be irretrievably lost in space".

Now frankly that second part sounds to me like baloney. If you're in orbit, and you do a retro burn, the decrease in energy forces you to drop into a lower-faster orbit. If you were to then "skip" off the atmosphere, there's no way in HELL you're going to wind up flying off into outer space. At best you'd wind up in a highly elliptical orbit, but once you do that burn, unless you do a burn to add energy back into your orbit, you're coming down baby, one way or another.

Which leads me to my next question - why not enter the atmosphere at as shallow an angle possible, in order to force the craft to repeatedly skip off the atmosphere, possibly for hours, skip - skip - skip - skip - skip - skip - skip, until you finally drop down to less hellish speeds before trying for a final reentry?

5. Aug 20, 2008

### mgb_phys

Every time you skip off the atmosphere you need a lot of fuel to turn around and head back into the atmosphere.

6. Aug 20, 2008

### D H

Staff Emeritus
You are correct in that whatever orbit the skip flings the spacecraft, the spacecraft will remain in Earth orbit, and it will eventually reenter. However, that eventually might be quite some time into the future. The Apollo vehicle was literally on its last gasps at the time of reentry. The vehicle did not have enough spare oxygen for a skip that put the vehicle on an orbit with a period of 7 to 8 days. The vehicle would eventually reenter, but the crew would have died of asphyxiation long before reentry.

You are talking about skip reentry. The guidance is extremely touchy and the ability of the vehicle to withstand repeated heating-cooling-heating cycles is dubious. The Space Shuttle has a skip reentry guidance capability. However, this capability has been used in simulations but never in the real world.

Skip reentry is deemed as a critical capability for returning from Mars because the return velocity from a Mars journey is significantly greater than the return velocity from a Moon mission. Since energy is proportional to velocity squared, a Mars return vehicle has an incredibe amount of speed that needs to be dumped.

Skip reentry currently has a rather low Technology Readiness Level (level 3 according to http://research.jsc.nasa.gov/presentations/EngChallengesJSC.ppt" [Broken]).

Last edited by a moderator: May 3, 2017
7. Aug 20, 2008

### MonstersFromTheId

That's an excellent presentation D H, where'd ya find that?

I take it that a "technology readiness level" of "3" means don't bet yer butt on it working w/o one heck of an insurance policy ;-).

Unfortunately, the one chart I really wanted to see there - "Altitude, G-Load, Heat Rate vs Time", is a tiny little insert at the bottom center of the embedded graphic, and it just pixelates out when you try to magnify it. Which I suppose wouldn't help anyway w/o a similar chart for a traditional reentry to compare it to, so - "oh well".

What I'd like to know is if skip reentries can be used as a means of lowering the intense temps faced on reentry despite the challenges faced in pulling one off.

My guess is that not only DO they lower the max temps faced, but that that's exactly why they're being considered for use in a return from Mars.

I'd imagine that a more traditional reentry after a return from Mars is technically feasible, but with SO much more energy to dump, coming up with a form of heat shielding that would take the much higher max temps faced is what drives considering a skip reentry in the first place.

Have I got that essentially right?

8. Aug 20, 2008

### D H

Staff Emeritus

That's right. From http://www.hq.nasa.gov/office/codeq/trl/trl.pdf" [Broken] (emphasis mine):
That is exactly the intent.

Maybe. The closest analogy is the Stardust mission, in which a very small vehicle returned to Earth with greater than escape velocity. Larger vehicles have a larger mass:surface area ratio, and a human-rated vehicle would have to have a very high mass:surface area ratio. A human-rated entry at hyperbolic speeds is at even a lower TRL than skip entry.

Last edited by a moderator: May 3, 2017
9. Apr 13, 2009