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Many sources (eg this one) show the Mars Sample Return Earth Return Orbiter with both ion and chemical propulsion, and also state that it will be launched on an Ariane 6, but I can't find a clear statement of how they are being used, and whether aero-braking, or aero-capture at Mars plays any role.

So my question is what, in terms of orbits and propulsion, is the mission profile? Specifically: what orbit will the Ariane 6 deliver it to? How will it enter Mars orbit? and low Mars orbit? how will it leave Mars? will it decelerate at all on return to near-Earth space, or is it on a direct entry trajectory?

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    $\begingroup$ If that little ball at the bottom of the image is the sample capsule from the surface, then you may have at least a partial answer to Will surface samples from Mars orbit it in a spherical capsule until captured? $\endgroup$
    – uhoh
    Jul 29, 2020 at 12:29
  • $\begingroup$ I don't understand why they won't dock a spacecraft with a fueled upper stage in Earth orbit, so that enough mass can be landed on Mars to bring the sample back directly. Gambling the sample in a risky autonomous Mars orbit maneuver doesn't sound sane to me. $\endgroup$
    – LocalFluff
    Aug 11, 2020 at 11:31
  • $\begingroup$ @LocalFluff I started trying to show that that would need a big complex liquid fueled rocket to launch from Mars with all the associated complications, but ended up asking space.stackexchange.com/questions/45972/… $\endgroup$ Aug 11, 2020 at 12:02

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The MSR Earth Return Orbiter mission is using a hybrid combination of Solar Electric Propulsion and Chemical Propulsion. Solar Electric Propulsion (SEP) is used on the "Main Module" and Chemical Propulsion (CP) is used on the "Orbit Insertion Module" (OIM). This hybrid design appears to have been the result of some extensive concept exploration and trade-offs, as indicated in this paper: http://electricrocket.org/2019/927.pdf

The Ariane 64 launches the spacecraft onto an earth escape trajectory. The SEP system is then used to complete the transfer to Mars. The spacecraft will arrive at Mars with a positive hyperbolic excess velocity, and the OIM is used to perform an impulsive Mars orbit insertion manoeuvre into a highly elliptical orbit. Following MOI, the OIM is jettisoned and the SEP system is used for all remaining major manoeuvres (including transfer down to LMO, Mars Escape, and the return transfer to Earth). The overall mission scenario is shown in the figure below:

Illustration of the ERO mission concept

Illustration of the ERO mission concept

Highly-efficient SEP systems allow for the reduction of launch mass, usually at the compromise of added transfer time. An all CP mission however is probably not feasible due to the large delta-v requirements (and hence propellant mass) associated with transferring to LMO and back which would likely exceed the launch capability of the Ariane 64.

The paper indicates that this hybrid propulsion concept has been selected because it is a good compromise between reducing launch mass with highly efficient SEP, while reducing mission duration with the OIM to shorten the outbound cruise.

The paper also gives a breakdown of the reference mission states and steps, which gives a helpful indication of which propulsion system is being used for which manoeuvre, and should answer your remaining questions. To read this table, just check what kind of propellant is being consumed for each state or step ("Xe" or corresponds to an EP burn, BiProp/RCS correspond to CP burns). Using the mass information provided you could approximate the delta-v considered for each manoeuvre/phase if you make some additional assumptions about the specific impulse of each propulsion system.

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Spacecraft state and timeline throughout the complete baseline mission

Reference: Sutherland, O., et al. "Mars Sample Return-Earth Return Orbiter: ESA's next Interplanetary Electric Propulsion Mission Concept." 36th International Electric Propulsion Conference, Vienna, Austria. 2019.

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