Solving Mars mission docking with Phobos

GW Johnson · 2019-11-14 09:32:48

tahanson43206 wrote:

For GW Johnson re #38
Thank you for your clarification regarding velocity change needed at Mars given a Hohmann transfer orbit as the starting point.
If I understand the post correctly, the velocity change would be on the order of 5 km/s, because of the gravitational attraction of Mars itself.
The Soviets attempted to solve this problem for a docking with Phobos, but (as I recall) their attempt failed after approach to Phobos succeeded.
I am interested in the problem of matching orbit with Phobos NOT using aerobraking.
At the risk of testing your patience further, would I be correct in assuming that matching orbit with Phobos would require a velocity change LESS than would be required for a landing on Mars, but GREATER than the 2.5 km/s velocity with which Mars is catching up with the vehicle?
I would like to find a solution that allows momentum to be ADDED to that already possessed by Phobos, in order to counter the gravitational drag of Mars which is slowing it down.
SearchTerm:HohmannTransfer Author:GW Johnson
(th)

Tahanson43206 re post 39:

Use the formula. For VMars - Vapo = Vinf = 1.85 km/s, say, and Mars escape 5.03 km/s,, then Vnear = (1.85^2+5.03^2)^0.5 = 5.36 km/s. That's not the most challenging for direct entry heat shield design, faster trajectories are very quickly far more challenging. But it is above Mars escape.

Now, if you want to try entering orbit or go to Phobos, look at Vnear = 5.36 km/s. For posigrade orbit or going to Phobos which has a posigrade orbit, on the side away from Earth, Phobos (or the orbit speed) is toward you, so that it adds to the delta vee you need. 5.36 + 3.5 = 8.86 km/s required to enter orbit on that side.

On the side facing Earth, the orbital motion is away from you, subtracting from your delta vee. THAT is what you want to do! Mars low circular orbit is about 3.5 km/s. You will have to decelerate wrt Mars by about 5.36 - 3.5 = 2.06 km/s to enter Mars orbit on the side facing Earth. IF your orbit is lined up with that orbit so the vectors are parallel. If not, it's more delta vee required, by vector addition.

Phobos is further out from Mars and moving slower, so its orbit speed's effect on your delta vee requirement is less. In the limit at zero orbit speed, delta vee is Vnear = 5.36 km/s. That's how fast Mars is running over you, once you are close.

GW

Last edited by GW Johnson (2019-11-14 09:36:21)

tahanson43206 · 2019-11-14 10:21:13

For forum readers: The purpose of this series is to (try to) learn how to dock at Phobos

For GW Johnson re Docking with Phobos:
Thank you for your encouragement, and for the details in Post #41

For SpaceNut … thank you for the original diagram of a Hohmann Transfer to Mars, and for the Instructables link, where I found a (to me remarkable) review of the steps to compute a Mars transfer, or by extension, any transfer.

I drew a little sketch to try to picture the situation.

With the vehicle from Earth coasting up from below Mars, I gather (correction welcome) that Mars is booming along in it’s orbit at 2.5 km/s relative to the vehicle.
However, according to Google (citation below), Phobos is revolving around Mars at 2.138 km/s.

What I’m NOT yet clear on is whether the plane of the orbit of Phobos is parallel to the Solar plane occupied by Earth and Mars.
Assuming for the moment that the Phobos orbital plane IS parallel, then I deduce that on the back side of Phobos’ orbit around Mars, it would be moving at 2.5 km/s – 2.138 km/s with respect to the vehicle.

In other words, if my understanding is correct, and if the planes are parallel, the amount of velocity needed to dock with Phobos would be .362 km/s.

Before proceeding further, I’d like to ask for confirmation that this docking delta-v change is correct.

For GW Johnson ... the factor I have omitted here is how much influence the pull of Mars itself is going to have on the vehicle as it maneuvers to try to dock with Phobos. My (not yet educated guess) is that Mars is going to be adding significantly to the velocity of the vehicle, but I am not clear on how much. The distance of the Phobos orbit from the center of Mars would decrease the gravitational force modestly, but perhaps ? not significantly.

For Louis …
I am interested in the problem you have posed, of finding a faster travel option for humans, but the foundation for any work in that direction would have to be based upon thorough understanding of the Hohmann Transfer as it applies to the Earth-Mars case.

Citation from Google:

velocity of phobos in mars orbit
Phobos orbits 6,000 km (3,700 mi) from the Martian surface, closer to its primary body than any other known planetary moon.
...
Phobos (moon)
Discovery
Semi-major axis 9376 km (2.76 Mars radii)
Eccentricity 0.0151
Orbital period 0.31891023 d (7 h 39.2 min)
Average orbital speed 2.138 km/s

Edit: This web site appears to contain a set of tables showing performance characteristics of various fuel and oxidizer combinations at sea level and in vacuum.

http://www.braeunig.us/space/propel.htm

(th)

Last edited by tahanson43206 (2019-11-14 12:58:57)

louis · 2019-11-14 13:00:35

Probably better if you start a separate thread as this is about Musk's proposals for an enormous transfer of mass from Earth to Mars using Starships rather than docking at Phobos.

tahanson43206 wrote:

For forum readers: The purpose of this series is to (try to) learn how to dock at Phobos
For GW Johnson re Docking with Phobos:
Thank you for your encouragement, and for the details in Post #41
For SpaceNut … thank you for the original diagram of a Hohmann Transfer to Mars, and for the Instructables link, where I found a (to me remarkable) review of the steps to compute a Mars transfer, or by extension, any transfer.
I drew a little sketch to try to picture the situation.
With the vehicle from Earth coasting up from below Mars, I gather (correction welcome) that Mars is booming along in it’s orbit at 2.5 km/s relative to the vehicle.
However, according to Google (citation below), Phobos is revolving around Mars at 2.138 km/s.
What I’m NOT yet clear on is whether the plane of the orbit of Phobos is parallel to the Solar plane occupied by Earth and Mars.
Assuming for the moment that the Phobos orbital plane IS parallel, then I deduce that on the back side of Phobos’ orbit around Mars, it would be moving at 2.5 km/s – 2.138 km/s with respect to the vehicle.
In other words, if my understanding is correct, and if the planes are parallel, the amount of velocity needed to dock with Phobos would be .362 km/s.
Before proceeding further, I’d like to ask for confirmation that this docking delta-v change is correct.
For GW Johnson ... the factor I have omitted here is how much influence the pull of Mars itself is going to have on the vehicle as it maneuvers to try to dock with Phobos. My (not yet educated guess) is that Mars is going to be adding significantly to the velocity of the vehicle, but I am not clear on how much. The distance of the Phobos orbit from the center of Mars would decrease the gravitational force modestly, but perhaps ? not significantly.
For Louis …
I am interested in the problem you have posed, of finding a faster travel option for humans, but the foundation for any work in that direction would have to be based upon thorough understanding of the Hohmann Transfer as it applies to the Earth-Mars case.
Citation from Google:
velocity of phobos in mars orbit
Phobos orbits 6,000 km (3,700 mi) from the Martian surface, closer to its primary body than any other known planetary moon.
...
Phobos (moon)
Discovery
Semi-major axis 9376 km (2.76 Mars radii)
Eccentricity 0.0151
Orbital period 0.31891023 d (7 h 39.2 min)
Average orbital speed 2.138 km/s
(th)

tahanson43206 · 2019-11-14 13:11:04

For louis re topic ...

Louis, you created this topic, so your decision rules ...

However, I'd like to try to make the case for keeping the subtopic of docking at Phobos.

If, after your review of your argument, you still want it removed, then (I'm hoping) SpaceNut can use his administrative tools to move it.

The purpose of my initiative here is to show that docking at Phobos will be FAR more cost effective for Mr. Musk to ship 10 million tons of mass to Mars, than would traditional aerobraking.

I propose to show that the amount of fuel to be expended docking at Phobos is equal to or less than the amount of fuel that would be required to land on Mars, even AFTER momentum has been bled off by aerobraking.

However, perhaps more importantly, the risks of mission failure due to failures on the part of automation to properly control the aerobraking are unknowable at this point, but I suspect they are non-Zero.

The procedure for moving cargo from Phobos to Mars has been discussed at some length earlier in the NewMars archive.

I await your decision.

(th)

tahanson43206 · 2019-11-14 18:28:14

For Louis re move ....

Thanks for allowing this branch to start in your topic!

For SpaceNut ... thanks for the very nice start for this new topic.

And thanks to GW Johnson for providing guidance for the initial phase.

(th)

SpaceNut · 2019-11-14 18:43:52

Not sure if the transit via ION drive to mars coridor would allow for the correct timing with minimal fuel being used for insertion to mars or that it would be harder to match the speed for a direct path to phobos.

tahanson43206 · 2019-11-14 19:05:37

For SpaceNut re this new topic ...

I note with great interest that you have created this new topic in such a way that GW Johnson is the owner/creator of the topic.

I think that is most fitting, and hope that GW Johnson is willing to accept that (perhaps unexpected) responsibility.

This topic can (and probably) should interact smoothly with the Orbital Mechanics topic.

If members of this forum are up to it, we may able to create a sequence of posts that would show the solution to the problem posed, for a wide variety of vehicles and propulsion methods.

While solving for the ion drive is not something I had considered when we started this topic, I welcome it's addition to the task list.

***
While we're adding things .... here's an option that I've not seen proposed anywhere, so it can go here as well as somewhere else.

A vehicle can be set on a 7 month Hohmann Transfer trajectory to Mars from Earth. This particular vehicle is designed to dock with Phobos using a propulsion system that burns Hydrogen with Oxygen as the oxidizer. However, the vehicle departs Earth with a "fuel" load of distilled water, and a small nuclear reactor. During the voyage, the water would be electrolyzed to gaseous Hydrogen and gaseous Oxygen. The gases would be stored in (suitable) balloons projecting from the vehicle.

There would be no need (in this scenario) to bother with cooling the gases. They will cool off nicely in the balloons, though whether conditions would permit liquefaction is not clear to me at this point.

in any case, the separate gases would be pulled into the engine to carry out the docking burn, and such course adjustment burns that might be required.

An unknown is the extent to which Hydrogen (in particular) would escape from the balloon.

Both balloons would be subject to damage by debris particles moving through space, so pin holes would be a source of concern.

If this vehicle were designed for permanent space-only operation, then the balloons could be replaced with permanent metal enclosures, such as used rocket bodies from launches to LEO.

(th)

SpaceNut · 2019-11-14 20:44:02

Interesting to use water for the same departure engines to use for the thrust to settle in to phobos for a landing. Since the tanks for departure are empty they can be reused where the water tank could be dumped once empty. The tanks for the fuel for departure are larger than the amount of fuel going back into them so boiloff should not be a problem and for being liquid thats a heat exchanger coil to dump the heat in the tanks to an external radiator. The reactor would give you plenty of power to circulate the coolant in the exchanger.

I bring up the ION drive as it was very successful for that mission to visit several asteriods ending with Ceres. The presence of a reactor for power is favorable and if you add in AG with plenty of consumables who cares how slow you get there.

tahanson43206 · 2019-11-15 05:10:36

For SpaceNut re #8 and new topic in general ...

Thank you again for creating this new topic, with GW Johnson as the "assigned" creator!

Your suggestion of using ion engines for part of the navigation propulsion for vessels docking at Phobos has a lot going for it. In particular, I am thinking of the fine tuning of the elliptical transfer orbit. The goal for a Phobos docking is to arrive at Mars at just the right time so that the moon is on the back swing with respect to the arriving vehicle, to minimize the delta-v that will be required to dock successfully.

Another benefit of the water idea is that the output of the burn will land on Phobos, and become part of the regolith there. As the number of vessels docking increases, and particularly as the mass of those vessels increases, the volume of water adsorbed by Phobos would become appreciable, so that at some point it would be worth collecting.

(th)

Calliban · 2019-11-15 08:56:19

Spectroscopy suggests that Phobos is a silicate rich body. It could be a useful place to build a refuelling station.

Phobos is also big enough that hydrostatic pressure at it centre is about 0.5bar. A sizable colony could be built at the centre of Phobos, provided one was willing to pay for equipment needed to drill some 10km into the body. Close to the centre, inhabited volumes would not need pressure vessels, as air pressure would be balanced by hydrostatic pressure. This means that a very large pressurised volume (perhaps a kilometre or more in diameter) could be excavated relatively cheaply.

The excavated materials could be used to produce fuel, air, reaction mass or even manufactured products.

Last edited by Calliban (2019-11-15 09:10:07)

tahanson43206 · 2019-11-15 14:05:18

For Calliban re #10

Thank you for your (to me very) interesting addition to the topic.

I checked with Mr. Google, and found: https://en.wikipedia.org/wiki/Ambient_pressure

My intention was to try to follow your idea ... If I understand it correctly, at the center of Phobos, the gravitational force of the body is strong enough to deliver half an atmosphere of pressure.

Would it be your recommendation to set the habitat pressure to .5 bar, or would you increase it to a full bar, counting upon the gravitation of Phobos to reduce the burden on the evacuated region?

One ** definite ** advantage of your proposal is that the radiation protection in such an evacuated habitat would be about as good as it gets.

(th)

Calliban · 2019-11-15 16:21:42

tahanson43206 wrote:

For Calliban re #10
Thank you for your (to me very) interesting addition to the topic.
I checked with Mr. Google, and found: https://en.wikipedia.org/wiki/Ambient_pressure
My intention was to try to follow your idea ... If I understand it correctly, at the center of Phobos, the gravitational force of the body is strong enough to deliver half an atmosphere of pressure.
Would it be your recommendation to set the habitat pressure to .5 bar, or would you increase it to a full bar, counting upon the gravitation of Phobos to reduce the burden on the evacuated region?
One ** definite ** advantage of your proposal is that the radiation protection in such an evacuated habitat would be about as good as it gets.
(th)

I think 0.5bar is the maximum pressure that could be achieved, as Phobos is a rubble pile with no cohesion. But within that limit, the gravitation of Phobos would balance internal pressure.

Phobos could be a very significant world in its own right.

Last edited by Calliban (2019-11-15 16:24:52)

SpaceNut · 2019-11-15 16:52:17

After boring like on earth you would add a liner on the walls to allow for building to anchor to. The outer opening would have an air lock system and exit means, once its been bored and lined. The internal boring can be out to what ever wall thinkness we would want and would have no relation to how wide the opening is to start gaining access to the underground that we will want to occupy.

tahanson43206 · 2019-11-17 07:53:39

For SpaceNut re topic ...

It came to me recently, as I mulled over GW Johnson's advice earlier in this topic, that the fuel that would be needed to dock with Phobos, regardless of how much may be required, CAN BE RECOVERED! Very little need be lost, if the trajectory of the arriving vessel is well chosen.

In the case of a propulsion system which employs methane and liquid oxygen, the output is Carbon Dioxide and Water. The individual molecules produced in the engine will be moving at significant velocity with respect to the vessel itself, and certainly with respect to Phobos, which is advancing toward the arriving vessel.

My proposal is to design a receiving field for the exhaust molecules. The surface of Phobos will be cold as compared to the incoming exhaust molecules, so they should be able to transfer their kinetic energy into heating whatever receiving surface is provided. That energy should be wicked away by the receiving surface and ultimately delivered to the body of Phobos itself, given enough time.

In the mean time, if my understanding of the situation is correct, the molecules should collect on the receiving surface for subsequent scraping up by robotic devices. The water and dry ice would be about as pure as they can get, since they would have been generated by combustion of highly purified inputs.

The collected materials can then be restored to fuel-ready status using solar energy, which at the surface of Phobos, is NOT obscured by the atmosphere of Mars.

Edit: from Wikipedia via Google:

Phobos is one of the least reflective bodies in the Solar System, with an albedo of just 0.071. Surface temperatures range from about −4 °C (25 °F) on the sunlit side to −112 °C (−170 °F) on the shadowed side.

(th)

Last edited by tahanson43206 (2019-11-17 07:55:08)

SpaceNut · 2019-11-17 10:00:42

Exhaust to catch phobos from behind would be showed on the moon on its next pass if it does not get pulled away from the orbit by mars.
If the rendevous is one that we lead phobos orbit then slowing for the moons gravity capture landing on its surface will do it for us.

GW Johnson · 2019-11-17 11:38:34

Good heavens! My name on a topic? I'm not sure what to do or say!

Just to let Tahanson43206 and the rest of y'all know, I've been working on a new posting at "exrocketman" dealing with Hohmann and faster orbits to Mars, and with how to estimate the arrival velocity requirements or direct entry speeds. I hope to post that in the next several days. It'll have illustrations that could be linked for display here, although I do not know how to do that myself.

The scope of that was planned to cover velocity requirements and travel times for Hohmann or faster orbits to Mars, and direct entry vs entry into low Mars orbit requirements. Looks like I need to add Phobos to it. I dunno whether that relates more to this, or the orbital mechanics thread, or both.

Bear in mind that what I do is the semi-automated by-hand/spreadsheet stuff that one does before refining trajectories in the computer. It's really how one decides what to run in the computer. They don't seem to teach that in schools and colleges anymore, unfortunately.

GW

tahanson43206 · 2019-11-17 18:27:10

For GW Johnson re #16

It is good to know that you are continuing to develop exrocketman and I'm looking forward to seeing the updates. Thanks for considering adding Phobos to your analysis! I'd very much like to see this thread end up with a precise and replicable model for how to plan for a Phobos docking. I'm betting that as folks get more and more serious about travelling to Mars, they will become favorably impressed by the distinct advantages of docking at Phobos.

In a recent post, Calliban offered a vision of a hollow center for Phobos, able to sustain an agreeable atmosphere for humans. I think that is a most interesting idea, and hope that it will receive consideration as planning advances.

(th)

tahanson43206 · 2019-11-17 18:31:07

For SpaceNut re #15

I am hoping that through the work of GW Johnson and (perhaps?) others, it may be possible to plan for a trajectory that delivers ALL (or at least Most) of the exhaust gases from a retro-burn to the surface of Phobos, and ideally, to a surface specifically prepared for the task of collecting the molecules expelled by approaching vehicles.

If Elon Musk were to think of this, it would fit in nicely (it seems to me) with his remarkable focus on reusability of rocket components. In this case, we would be extending the idea to the fuel itself.

(th)

SpaceNut · 2019-11-17 19:22:37

Then it should come as no surprise that Nasa is still insisting that putting boots on Mars by the end of the 2030s is not just a pipe dream for astronauts on the Red Planet by 2039. Nasa is taking a stepwise approach that includes a manned 2033 trip to the Mars moon Phobos.
A Crewed Mission to Mars: How NASA Could Do It
Mission to Phobos – The precursor to human Mars landing
Manned Mars Plan: Phobos by 2033, Martian Surface by 2039?

Several space tugs, powered by solar-electric propulsion, would be placed in Mars orbit with supplies that future crews could use for landings. The mission to Phobos — one of the Red Planet's two miniscule moons; Deimos is the other — would require four SLS launches, while a six-day stay on Mars on top of that would bring the total to six liftoffs.
Keeping the cost of manned Mars exploration down will also require in-situ resource utilization — living off the Martian land as much as possible, said Oliver de Weck, an aeronautics and astronautics professor at the Massachusetts Institute of Technology, in a separate panel.

Reading about Delta-V required to go to Phobos is 20% less than the moon.

https://ntrs.nasa.gov/archive/nasa/casi … 008233.pdf
MANNED MARS EXPLORER PROJECT - NASA

https://ntrs.nasa.gov/archive/nasa/casi … 012392.pdf
Trajectory Design for the Phobos and Deimos & Mars ...

https://cpb-us-w2.wpmucdn.com/sites.coe … pq2sz2.pdf
Manned Sample Return Mission to Phobos: a Technology ...

https://www.tasc-group.com/uploads/6/1/ … y18m06.pdf
Trajectory design and guidance for landing on Phobos

https://www.nasa.gov/sites/default/file … derson.pdf
Phobos First - NASA

SpaceNut · 2019-11-17 19:37:35

Quick read boil down is pick a launch system and know what are the payload constraints
Know what is required for safety and survivability ex AG, Life support, radiation protection ect
Plan mission stay time form what we can take with us
Mission statement and goals for science
Come up with an estimate cost for mission

SpaceNut · 2019-11-17 20:17:35

Just a little more digging for data for planning a mission.

Delta V for a low thrust trajectory to Mars

http://members.marssociety.org/inspirat … and%29.pdf
Project of a manned, Mars flyby mission in 2018

https://ntrs.nasa.gov/archive/nasa/casi … 021195.pdf
Project APEX: Advance Manned Exploration of the Martian Moon Phobos

https://spaceflight-simulator.fandom.co … rn_Mission

tahanson43206 · 2019-11-17 20:50:05

For SpaceNut re #21 and specifically the Caltech study ...

https://cpb-us-w2.wpmucdn.com/sites.coe … pq2sz2.pdf

Two teams of 16 students, with varying backgrounds and
nationalities, were allocated five days to design a mission to
land at least one human on a Martian moon and return them,
along with a sample, safely to Earth with a launch date no later
than January 1, 2041.

Assuming the paper cited reflects the time frame described above, to my eye, this is an astonishing performance.

I didn't read every word, but did scan the entire document, with some sections read closely.

One wrinkle I found interesting was the decision not to land the primary vehicle on Phobos, but to set up a parking orbit and send a manned probe to the moon.

Astronaut selection was considered, along with radiation dangers and the need for physical conditioning.

(th)

SpaceNut · 2019-11-18 17:40:34

Mars Dogleg - Phobos refueling station

tahanson43206 · 2019-11-20 12:23:09

Clarification invited:

In pursuing the benefits of docking at Phobos, today I'm introducing the concept of a circular track running around the outer surface of Phobos.

The intention is to emulate the Stanford Torus, which included radiation protection, and a diameter of 1 mile to yield a rotation rate of 1 rpm (as I recall).

In the case of Phobos, the diameter is almost exactly 14 miles, and the circumference would be about 44 miles.

http://www.meracalculator.com/physics/c … -force.php

The web site below came up with a rotation rate of 0.28165 rpm.

https://www.omnicalculator.com/physics/ … ugal-force

What I'm uncertain about at this point is whether the 9.8 meters/per second acceleration I'm looking for is experienced by all masses moving with the track at that distance.

Can/would another forum contributor take a look at the situation?

This track, if constructed, would (presumably) provide 1 gravity of acceleration for passengers, and if well shielded by regolith, protection from radiation as well.

As stated elsewhere in the forum archive, there exists a patent for a fixed(permanent) magnet transportation system, which, if applied to this track, would insure a friction free rotating environment for the compartments making up the system.

Edit: The track would be engineered to be perfectly circular, to minimize unwanted perturbation of the movement of the compartments.

Edit: The track would operate in the vacuum of space in the vicinity of Phobos, to minimize drag on the compartments.

In another topic, Calliban recently suggested an open space in the center of Phobos where an "Earth normal" atmosphere could be maintained. This open space, while gravity free, would provide a very nice recreation area for passengers and crew who would otherwise spend most of their time in the rotating track compartments.

(th)

Last edited by tahanson43206 (2019-11-20 12:27:50)

SpaceNut · 2019-11-20 17:48:49

The moving car on a track has been discussed in a lunar train concept in the past. The cetripical force of motion would push the occupant to the top and rear of the car from motion. Meaning that the brain will need time to get use to walking on the ceiling and not the floor at radius. There is going to be a gradiant from head to toe at an speed and diameter but it gets less with the larger size diameter.
44 miles is 70.8111 kilometer
the rpm distance covered is 19.944 kilometers in the minute time frame
converting back to english units we get 743.558 miles per hour (mph)

http://hyperphysics.phy-astr.gsu.edu/hbase/cf.html
lets work it backwards for what we know that we would want Centripetal force = mass x velocity2 / radius

https://www.engineeringtoolbox.com/cent … _1285.html
ac = v2 / r
ac = (2*nrpm / 60)2 * r
nrpm = revolutions per min (rev/min, 1/min)
ac = 9.81 m/s2
r = 11.2654 km
v2 = 110400 m2/s2
http://www.math.com/students/calculator … e-root.htm
v = 332.65 m/s

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Solving Mars mission docking with Phobos

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Re: Solving Mars mission docking with Phobos

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Re: Solving Mars mission docking with Phobos

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Re: Solving Mars mission docking with Phobos

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Re: Solving Mars mission docking with Phobos

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Re: Solving Mars mission docking with Phobos

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Re: Solving Mars mission docking with Phobos

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Re: Solving Mars mission docking with Phobos

#18 2019-11-17 18:31:07

Re: Solving Mars mission docking with Phobos

#19 2019-11-17 19:22:37

Re: Solving Mars mission docking with Phobos

#20 2019-11-17 19:37:35

Re: Solving Mars mission docking with Phobos

#21 2019-11-17 20:17:35

Re: Solving Mars mission docking with Phobos

#22 2019-11-17 20:50:05

Re: Solving Mars mission docking with Phobos

#23 2019-11-18 17:40:34

Re: Solving Mars mission docking with Phobos

#24 2019-11-20 12:23:09

Re: Solving Mars mission docking with Phobos

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Re: Solving Mars mission docking with Phobos

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