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What are the reentry velocity options available to Starship on return from Mars? I assume the approach will be significantly faster than a Hohmann transfer orbit due to timing of transfer windows and the desire to shorten the mission. And even a Hohmann transfer velocity is higher than cislunar or LEO reentries.

Is there any advantage to using aerobraking or aerocapture? Lunar gravity assist? Is Starship’s thermal protection planned to be robust enough for a direct reentry?

I assume the maximum tolerable atmospheric entry velocity depends on how Starship is loaded. If it is significantly loaded (as in a return crewed mission) the added kinetic energy of payload and more landing fuel will increase the thermal load.

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    $\begingroup$ One big problem with any form of lunar gravity assist would be timing. It would restrict the launch window. After a few days it would no longer be in the correct position this would complicate contingency planning. Although aero capture may well limit the period of heating it also tends to leave the vehicle in a very large elliptical orbit and has to be repeated. It could take months to circularise and return for a regular reentry. $\endgroup$
    – Slarty
    Oct 29, 2021 at 7:20

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Starship's limited $\Delta V$ means that it can't stray too far from minimum energy transfers.

Here is a porkchop plot of the next decades' worth of Mars to Earth trajectories subject to $\Delta V < 6.9$ $km/s$ (ignoring fuel for landing) from a low Mars orbit (150 km), notice the sparsity above the black 'equal date' line:

Loosely constrained M-E porkchop

Those are some ludicrous entry speeds at the high end of the scale, though they are uncommon. If we constrain the entry velocity (inertial) to 12.9 km/s, the highest ever for something human made (Stardust, 2006), we get something like this in the 2028-29 range:

Tightly constrained M-E porkchop

The minimum $V_{Entry}$ for the decade is $11.4$ $km/s$, though given some of the more efficient (~$2$ $km/s$ $\Delta V$) transfers its theoretically possible to perform a pre-entry 'braking burn' with the excess fuel remaining to reduce entry velocity significantly.

Crew Dragon uses PICA-X, a SpaceX derivative (& improvement) of PICA used by Stardust. It is an ablative heatshield material. I suspect the current Starship uses some kind of PICA-X like material given this statement on SpaceX's Starship page:

Starship will enter Mars’ atmosphere at 7.5 kilometers per second and decelerate aerodynamically. The vehicle’s heat shield is designed to withstand multiple entries, but given that the vehicle is coming into Mars' atmosphere so hot, we still expect to see some ablation of the heat shield (similar to wear and tear on a brake pad) [...]

Given Stardust's success (very cool video of Stardust entering the atmosphere), it's probably safe to suspect that Starship's heatshield could handle a direct entry, if only once.

Aerobraking is a difficult problem for the ablative heatshield. As explored in my answer to What is the heat shield refurbishment procedure for a crew Dragon capsule? PICA(-X) is highly capable, but the underlying mechanism of ablation does not lend it self to rapid reuse (i.e., repeated aerobraking passes). Here is a series of plots detailing three trajectories (all $11.4$ $km/s$ initial entry velocity): direct, ~lunar distance aerocapture, 5 G's max aerocapture along with a LEO reference.

Starship entry trajectories

The top row plots show the flight trajectories and the bottom row of plots can be interpreted as 'heat shield strain' as explained in my linked answer (heat flux VS stagnation pressure).

It can be seen that both high (instantaneous) heating and high deceleration can be avoided using aerobraking, though the integrated heat load, a strong determinant of heat shield thickness, remains more or less the same (~$200$ $kJ/cm^2$) for each Mars return architecture, ~16 times greater than a LEO return.

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    $\begingroup$ Excellent answer. It looks like direct reentry is going to severely challenge SpaceX's goal for prompt turn-around. Do you think a single aerobraking pass would reduce reentry speed to cislunar-type reentry speed? I suppose this would necessitate, by definition, a ~month long delay. $\endgroup$
    – Woody
    Dec 4, 2021 at 14:56
  • $\begingroup$ @Woody an aerobraking pass could remove any amount of speed depending on how deep in the atmosphere you 'aim'. I'll add some details about aerobraking to my answer though. $\endgroup$ Dec 6, 2021 at 12:20
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    $\begingroup$ @Woody prompt turnaround isn't an issue for Mars ships. Even if the multi-year mission involving a long stay on the surface of Mars wasn't enough, orbital mechanics mean a substantial delay before the next mission starts. $\endgroup$ Dec 7, 2021 at 4:21
  • $\begingroup$ @BrendanLuke15 the 6.9 km/s figure assumes a full 100 t payload which is unlikely for Starships returning from Mars. Also, I don't think it's been publicly announced what they're using now for Starship's main TPS, but it isn't PICA-X...it appears to be some kind of sintered silica fiber tile over a fibrous blanket. $\endgroup$ Dec 7, 2021 at 4:43

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