2
$\begingroup$

Pretty much like the title says. Beyond a simple (one or more) retro burn into unpowered plummeting into the atmosphere, is some form of powered descent a viable method of reentry from orbit on celestial bodies that have an atmosphere, such as Earth, Mars or the moons of the gas giants? What are the limitations of such an approach?

I'm primarily thinking about starting altitudes corresponding to orbits similar to low Earth orbit and higher, including transfer orbits from elsewhere. I am not asking about landing (or simply unpowered, partial-ballistic-trajectory jettison) starting from altitudes where there still is an appreciable atmosphere.

For the purposes of this question, consider reasonably modern technology and missions profiles; missions launched perhaps around year 2000 or so and later. A historical perspective is nice, but not the main focus of this question.

Improve this question
$\endgroup$
5
  • 1
    $\begingroup$ What systems are you trying to avoid? If you want to avoid just deployable decelerators, e.g. parachutes, that's one answer. If you're also trying to avoid heatshields (not sure why), then that's a very different answer. $\endgroup$
    – Mark Adler
    Jul 30, 2015 at 15:20
  • $\begingroup$ @MarkAdler I was mostly curious if it's a viable method at all. Full powered descent is an obvious choice on bodies without an atmosphere, which made me curious about the possibility of doing it on bodies with an atmosphere. Judging by the answers posted thus far, the answer pretty much boils down to (what I more or less expected) "sure you can do it, but other than niche cases, why would you?". $\endgroup$
    – user
    Jul 30, 2015 at 18:14
  • $\begingroup$ On a body without an atmosphere, a full powered descent starts a few kilometers from the surface. You simply can't do that on a body with an atmosphere, since you will have already gone through most of it by then. You would have needed a heatshield to survive that, and by the way drag slowed you down reducing your velocity, quite a bit even at Mars, as compared to that same altitude with no atmosphere. So I don't know what you are imagining as a powered descent. $\endgroup$
    – Mark Adler
    Jul 31, 2015 at 23:30
  • $\begingroup$ Are you going to try to effectively "land" at the top of the atmosphere at some relatively low velocity, and then continue to power down at a constant velocity to near the surface? $\endgroup$
    – Mark Adler
    Jul 31, 2015 at 23:30
  • $\begingroup$ Related: Why would SpaceX not use parachutes for the final descent of the first stage? $\endgroup$
    – user
    Dec 26, 2015 at 22:39

4 Answers 4

Reset to default
5
$\begingroup$

Aerodynamic descent (chutes, balloons, wings, lifting bodies, etc.) is generally much more weight-efficient than powered descent, so you'll nearly always use it for the bulk of your deceleration to surface in an atmosphere.

However, there are two big factors that make powered descent attractive as a supplement to aerodynamic systems:

  • Precision landings. Once a chute is out, you have limited control over where you land; wings can take you where you want to go but incur significant weight penalties. Using a rocket for the very last portion of the descent can give you more precise control of the landing site, as with the Falcon 9R.
  • Thin atmospheres. Mars doesn't have enough atmosphere to make a purely aerodynamic descent practical for large landers; Curiosity needed a rocket-powered skycrane to get down safely.
Improve this answer
$\endgroup$
1
  • $\begingroup$ I'm accepting this answer because it both says why powered descent is not more common, as well as lists situations in which one might actually want to use a powered descent mission profile. GdD's answer explaining in more detail why it is impractical in many cases is also good IMO. $\endgroup$
    – user
    Jul 31, 2015 at 9:26
12
$\begingroup$

Assuming you are asking whether you could use a powered re-entry instead of a thermal protection system (aka heat shield) in re-entry, then it's certainly theoretically possible, however it's totally impractical. What you are talking about is essentially a reverse launch, slowing a rocket down from orbital speeds down to relative zero on the surface of the earth. It takes a huge amount of energy to put something into orbit and it will take a huge amount of energy to land it, so a landing rocket would have to be big. It would be incredibly expensive to put a payload like that into orbit in the first place. If earth had no atmosphere it would be much harder to have manned space flight.

Heat shields and parachutes are an extremely efficient way to re-enter, you get so much effect for very little weight. This is why they are still in use, there's simply nothing better.

Improve this answer
$\endgroup$
1
  • 1
    $\begingroup$ I would have accepted this too if it was possible to accept more than one answer. Your and Russell Borogove's answer forms a very nice pair that covers all aspects I was hoping for answers to cover when asking my question. As it stands, you will have to make do with an upvote. $\endgroup$
    – user
    Jul 31, 2015 at 9:28
5
$\begingroup$

There are a number of systems that have been used that essentially use a heat shield and rockets to slow down, no parachutes or similar systems. The heat shield acts as a protection during the reentry phase. Entering the atmosphere will slow you down, no matter what you do. Then rockets are used to finish the job, so to speak.

Earth:

  • The Dragon V2 plans to use this form of re-entry.

Mars

  • Viking 1 and 2 both employed similar systems
  • Human missions to Mars have been proposed using this technique, as it is the only known method to land large amounts of payload on Mars.
Improve this answer
$\endgroup$
1
  • 1
    $\begingroup$ The Soyuz also uses a similar "soft landing" rocket at the very end of its landing, albeit after a parachute. $\endgroup$
    – Sarah Bailey
    Jul 30, 2015 at 14:11
2
$\begingroup$

A heat shield and a parachute are attempts to use the atmosphere to get the vehicle to its terminal velocity.

On Earth that suffices for something like Dragon V2 to need only fairly small amounts of fuel and reasonably sized engines (SuperDraco) to slow down the final amount.

The utility of a parachute/heatshield depend on what terminal velocity the vehicle will get due to shape and atmospheric density.

Otherwise, you need an inordinate amount of fuel for that final step to slow down. Eventually that becomes untenable.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.