Deorbiting q, low speed reentry?

[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?

 

[russ_watters]

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.

 

[Janus]

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!

 

[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 let's 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?

 

[mgb_phys]

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?

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

 

[D H]

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.

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.

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?

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" ).

 

[MonstersFromTheId]

That's an excellent presentation D H, where'd you 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?

 

[D H]

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

Google is my friend: http://www.google.com/search?&q=skip+entry+technical+readiness+site:nasa.gov"

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 ;-).

That's right. From http://www.hq.nasa.gov/office/codeq/trl/trl.pdf" (emphasis mine):

TRL 3: Characteristic proof of concept. Active research and development is initiated. This includes analytical studies and laboratory studies to physically validate analytical predictions of separate elements of the technology. Examples include components that are not yet integrated or representative.

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.

That is exactly the intent.

I'd imagine that a more traditional reentry after a return from Mars is technically feasible

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.

 

[scibuff]

Just to add some numbers:

Apollo 13 re-entred the Earth's atmosphere at 11.037 km/s (http://history.nasa.gov/SP-4029/Apollo_13a_Summary.htm) at the altitude of 121.9 km. The escape velocity for the Earth's SURFACE is 11.186 km/s but only 11.075 km/s at the point of the re-entry. You are correct in saying that even if the re-entry angle was too shallow and the CM (command module) skipped across the atmosphere back into the space, it would have lost a lot of kinetic energy, making its orbit highly eccentric (still elliptical) yet the orbital period would be several days.