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It has been proposed that a human mission to Mars could park the transfer habitat in high Martian orbit while only a small capsule lands on the surface or on a moon. This in order to minimize the mass entering the deep gravity well of Mars, saving fuel. And aerobraking could help a lot to get the spacecraft captured in Mars orbit. But do those two ideas work together? Aerobraking requires very low periapsis. Can aerobraking itself be used to raise periapsis, or would it need a costly maneuver terms of fuel?

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The idea with aerobraking is usually to do something like the following:

  1. Make your orbital insertion burn as usual, but instead of slowing all the way down into your final orbit, slow down just enough to get captured by the planet. (This burn is executed close to the planet, using the Oberth effect to reduce the amount of fuel required.) You'll end up in a highly elliptical orbit with periapsis (lowest-altitude point) close to the planet.
  2. On every succeeding orbit make a small correction burn at apoasis (highest-altitude point). The ground team calculates how deep into the atmosphere the spacecraft can safely fly, and computes the correction burn to adjust the periapsis altitude accordingly. On each pass through the atmosphere the drag slows you down slightly, reducing your apoapsis altitude.
  3. When you have the correct apoasis altitude, make a burn at apoasis to bring the periapsis back up out of the atmosphere.

That gets the spacecraft into a low Martian orbit with substantially less fuel than would be required to enter into that orbit directly. Note that step 1 is optional: you can actually use the atmosphere to capture into orbit, but this is a rather advanced version of aerobraking called aerocapture. It hasn't been attempted yet because of both the precision required and the stress on the spacecraft.

The Mars Reconnaissance Orbiter (MRO) was an example of a spacecraft that used aerobraking at Mars. Its initial orbit was 300 km x 45,000 km (periapsis altitude x apoapsis altitude). It then followed a long timeline of aerobraking maneuvers over the course of about six months lower its orbit to 250 km x 316 km. Note that as we get better knowledge of Mars's atmosphere, we can do lower dips, making things quicker.

Here's a rough depiction of how MRO's orbit changed during the aerobraking procedure:

enter image description here

Aerobraking doesn't raise the periapsis by itself, but the periapsis-raising maneuver isn't that costly—especially considering how much fuel is saved overall. Generally speaking, Aerobraking can only take energy away from a spacecraft, making it impossible to do something like raising an orbit (which requires an increase in energy).

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  • $\begingroup$ So "High Mars Orbit", in space flight plans, is generally a very eccentric orbit with periapsis raised as little as necessary to avoid the atmosphere? And maybe with aposis as far out in the Hill sphere as possible? Enjoying the Oberth effect at periapsis and being halfway home at apoapsis, is that what it is about? $\endgroup$ – LocalFluff Sep 6 '15 at 11:51
  • $\begingroup$ I'll add in a link to what MRO's orbital insertion was, which is pretty typical. $\endgroup$ – PearsonArtPhoto Sep 6 '15 at 12:13
  • $\begingroup$ Note that though the question used the word "aerobraking", it is actually about aerocapture, not aerobraking a la MRO, and what was meant by a "high orbit" for a human mission. The questioner was thinking a high circular orbit, and wondering how you raise periapsis so far after an aerocapture. However it's a high elliptical orbit with a low periapsis, so only a very slight raise of periapsis is needed. $\endgroup$ – Mark Adler Sep 7 '15 at 2:21
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The "high Mars orbit" being referred to is an elliptical orbit with a low periapsis and a high apoapsis. I usually see something like a one-day period orbit. The idea is to reduce the amount of fuel needed to depart Mars and return to Earth, which however puts more of a burden on the Mars Ascent Vehicle, which now has to reach this highly elliptical orbit. It is an overall mass trade that tends to favor elliptical orbit rendezvous for chemical systems, especially when the components should have similar Earth departure masses. (I don't think it will happen this way though, since we will very likely use electric propulsion systems, which will favor low Mars orbit rendezvous.)

The aerobraking to help capture into Mars orbit from a hyperbolic approach is referred to as aerocapture. Yes, an aerocapture works quite nicely to capture into an elliptical orbit. Though you don't want it to be so elliptical that your aerocapture uncertainty could result in not be being captured at all and flying by Mars. That would be bad.

After the aerocapture event, a propulsive maneuver must be performed at the apoapsis to raise the periapsis and prevent a second atmospheric entry. (Or at least adjust it, in the case of a high aerobraking pass to fine tune the orbit period.) That maneuver is small.

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  • $\begingroup$ Do you think that the first crewed mission to Mars, in say 20 years, will use mainly solar electric propulsion, is it scaling up that powerfully? $\endgroup$ – LocalFluff Sep 6 '15 at 16:56
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    $\begingroup$ That's another question, that's highly speculative. It would make an interesting chatroom question, however... $\endgroup$ – PearsonArtPhoto Sep 6 '15 at 17:10

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