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tahanson43206

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Registered: 2018-04-27

Posts: 18,142

Re: Solving Mars mission docking with Phobos

For SpaceNut... Thank you for finding and posting the link to the Phobos/Deimos/Waypoints study ... I think it is from 2007.  It provides the level of detail that I hope GW Johnson will be willing to scan.  The bottom line appears to be that a particular orbit called a "One sol" orbit is most efficient/cost-effective. On the other hand, apparently both Phobos and Deimos can serve as Waypoints if they can provide propellant for the missions. I note that the mass to be delivered is 40 tons, which GW Johnson has studied at some length, GW's work was published in this forum in the form of links to papers and spreadsheets, with text.

From the article we have:

Conclusion
The purpose of this study was to determine if Phobos and Deimos are viable waypoints for Mars surface
missions. The trade space considered fixed arrival masses as well as fixed payload masses and considered sensitivity
to landing latitude and aerocapture compared to the historical 1 Sol and 500 km circular reference orbits. Those
historic orbits have many advantages, namely the low periapsis altitude compared to the moons’ orbits and the
assumption that they can be established in any inclination required to reach a designated landing site. The results
indicate that, if all the propellant required to perform post aerocapture maneuvers must be carried to Mars, then
Deimos looks like a more attractive waypoint primarily due to its lower orbital velocity. For equatorial landing sites,
both Phobos and Deimos offer about the same performance in terms of arrival mass or payload capability.
There are options for a SEP in-space transportation stage to deliver payloads (~20 t) to the moons orbits.
However, ISRU propellant generation or preplaced propulsion stages may be needed to enable a Mars surface
mission. If ISRU can be performed on the moons to make propellant, then the moons become a more attractive
option in terms of payload delivery mass. The results indicate that approximately 25 t and 18 t of ISRU propellant
production would be needed at Phobos and Deimos, respectively, for a single Mars surface lander to 40 deg latitude.
However, if only equatorial sites are considered, each moon would need to produce approximately 9 t of ISRU
propellant. ISRU production rates, emplacement and operations technologies are not considered here.
This study illustrates a few of the mass advantages of the 1 sol and 500 km circular orbit and reinforces the
reasons they remain in the EMC trade space. While the results do not exclude the possibility of utilizing the Mars
moons as staging points, it is noted that doing so comes at a cost that can be mitigated with advancements in ISRU,
engines and other technologies.

(th)

GW Johnson

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From: McGregor, Texas USA

Registered: 2011-12-04

Posts: 5,615

Website

Re: Solving Mars mission docking with Phobos

I rather doubt that aerocapture at Mars,  mentioned as baseline in the plan quoted in the previous post,  will ever prove very practical.  That is because the upper atmosphere densities at Mars vary through a factor near 4 erratically.  There are some figures for how variable that atmosphere really is,  in the Justus and Braun EDL paper.  They recommend using the MarsGram models of the Martian atmosphere,  and not the averages in their paper,  when planning missions.

With that much variation,  you never know if you will really see enough drag to actually aerocapture.  Which in turn means you must be prepared to essentially substitute an immediate engine burn for an aerocapture that failed to capture.  If you have to be prepared to burn anyway,  then what's the point of attempting aerocapture in the first place?

Direct entry is not so sensitive to density variations,  if (and only if) you can develop some hypersonic lift to modify your trajectory real-time as you come down.  A part of the smaller sensitivity is diving deeper to slow far below orbital-class speeds.  Down there,  the density is not anywhere near so erratically variable.  It is seasonally quite variable,  but that is pretty much predictable (which is what the MarsGram models are all about).

The other issue is the stated need for ISRU propellant from Phobos or Deimos to make trips to the surface.  I really,  really doubt there are any volatile ices to be had on either moon.  They resemble nothing so much as the dry,  loose rubble-pile C-type asteroids we have seen.

The way around that is not during exploration or initial base-building.  It comes later as the settlement grows to significant size,  requiring lots of two-way trips from Earth for various purposes.  You do the ISRU down on Mars and base the shuttle ships down there,  making round trips single-stage to orbit and back.  The 1-way unfactored dV to theoretically reach the moons is near Mars surface escape speed.  To reach only low orbit,  it is nearer Mars surface circular orbit speed,  some factor 1.414 slower.

That last is what you must trade off against the cost of building your orbital base,  as a structure on the moon,  or as a free-flying complete space station in low orbit.  That may not be only a tradeoff in terms of money.  There are psychological things,  and other things,  to consider.

GW

Last edited by GW Johnson (2024-06-25 09:34:26)

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GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"