Author Topic: Phobos and Deimos on the return to Earth  (Read 4607 times)

Offline redliox

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Phobos and Deimos on the return to Earth
« on: 03/25/2017 11:01 PM »
While I'd love to see exploration of the Martian moons included with journeying to the red planet, the current thinking to reach them or to perform an orbital mission requires a huge amount of buildup for a single expedition...most of which amounts to delivering fuel thanks to physics.  By comparison a surface mission at least offers the advantage of fueling from Mars whereas the moons don't have resource utilization options.  Instead of visiting either moon on the inbound phase of a mission, what about visiting them on the outbound phase, just before Earth return?

I was imagining something akin to Mars Direct or SpaceX's ITS where the mission focuses on the surface and fueling from Martian resources.  The moons come into play on the outbound, Earth-return phase of the mission.  This would be the rough layout:

1) Crew vehicle launches from Mars directly into a transfer orbit
2) Crew vehicle adjusts from transfer to circular orbit (one that near-matches target moon).
3) Crew vehicle rendezvouses with a Moon vehicle.
4) Exploration of moon via Moon vehicle.
5) Crew vehicle departs for Earth.

The Moon vehicle would have been positioned before the mission and possibly reusable for later ones.  Main questions I have lie with how much trouble it'd be to:
1) Launch from Mars surface directly into a transfer orbit matching either Phobos or Deimos orbit
2) Doing a burn that circularizes the transfer to a circular orbit
3) Departing to Earth from said circular orbit
I'd presume a near-Phobos orbit would have a period of 7.5 hours and 30 hours for near-Deimos.

I am aware the disadvantage of visiting the moons is, either on the outbound or inbound, circularizing/matching.  I am simply trying to compute the needs and delta-v of the scenario I explained above.  I'd especially like to compare visiting Phobos to visiting Deimos.  I would only expect to visit one or the other, not both since that'd further increase the fuel load for orbital maneuvering.  We'd be leveraging ISRU from Mars, but the priority would be reserving enough propellant for Earth return after visiting a single moon.

Summon the delta-v masters!  (since mathematician isn't flamboyant enough)
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Online guckyfan

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Re: Phobos and Deimos on the return to Earth
« Reply #1 on: 03/26/2017 10:06 AM »
Best may be doing Phobos and Deimos from the surface of Mars, then returning to Mars for refuelling again.

Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #2 on: 03/26/2017 01:15 PM »
Best may be doing Phobos and Deimos from the surface of Mars, then returning to Mars for refuelling again.

Show the numbers then.  'Best' is an opinion, not a fact.
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Online guckyfan

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Re: Phobos and Deimos on the return to Earth
« Reply #3 on: 03/26/2017 01:26 PM »
Best may be doing Phobos and Deimos from the surface of Mars, then returning to Mars for refuelling again.

Show the numbers then.  'Best' is an opinion, not a fact.

You do know the meaning of "may"?

My reasoning is simple. Getting from Phobos or Deimos to the surface of Mars requires less delta-v than going to earth.

Offline ThereIWas3

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Re: Phobos and Deimos on the return to Earth
« Reply #4 on: 03/26/2017 01:47 PM »
And returning to Mars lets you top-up the tanks for the trip back to Earth.   Doing the moons on the way to Earth means you have to carry the entire to-Earth fuel load, plus what is needed for the moon arrival and departure.
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Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #5 on: 03/26/2017 01:59 PM »
And returning to Mars lets you top-up the tanks for the trip back to Earth.   Doing the moons on the way to Earth means you have to carry the entire to-Earth fuel load, plus what is needed for the moon arrival and departure.

Heatshields tend to be disposable, heavy things bear in mind.  I'm not debating multiple refueling stops and orbital forays.  I DID spell out in the thread title for specifying return-to-Earth.
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Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #6 on: 03/26/2017 02:14 PM »
My reasoning is simple. Getting from Phobos or Deimos to the surface of Mars requires less delta-v than going to earth.

Getting to Low Mars Orbit or High Mars Orbit from the moons is also easier than getting to either the Martian surface or Earth if the objective is to conserve delta-v, especially Low Orbit with aerobraking.
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Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #7 on: 03/26/2017 02:40 PM »
Reference thanks to Dalhousie: http://old.marssociety.org.au/library/willson-et-al.pdf
In this case they specify Phobos.
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Online guckyfan

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Re: Phobos and Deimos on the return to Earth
« Reply #8 on: 03/26/2017 02:54 PM »
Heatshields tend to be disposable, heavy things bear in mind.  I'm not debating multiple refueling stops and orbital forays.  I DID spell out in the thread title for specifying return-to-Earth.

A heatshield for any vehicle flying between Earth and Mars should be of the advanced types in the class of PicaX and be able to do a number of reentries. Especially a less stressing one like returning to Mars from orbit.

But you did specify on the return to Earth, so my suggestion was in some way OT, though I believe it is the better option. I won't continue arguing it here.

Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #9 on: 03/26/2017 02:59 PM »
Heatshields tend to be disposable, heavy things bear in mind.  I'm not debating multiple refueling stops and orbital forays.  I DID spell out in the thread title for specifying return-to-Earth.

A heatshield for any vehicle flying between Earth and Mars should be of the advanced types in the class of PicaX and be able to do a number of reentries. Especially a less stressing one like returning to Mars from orbit.

But you did specify on the return to Earth, so my suggestion was in some way OT, though I believe it is the better option. I won't continue arguing it here.

Correct; there are many scenarios.  Refueling and reusability are great ideas.  I was simply trying to keep things on topic with no offense intended.
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Offline dror

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Re: Phobos and Deimos on the return to Earth
« Reply #10 on: 03/26/2017 06:06 PM »
IF some ISRU can be done on phobos and you can get some H2+O2 fuel on it, this could save some significant ISRU on Mars
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Offline Hop_David

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Re: Phobos and Deimos on the return to Earth
« Reply #11 on: 03/26/2017 06:26 PM »
While I'd love to see exploration of the Martian moons included with journeying to the red planet, the current thinking to reach them or to perform an orbital mission requires a huge amount of buildup for a single expedition...most of which amounts to delivering fuel thanks to physics.  By comparison a surface mission at least offers the advantage of fueling from Mars whereas the moons don't have resource utilization options.  Instead of visiting either moon on the inbound phase of a mission, what about visiting them on the outbound phase, just before Earth return?

I was imagining something akin to Mars Direct or SpaceX's ITS where the mission focuses on the surface and fueling from Martian resources.  The moons come into play on the outbound, Earth-return phase of the mission.  This would be the rough layout:

1) Crew vehicle launches from Mars directly into a transfer orbit
2) Crew vehicle adjusts from transfer to circular orbit (one that near-matches target moon).
3) Crew vehicle rendezvouses with a Moon vehicle.
4) Exploration of moon via Moon vehicle.
5) Crew vehicle departs for Earth.

The Moon vehicle would have been positioned before the mission and possibly reusable for later ones.  Main questions I have lie with how much trouble it'd be to:
1) Launch from Mars surface directly into a transfer orbit matching either Phobos or Deimos orbit
2) Doing a burn that circularizes the transfer to a circular orbit
3) Departing to Earth from said circular orbit
I'd presume a near-Phobos orbit would have a period of 7.5 hours and 30 hours for near-Deimos.

I am aware the disadvantage of visiting the moons is, either on the outbound or inbound, circularizing/matching.  I am simply trying to compute the needs and delta-v of the scenario I explained above.  I'd especially like to compare visiting Phobos to visiting Deimos.  I would only expect to visit one or the other, not both since that'd further increase the fuel load for orbital maneuvering.  We'd be leveraging ISRU from Mars, but the priority would be reserving enough propellant for Earth return after visiting a single moon.

Summon the delta-v masters!  (since mathematician isn't flamboyant enough)

Phobos and Mars are near equatorial. If launching from the equator, there's little plane change expense.

Here's a few numbers just based on the vis viva equation, not looking at gravity loss during vertical ascent to get above Mars' thickest atmosphere:

Mars to 300 km LMO: 3.7 km/s
Mars to Phobos: 4.9 km/s
Mars to Deimos: 5.4 km/s

Deimos would be more interesting if we were using ion engine interplanetary transfer vehicles such as the Hermes in The Martian.

A rough rule of thumb for delta V with low thrust engines is subtracting speed of destination orbit from speed of departure orbit. An ion craft would enter Mars sphere of influence with a C3=0 traveling close to 0 km/s wrt to Mars. Deimos travels 1.4 km/s. A 300 km LMO is about 3.4 km/s. So rendezvous with Deimos would save about 2 km/s delta V over parking in LMO.

"Wait a minute!" someone might say. "You can use aerobraking to park in LMO. So LMO isn't 3.4 km/s." To achieve a decent alpha, Hermes would need either acres of solar panels or acres of light weight radiator are to dump a nuke power source's waste heat. In either case the power source would be large and fragile and not able to endure aerobraking.

Weir gave Hermes a 2 mm/s^2 acceleration. That assumes an implausibly good alpha (in my opinion) but we'll go with it. An extra 2 km/s at 2 mm/s^2 acceleration would take a million seconds or about 12 days. I believe 1 mm/s^2 is perhaps plausible in which case it'd take an extra 24 days to spiral down to LMO.

An interplanetary ship harbored in a planet's orbit must depart as well as arrive. Harboring at Deimos vs LMO would save 4 km/s of propellent and 24 to 48 days of time depending on what acceleration an ion craft could achieve.

For a similar reason I advocate harboring something like the Hermes at EML2 rather than LEO. Parking at the edge of earth's gravity rather than LEO would save Hermes 14 km/s climbing in and out of earth's gravity well. And skipping the climb to near bottom and back would save 80 days if acceleration is 2 mm/s^2. 160 days if we use a more plausible 1 mm/s^2.

I'm attaching a graphic illustrating my day dream where ion ships are harbored as close as possible to the edge of planetary gravity wells.
« Last Edit: 03/26/2017 06:27 PM by Hop_David »

Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #12 on: 03/26/2017 06:40 PM »
Phobos and Mars are near equatorial. If launching from the equator, there's little plane change expense.

Here's a few numbers just based on the vis viva equation, not looking at gravity loss during vertical ascent to get above Mars' thickest atmosphere:

Mars to 300 km LMO: 3.7 km/s
Mars to Phobos: 4.9 km/s
Mars to Deimos: 5.4 km/s

Deimos would be more interesting if we were using ion engine interplanetary transfer vehicles such as the Hermes in The Martian.

A rough rule of thumb for delta V with low thrust engines is subtracting speed of destination orbit from speed of departure orbit. An ion craft would enter Mars sphere of influence with a C3=0 traveling close to 0 km/s wrt to Mars. Deimos travels 1.4 km/s. A 300 km LMO is about 3.4 km/s. So rendezvous with Deimos would save about 2 km/s delta V over parking in LMO.

"Wait a minute!" someone might say. "You can use aerobraking to park in LMO. So LMO isn't 3.4 km/s." To achieve a decent alpha, Hermes would need either acres of solar panels or acres of light weight radiator are to dump a nuke power source's waste heat. In either case the power source would be large and fragile and not able to endure aerobraking.

Weir gave Hermes a 2 mm/s^2 acceleration. That assumes an implausibly good alpha (in my opinion) but we'll go with it. An extra 2 km/s at 2 mm/s^2 acceleration would take a million seconds or about 12 days. I believe 1 mm/s^2 is perhaps plausible in which case it'd take an extra 24 days to spiral down to LMO.

An interplanetary ship harbored in a planet's orbit must depart as well as arrive. Harboring at Deimos vs LMO would save 4 km/s of propellent and 24 to 48 days of time depending on what acceleration an ion craft could achieve.

For a similar reason I advocate harboring something like the Hermes at EML2 rather than LEO. Parking at the edge of earth's gravity rather than LEO would save Hermes 14 km/s climbing in and out of earth's gravity well. And skipping the climb to near bottom and back would save 80 days if acceleration is 2 mm/s^2. 160 days if we use a more plausible 1 mm/s^2.

I'm attaching a graphic illustrating my day dream where ion ships are harbored as close as possible to the edge of planetary gravity wells.

Clearly you favor ion propulsion; the only thing I have against it is it's too slow for human transport although suitable for most probes.

A critical question I have is do those numbers, 4.9 and 5.4 for Phobos and Deimos respectively, include the burns to circularize an orbit?  I assume your ion engines would spiral outward from a low orbit but, assuming, purely chemical rockets do the orbital maneuvers, basically periapsis raises, alter those numbers significantly?
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Offline Hop_David

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Re: Phobos and Deimos on the return to Earth
« Reply #13 on: 03/26/2017 07:00 PM »
A critical question I have is do those numbers, 4.9 and 5.4 for Phobos and Deimos respectively, include the burns to circularize an orbit?

Yes. I'm hoping people will use my Hohmann spreadsheet. I'll walk through the steps with a transfer orbit from Mars surface to Phobos.

Altitude periapsis transfer orbit is 0. Altitude apoapsis is same as altitude of Phobos orbit, 5981 km.

Elliptical transfer orbit is moving 4.3033 km/s at periapsis (Mars surface). To circularize at apoapis takes .5787 km/s. I add these together to get 4.88206 km/s. Which I call 4.9 km/s since my model makes simplifying assumptions that reduce precision.

Attached is a screen capture of what I'm talking about. I locked most the spreadsheet to keep myself from accidentally overwriting important cells. The blue area is left unlocked so I can use it as scratch paper.

Offline Rei

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Re: Phobos and Deimos on the return to Earth
« Reply #14 on: 03/26/2017 07:08 PM »
Phobos and Mars are near equatorial. If launching from the equator, there's little plane change expense.

Of course, Mars's axial tilt is ~25 degrees, so the orbits are not aligned with the ecliptic.

Quote
"Wait a minute!" someone might say. "You can use aerobraking to park in LMO. So LMO isn't 3.4 km/s." To achieve a decent alpha, Hermes would need either acres of solar panels or acres of light weight radiator are to dump a nuke power source's waste heat. In either case the power source would be large and fragile and not able to endure aerobraking.

How did the modules that make up Hermes survive launch from Earth if they're that fragile? They were launched retracted, right? Nowadays, large solar power systems are generally deployed on retractable booms or fans. If you needed to, there's no reason you couldn't engineer the deployment to be reversible, so you can retract for aerobraking.

Not that aerobraking is by any stretch a simple process.  But...

Quote
An interplanetary ship harbored in a planet's orbit must depart as well as arrive. Harboring at Deimos vs LMO would save 4 km/s of propellent and 24 to 48 days of time depending on what acceleration an ion craft could achieve.

On the other hand, if your objective is to have a large heavy transfer stage, and a small but high dV ascent/descent stage , it raises the possibility question, why not go all the way and put the heavy craft on a cycler trajectory?

Offline Hop_David

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Re: Phobos and Deimos on the return to Earth
« Reply #15 on: 03/26/2017 07:11 PM »
Clearly you favor ion propulsion; the only thing I have against it is it's too slow for human transport although suitable for most probes.

If a ship's acceleration is a tiny fraction of local gravity field, path will be gradually expanding spiral. If ship's acceleration is a significant fraction of local gravity field, burns can be modeled as impulsive burns allowing ship to enter and exit an elliptical transfer path.

In LEO local acceleration is is around 9 km/s^2. Obviously 1 mm/s^2 is a tiny fraction of that.

But heliocentric orbits are a different story. At 1 A.U. sun's acceleration is about 6 mm/s^2. At Mars, less than 3 mm/s. So a ship with 1 mm/s^2 acceleration would do something closer to an elliptical transfer orbit rather than the slow ion spirals we leaving LEO.

tl;dr Ion engines suck at climbing in and out of planetary gravity wells. But make good time for moving between heliocentric orbits.

Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #16 on: 03/26/2017 07:12 PM »
A critical question I have is do those numbers, 4.9 and 5.4 for Phobos and Deimos respectively, include the burns to circularize an orbit?

Yes. I'm hoping people will use my Hohmann spreadsheet. I'll walk through the steps with a transfer orbit from Mars surface to Phobos.

Altitude periapsis transfer orbit is 0. Altitude apoapsis is same as altitude of Phobos orbit, 5981 km.

Elliptical transfer orbit is moving 4.3033 km/s at periapsis (Mars surface). To circularize at apoapis takes .5787 km/s. I add these together to get 4.88206 km/s. Which I call 4.9 km/s since my model makes simplifying assumptions that reduce precision.

Attached is a screen capture of what I'm talking about. I locked most the spreadsheet to keep myself from accidentally overwriting important cells. The blue area is left unlocked so I can use it as scratch paper.

Impressive.  I see that the periapsis burn for a Phobos orbit just under 600 m/s.  I will take your word that you included the same burn for Deimos in its number, but how does that one likewise compare?

I ask these kinds of questions because, whenever I get commentary on mission setups, I get numbers like these flung in my face among technical concerns.  The biggest challenge for the moons, in this case at least, is ensuring there's reserve fuel for the return to Earth afterwards.

Does leaving Mars from a circling orbit effect departure any differently from the Martian surface or an elliptical orbit?
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Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #17 on: 03/26/2017 07:31 PM »
Gave the spreadsheet a try, in this case for Deimos.

Assuming I used it correctly, a periapsis raise results in 680 m/s required to put the Martian vehicle into a Deimos orbit.  Also reading your spreadsheet, I further see that the transfer orbit's period would be 13.12 hours, meaning after launch from Mars the vehicle (in this case for Deimos of course) the raise burn happens just over 6 hours later.

Is this correct so far?
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Offline Hop_David

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Re: Phobos and Deimos on the return to Earth
« Reply #18 on: 03/26/2017 07:32 PM »
How did the modules that make up Hermes survive launch from Earth if they're that fragile?

If we had dexterous robotic avatars, something like Hermes could be launched in several payloads and assembled in orbit. I regard this as plausible. Launching the whole shebang from earth's surface is less plausible.

They were launched retracted, right? Nowadays, large solar power systems are generally deployed on retractable booms or fans. If you needed to, there's no reason you couldn't engineer the deployment to be reversible, so you can retract for aerobraking.

We're taking about acres of something like cellophane here. Retracting and deploying them each trip would be a nightmare.

On the other hand, if your objective is to have a large heavy transfer stage, and a small but high dV ascent/descent stage , it raises the possibility question, why not go all the way and put the heavy craft on a cycler trajectory?

I don't think an Aldrin cycler is plausible. All Mars cycler schemes I've seen rely on the notion that earth Mars synodic period is 2 1/7 years giving a cycler geometry that repeats every 15 years. The fact that it isn't quite 2 1/7 complicates things. Moreover Mars eccentricity and inclination also requires course corrections. And the big kicker is you need to rotate the Aldrin cycler line of apsides 50º every orbit. It would take a lot of propellent to maintain the cycler orbit, especially if the cycler is massive.

Also the Aldrin cycler has an aphelion clear out in the Main Belt. During fly by the cycler's velocity vector is almost 70º from Mars' velocity vector. This makes for a big delta V budget for the taxis traveling between Mars and cycler. Around 9 km/s, if memory serves.

The VISIT 1 and 2 cyclers are lower maintenance and the Taxis don't need whopping big delta V budgets. But these cyclers don't pass by each planet each synodic period. If I remember right, sometimes passengers would need to sit tight for as long as 7.5 years.
« Last Edit: 03/26/2017 07:33 PM by Hop_David »

Offline Hop_David

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Re: Phobos and Deimos on the return to Earth
« Reply #19 on: 03/26/2017 08:03 PM »
Gave the spreadsheet a try, in this case for Deimos.

Assuming I used it correctly, a periapsis raise results in 680 m/s required to put the Martian vehicle into a Deimos orbit.  Also reading your spreadsheet, I further see that the transfer orbit's period would be 13.12 hours, meaning after launch from Mars the vehicle (in this case for Deimos of course) the raise burn happens just over 6 hours later.

Is this correct so far?

You have the right peri and apo apsis for a transfer from Mars surface (the 0 km altitude) to Deimos (the 200062 km altitude). Your'e reading the period right. And you're correct that transfer is half that.

Not sure if you're interpreting the delta V burns correctly. To get off Mars look at Ellipse periapsis velocity (in this case it's  4.692 km/s). And then add to the the circularize burn at apoapsis. In this case it's .6717. I think you're getting the 680 meters per second from ellipse V at apoapsis (.6797 Km/s)? That's the speed the ship is moving at the height of it's orbit. To circularize it needs to be moving 1.3515 km/s. 1.35 - .68 is more like .67 km/s.

So circularizing at the top of transfer orbit is apoapsis circular minus apoapsis ellipse speed. And dV need to get off Mars is just ellipse speed at periapsis.

Finding relevant numbers is like looking for needles in a haystack so I greyed out some of the irrelevant stuff. The dV from Mars surface to Deimos would be about 4.7 + .67 km/s.

Offline Hop_David

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Re: Phobos and Deimos on the return to Earth
« Reply #20 on: 03/26/2017 08:19 PM »
Does leaving Mars from a circling orbit effect departure any differently from the Martian surface or an elliptical orbit?

It can make a huge difference. If departing from a capture orbit (periapsis just above atmosphere and apoapsis just within Sphere Of Influence), you're traveling a hair under escape velocity at periapsis. And escape velocity is 1.414 faster than circular orbit velocity.

I talk about this at Inflated Delta Vs

There's a problem, though. For departing from a capture orbit, the periapsis of the capture orbit needs to be at the right place and time during a launch window. There's no specific capture orbit that will serve general needs.

However from a high circular orbit it's possible to time a braking burn so your elliptical orbit's periapsis is at the right place at the right time. I talk about this at What about Mr. Oberth?

For arriving at a planet with an atmosphere you can do a small burn to brake into a capture orbit and let aerobraking circularize to a low circular orbit. (I'm talking about chemical rockets here).
« Last Edit: 03/26/2017 08:23 PM by Hop_David »

Offline Rei

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Re: Phobos and Deimos on the return to Earth
« Reply #21 on: 03/27/2017 12:41 AM »
They were launched retracted, right? Nowadays, large solar power systems are generally deployed on retractable booms or fans. If you needed to, there's no reason you couldn't engineer the deployment to be reversible, so you can retract for aerobraking.

We're taking about acres of something like cellophane here. Retracting and deploying them each trip would be a nightmare.

Retraction of deployed solar arrays, etc is a long studied problem.  Eg:

https://arc.aiaa.org/doi/abs/10.2514/3.26722?journalCode=jsr

It's seldom done because there's usually little need, but it certainly can be done. And given the dV advantage for aerobraking, it's hard to say it's not worth it.

Quote
On the other hand, if your objective is to have a large heavy transfer stage, and a small but high dV ascent/descent stage , it raises the possibility question, why not go all the way and put the heavy craft on a cycler trajectory?

I don't think an Aldrin cycler is plausible. All Mars cycler schemes I've seen rely on the notion that earth Mars synodic period is 2 1/7 years giving a cycler geometry that repeats every 15 years. The fact that it isn't quite 2 1/7 complicates things. Moreover Mars eccentricity and inclination also requires course corrections. And the big kicker is you need to rotate the Aldrin cycler line of apsides 50º every orbit. It would take a lot of propellent to maintain the cycler orbit, especially if the cycler is massive.

Also the Aldrin cycler has an aphelion clear out in the Main Belt. During fly by the cycler's velocity vector is almost 70º from Mars' velocity vector. This makes for a big delta V budget for the taxis traveling between Mars and cycler. Around 9 km/s, if memory serves.

The VISIT 1 and 2 cyclers are lower maintenance and the Taxis don't need whopping big delta V budgets. But these cyclers don't pass by each planet each synodic period. If I remember right, sometimes passengers would need to sit tight for as long as 7.5 years.

Honestly, I'm not much of a fan of cyclers either. But it seemed you were going fairly far in the direction of heavy/low-dV xfer stage with a light/high-dV ascent/descent stage, and Aldrin cycler correction dVs are a less than capture / escape dVs - under 2.7km/s per synodic period. But if that's not what you were trying to optimize toward, my apologies  :)
« Last Edit: 03/27/2017 12:42 AM by Rei »

Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #22 on: 03/27/2017 04:33 AM »
Does leaving Mars from a circling orbit effect departure any differently from the Martian surface or an elliptical orbit?

It can make a huge difference. If departing from a capture orbit (periapsis just above atmosphere and apoapsis just within Sphere Of Influence), you're traveling a hair under escape velocity at periapsis. And escape velocity is 1.414 faster than circular orbit velocity.

I talk about this at Inflated Delta Vs

There's a problem, though. For departing from a capture orbit, the periapsis of the capture orbit needs to be at the right place and time during a launch window. There's no specific capture orbit that will serve general needs.

However from a high circular orbit it's possible to time a braking burn so your elliptical orbit's periapsis is at the right place at the right time. I talk about this at What about Mr. Oberth?

At Earth I can see how the gravity wells of both Earth and Luna can give plenty of options for utilizing the Oberth effect.  How much of an Oberth effect be obtained from tweaking orbits matching Phobos and Deimos respectively?
« Last Edit: 03/27/2017 06:18 AM by redliox »
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Re: Phobos and Deimos on the return to Earth
« Reply #23 on: 03/27/2017 06:28 AM »
Two performance values to consider:
1) delta-v
2) mars ascent vehicle availability for a return launch

Correct naturally.  Mostly this thread is about crunching the numbers to pinpoint the delta-v required for an outgoing visit to either Phobos or Deimos.  An inbound flight is possible, but would have the disadvantage of requiring to haul fuel from Earth whereas an outbound from Mars could take advantage of ISRU from the red planet.  However, again, most it is about crunching numbers not hardware per say.

Clearly launching to a circular orbit about 300km is easiest.  But the rendezvous window could be a kicker (availability to launch may need to be very high w/no launch scrubs allowed).  It seems we cannot get near 100% as it is on Earth with every resource needed provided within reach of the launch pad.

Launching to a Phobos/Deimos station seems more practical for the ascent vehicle design, but it requires more weight to be landed.  It seems that any surface mission on mars requires landing half of Hoboken, and the design concept doesn't close.  There just isn't a practical way to land

Talk to Hop_David about the Oberth effect and the surprises orbital mechanics can bring.  The main issues of handling Deimos and Phobos is rendezvous, not the landing itself.  Orbits have to be matched and circularized.  Details like that are the reason Zubrin wasn't enthusiastic about incorporating moon visits with Mars direct.

If recent news accounts are accurate, Trump wants a plan in a few months around the fall.  When the report shows no practical solution...

No politics in this thread please, just math related to visiting the Martian moons from the Mars en route to Earth return...please.
« Last Edit: 03/27/2017 06:29 AM by redliox »
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Re: Phobos and Deimos on the return to Earth
« Reply #24 on: 03/27/2017 07:40 AM »
Alright, trying to compute how to get some form of Oberth Effect from the orbits of Phobos and Deimos.  I assumed a 500 km orbit about Mars to get deep in the gravity well but away to avoid even minute atmospheric drag.  I am unsure if I am reading the chart correctly but I posted what I got so the numbers can be explained...

At the 500 km periapsis (from a circular orbit naturally) it states the orbital velocity is 3.3158 km/s and that escape is 4.6893 km/s.  With the elliptical-Phobos orbit I see ellipse periapsis velocity at 3.9 km/s with a burn of 1.45 km/s to hit hyperbolic velocity.  With the elliptical-Deimos orbit the ellipse periapsis velocity is ~4.3 km/s requiring a burn of just above 1 km/s.

So, under the conditions to setup for Oberth Effect, an escape from Mars would be reasonably possible from either moon if you had a reserve of 1.5 km/s to spare...assuming I'm interpreting this right.  Beyond that I'd wish a double-check on the burns to adjust from Deimos and Phobos to reach the 500 km periapsis.
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Offline Russel

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Re: Phobos and Deimos on the return to Earth
« Reply #25 on: 03/27/2017 12:41 PM »
When I think about Mars, the one fixed point I keep returning to is the need for a light weight ascent vehicle.

What I don't have an absolute handle on is whether the ascent vehicle should have the capability to go beyond low Mars orbit. If it does then the options are either a near-escape orbit and rendezvous with an Earth return vehicle, or something similar but focused around one of the moons.

I probably lean towards having something more substantial (with good life support) being in a circular low Mars orbit. But I could be persuaded for that vehicle to sit at an L-point, probably near Phobos. The question is what would be the advantage in this to justify the extra fuel? It may require more time in flight but I'm not sure.

Of course, if you've got plenty of fuel sitting around in orbit, one use for a versatile ascent vehicle is to visit a moon, or two as an optional part of the mission. Hence I tend to agree that this might occur towards the end of the mission.  So its being used as a taxi for short term visits.

Anyhow that's where I'm at..

Offline Hop_David

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Re: Phobos and Deimos on the return to Earth
« Reply #26 on: 03/27/2017 01:56 PM »
Retraction of deployed solar arrays, etc is a long studied problem.  Eg:

https://arc.aiaa.org/doi/abs/10.2514/3.26722?journalCode=jsr

It's seldom done because there's usually little need, but it certainly can be done. And given the dV advantage for aerobraking, it's hard to say it's not worth it.

Economical, reusable SSTO spaceships are also worthwhile. As is warp drive. The question is, is it doable?

Again, we're not talking about solar array wings like Hubble uses or even the I.S.S.. To get the needed power and alpha for an interplanetary ion propelled vehicle we need a very large and very light weight power source.

And you're going to fold up and then redeploy this very large and fragile power source every trip? How many moving parts? What is the life expectancy or these acres of Seran Wrap solar arrays?

I regard an ion ship capable of 1 mm/s^2 plausible. But such a ship also able to endure the punishment of aerobraking is less plausible science fiction.

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Re: Phobos and Deimos on the return to Earth
« Reply #27 on: 03/27/2017 02:38 PM »
At the 500 km periapsis (from a circular orbit naturally) it states the orbital velocity is 3.3158 km/s and that escape is 4.6893 km/s.  With the elliptical-Phobos orbit I see ellipse periapsis velocity at 3.9 km/s with a burn of 1.45 km/s to hit hyperbolic velocity.  With the elliptical-Deimos orbit the ellipse periapsis velocity is ~4.3 km/s requiring a burn of just above 1 km/s.

So, under the conditions to setup for Oberth Effect, an escape from Mars would be reasonably possible from either moon if you had a reserve of 1.5 km/s to spare...assuming I'm interpreting this right.  Beyond that I'd wish a double-check on the burns to adjust from Deimos and Phobos to reach the 500 km periapsis.

I'm happy you're paying close attention to the Insertion burn from periapsis cell. However be careful that you don't regard this as the total delta V budget.

Besides the periapsis burn you must also do a braking to drop from the moon to the 500 km periapsis. To get this braking burn look at the Apoapsis circulize burn. (Just noticed I misspelled "circularize" - d'oh!). Since orbits are time reversible, the circularize and decircularize braking burns are the same.

So. To get to earth from Phobos and using the Oberth benefit, you first do a .4997 braking burn. Then at periapsis you do a 1.4444 km/s burn to give you Trans Earth Insertion (TEI). Total dV is 1.9441 km/s.

It's worth noting TEI directly from Phobos is 1.8818 km/s. So in this this case a braking burn to enjoy a periapsis Oberth kick isn't worth it.


From Deimos the braking burn is .6301 to drop to a 500 km periapsis. Add that to the Insertion burn from periapsis and you get 1.6734 km/s.

TEI directly from Deimos is 1.9150 km/s. So in this case a braking burn to enjoy a periapsis Oberth kick is worth it.

Again, my spreadsheet isn't accurate to 4 significant digits. I write out the numbers so it's easier to compare if we're getting the same results.
« Last Edit: 03/27/2017 02:43 PM by Hop_David »

Offline Rei

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Re: Phobos and Deimos on the return to Earth
« Reply #28 on: 03/27/2017 04:07 PM »
Retraction of deployed solar arrays, etc is a long studied problem.  Eg:

https://arc.aiaa.org/doi/abs/10.2514/3.26722?journalCode=jsr

It's seldom done because there's usually little need, but it certainly can be done. And given the dV advantage for aerobraking, it's hard to say it's not worth it.

Economical, reusable SSTO spaceships are also worthwhile. As is warp drive. The question is, is it doable?

Again, we're not talking about solar array wings like Hubble uses or even the I.S.S.. To get the needed power and alpha for an interplanetary ion propelled vehicle we need a very large and very light weight power source.

And you're going to fold up and then redeploy this very large and fragile power source every trip? How many moving parts? What is the life expectancy or these acres of Seran Wrap solar arrays?

Well, for one you could read the paper.  Or if you'd rather a video, here's today's state of the art, ATK Megaflex (150W/kg):



Note how several boom segments repeatedly go backwards, some almost to the point of being fully collapsed. When they're done with a test deployment, they just run the booms in reverse to repack it. 

And this is a system for which reversibility was not a design goal.

Your notion that solar cells would be so fragile that they can't be repacked is unrealistic because then they'd be too fragile to deal with incidental impulses. And let's just say that you have to add some mass to allow for repacking - so what? Who cares if reversibility adds some mass when you're saving kilometers per second dV by doing so?
« Last Edit: 03/27/2017 07:06 PM by Rei »

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Re: Phobos and Deimos on the return to Earth
« Reply #29 on: 03/27/2017 05:19 PM »
At the 500 km periapsis (from a circular orbit naturally) it states the orbital velocity is 3.3158 km/s and that escape is 4.6893 km/s.  With the elliptical-Phobos orbit I see ellipse periapsis velocity at 3.9 km/s with a burn of 1.45 km/s to hit hyperbolic velocity.  With the elliptical-Deimos orbit the ellipse periapsis velocity is ~4.3 km/s requiring a burn of just above 1 km/s.

So, under the conditions to setup for Oberth Effect, an escape from Mars would be reasonably possible from either moon if you had a reserve of 1.5 km/s to spare...assuming I'm interpreting this right.  Beyond that I'd wish a double-check on the burns to adjust from Deimos and Phobos to reach the 500 km periapsis.

I'm happy you're paying close attention to the Insertion burn from periapsis cell. However be careful that you don't regard this as the total delta V budget.

Besides the periapsis burn you must also do a braking to drop from the moon to the 500 km periapsis. To get this braking burn look at the Apoapsis circulize burn. (Just noticed I misspelled "circularize" - d'oh!). Since orbits are time reversible, the circularize and decircularize braking burns are the same.

Your help puts me in your debt H_D  :)  I think all the delta-v numbers are now calculated...

So. To get to earth from Phobos and using the Oberth benefit, you first do a .4997 braking burn. Then at periapsis you do a 1.4444 km/s burn to give you Trans Earth Insertion (TEI). Total dV is 1.9441 km/s.

It's worth noting TEI directly from Phobos is 1.8818 km/s. So in this this case a braking burn to enjoy a periapsis Oberth kick isn't worth it.


From Deimos the braking burn is .6301 to drop to a 500 km periapsis. Add that to the Insertion burn from periapsis and you get 1.6734 km/s.

TEI directly from Deimos is 1.9150 km/s. So in this case a braking burn to enjoy a periapsis Oberth kick is worth it.

Based on the numbers assembled, there appear to be 3 routes a mission could take involving visiting Phobos and Deimos post-Martian-surface-activity:

1) Phobos with a direct TEI
2) Deimos with a direct TEI
3) Deimos with an Oberth maneuver prior to TEI

An Oberth-assisted Phobos path is excluded as the Oberth Effect as it burns 100+ m/s more effort than direct TEI.

Scenario 1 Breakdown: Phobos with a direct TEI
1: Launch from Mars to Phobos Transfer Orbit: 4.3 km/s
2: Periapsis Raise: 0.6 km/s
3: Phobos Ops: 0.16 km/s
4: Direct Trans Earth Injection: 1.9 km/s
Total Delta-V: ~7 km/s

Scenario 2 Breakdown: Deimos with a direct TEI
1: Launch from Mars to Deimos Transfer Orbit: 4.7 km/s
2: Periapsis Raise: 0.7 km/s
3: Deimos Ops: 0.16 km/s
4: Direct Trans Earth Injection: 1.92 km/s
Total Delta-V: ~7.5 km/s

Scenario 3 Breakdown: Deimos with Oberth Effect maneuvering
1: Launch from Mars to Deimos Transfer Orbit: 4.7 km/s
2: Periapsis Raise: 0.7 km/s
3: Deimos Ops: 0.16 km/s
4: Periapsis Brake:  0.64 km/s
5: Trans Earth Injection: 1.68 km/s
Total Delta-V: ~7.9  km/s

Double-check the math naturally, although I believe I'm following charts (including Hop_David's) correctly.  The 160 m/s for operations at Deimos/Phobos is generously conservative, perhaps still easily possible with 50 m/s thanks to microgravity.  Assuming the numbers as is are correct, I think a few conclusions can be drawn from the delta-v calculations:

1) Phobos, even without the Oberth Effect, is the lowest overall for delta-v needs.
2) While more applicable to Deimos, trying to employ the Oberth Effect results in needing 400 m/s more than without.
3) Because the direct TEI delta-v for both Phobos and Deimos are very close, a common means of departure with a budget of 2 km/s would work well despite the moons' different orbits.
« Last Edit: 03/27/2017 06:38 PM by redliox »
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Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #30 on: 03/27/2017 06:56 PM »
When I think about Mars, the one fixed point I keep returning to is the need for a light weight ascent vehicle.

What I don't have an absolute handle on is whether the ascent vehicle should have the capability to go beyond low Mars orbit. If it does then the options are either a near-escape orbit and rendezvous with an Earth return vehicle, or something similar but focused around one of the moons.

I probably lean towards having something more substantial (with good life support) being in a circular low Mars orbit. But I could be persuaded for that vehicle to sit at an L-point, probably near Phobos. The question is what would be the advantage in this to justify the extra fuel? It may require more time in flight but I'm not sure.

I think Hop_David's charts can help answer where best to apply your light MAV ideas.  I believe your idea is possible, but the limit of it is keeping the astronauts comfortable during.  In the case of this thread's subject, it would specifically be during the ascent from Mars and venturing out to Phobos or Deimos during the transfer orbit.

Now the trick is time itself.  A transfer orbit that links the surface to Phobos is roughly 5 hours, with Deimos just over 13 hours.  The MAV would be in that orbit presumably just for the ascent, then perform a maneuver to circulize the orbit (or at the least raise it so there isn't a horrific return to the surface); but the main fact is your crew spends half of these respective orbits in the mini-MAV.  For Phobos 2.5 hours is reasonable, for Deimos 6.5 which isn't as reasonable.

So your MAV design would need to incorporate the following factors:
1) Launch with enough delta-v to march the target moon - 4.3 km/s for Phobos and 4.7 km/s for Deimos
2) Perform a second burn to raise periapsis - 0.6 km/s for Phobos and 0.7 km/s for Deimos
3) Keep the crew alive and (somewhat) comfortable for at least 3 hours for Phobos and 7 hours for Deimos

Your lighter MAV would be able to accommodate extra fuel for maneuvers like a periapsis raise readily, but the trick is how quickly can the MAV dock with the awaiting Hermes-eque orbiter.  So I think your idea works best for lower orbits and even Phobos but not so well for Deimos.
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Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #31 on: 03/27/2017 07:39 PM »
Economical, reusable SSTO spaceships are also worthwhile. As is warp drive. The question is, is it doable?

Again, we're not talking about solar array wings like Hubble uses or even the I.S.S.. To get the needed power and alpha for an interplanetary ion propelled vehicle we need a very large and very light weight power source.

And you're going to fold up and then redeploy this very large and fragile power source every trip? How many moving parts? What is the life expectancy or these acres of Seran Wrap solar arrays?

Well, for one you could read the paper.  Or if you'd rather a video, here's today's state of the art, ATK Megaflex (150W/kg):



Note how several boom segments repeatedly go backwards, some almost to the point of being fully collapsed. When they're done with a test deployment, they just run the booms in reverse to repack it. 

And this is a system for which reversibility was not a design goal.

Your notion that solar cells would be so fragile that they can't be repacked is unrealistic because then they'd be too fragile to deal with incidental impulses. And let's just say that you have to add some mass to allow for repacking - so what? Who cares if reversibility adds some mass when you're saving kilometers per second dV by doing so?

When you're talking saving mass with ion propulsion it can be a tricky question.  What you're doing, to a degree, is trading the immense mass of fuel chemical rocketry burns and exchanging it for a power system; if your power supply is big and heavy it is going to ruin the fuel/mass-saving merits of electric propulsion you wish to achieve.  Fortunately, the trend in technology leads to efficient and lighter electronics and likewise solar cells.

Hop does have a point about moving parts jamming.  However, if you're advocating a form of reusable and storable ion propulsion my suggestion would be to make said arrays detachable if necessary, just as the Hubble array that once jammed was literally tossed into space.  If you intend to bring the vehicle back to either Earth or Earth orbit one could swap out the arrays periodically.

My opinion on electric propulsion/ion drive in this thread is slightly indifferent.  Because of the Martian gravity well, its application is probably limited toward Deimos partly because you'll need to spiral inward.  Otherwise the trouble is gaining speed.  Previously Hop made this calculation...

Weir gave Hermes a 2 mm/s^2 acceleration. That assumes an implausibly good alpha (in my opinion) but we'll go with it. An extra 2 km/s at 2 mm/s^2 acceleration would take a million seconds or about 12 days. I believe 1 mm/s^2 is perhaps plausible in which case it'd take an extra 24 days to spiral down to LMO.

An interplanetary ship harbored in a planet's orbit must depart as well as arrive. Harboring at Deimos vs LMO would save 4 km/s of propellent and 24 to 48 days of time depending on what acceleration an ion craft could achieve.

For a similar reason I advocate harboring something like the Hermes at EML2 rather than LEO. Parking at the edge of earth's gravity rather than LEO would save Hermes 14 km/s climbing in and out of earth's gravity well. And skipping the climb to near bottom and back would save 80 days if acceleration is 2 mm/s^2. 160 days if we use a more plausible 1 mm/s^2.

While 24 to 80 days isn't unreasonable given the average conjunction-class Mars mission lasts 3 years, the problem is most launch windows are lucky to last 2 months.  The longer your engines need to build up speed, the less your chance of returning to Earth (since that too is a focus on this thread).  So the best idea, as Hop would agree with, is to employ ion drives as far from Mars as possible.

My thoughts on how to apply ion drives to Mars is to focus them on orbital elements, especially Deimos or say synchronous orbit.  The crew's primary vehicle could be either styled after the Mars Direct ERV or SpaceX's ITS, chemical-fueled and independent, whereas a vehicle for exploring Deimos/Phobos could be chiefly ion supplemented with some hypergolic fuel.
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Offline Russel

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Re: Phobos and Deimos on the return to Earth
« Reply #32 on: 03/28/2017 01:43 AM »
When I think about Mars, the one fixed point I keep returning to is the need for a light weight ascent vehicle.

What I don't have an absolute handle on is whether the ascent vehicle should have the capability to go beyond low Mars orbit. If it does then the options are either a near-escape orbit and rendezvous with an Earth return vehicle, or something similar but focused around one of the moons.

I probably lean towards having something more substantial (with good life support) being in a circular low Mars orbit. But I could be persuaded for that vehicle to sit at an L-point, probably near Phobos. The question is what would be the advantage in this to justify the extra fuel? It may require more time in flight but I'm not sure.

I think Hop_David's charts can help answer where best to apply your light MAV ideas.  I believe your idea is possible, but the limit of it is keeping the astronauts comfortable during.  In the case of this thread's subject, it would specifically be during the ascent from Mars and venturing out to Phobos or Deimos during the transfer orbit.

Now the trick is time itself.  A transfer orbit that links the surface to Phobos is roughly 5 hours, with Deimos just over 13 hours.  The MAV would be in that orbit presumably just for the ascent, then perform a maneuver to circulize the orbit (or at the least raise it so there isn't a horrific return to the surface); but the main fact is your crew spends half of these respective orbits in the mini-MAV.  For Phobos 2.5 hours is reasonable, for Deimos 6.5 which isn't as reasonable.

So your MAV design would need to incorporate the following factors:
1) Launch with enough delta-v to march the target moon - 4.3 km/s for Phobos and 4.7 km/s for Deimos
2) Perform a second burn to raise periapsis - 0.6 km/s for Phobos and 0.7 km/s for Deimos
3) Keep the crew alive and (somewhat) comfortable for at least 3 hours for Phobos and 7 hours for Deimos

Your lighter MAV would be able to accommodate extra fuel for maneuvers like a periapsis raise readily, but the trick is how quickly can the MAV dock with the awaiting Hermes-eque orbiter.  So I think your idea works best for lower orbits and even Phobos but not so well for Deimos.

Yeah that's reasonable. The question I have is what is the equivalent length of time spent transferring from the surface to a circular low Mars orbit? And what are the issues with launch windows when comparing going to a specific low Mars orbit and going to Phobos?

Perhaps I should put this into concrete terms. Suppose you have a permanent base in low Mars orbit. A modest habitat and fuel store. The MAV makes its initial ascent from the surface to the base in low Mars orbit. That's the first step in a return to Earth (I'll leave out the other main line detail). Once you have the crew safely on board the base in low Mars orbit you have the option of making an round trip excursion to Phobos. In this case you can use the MAV with a 2 person crew.

The point here is that with a limited crew, the MAV can provide more comfort and more durability. You're also in a position to make this part of the mission optional so that the fuel you need for the excursion can also be a contingency.

Three basic philosophical points here.

1. I've grown less worried about having lots of fuel, at least in Mars orbit. Musk is right about one thing. Its about economy of scale and while its nice to be mindful of total mass, I don't think that mass of fuel is as big an issue as has been thought.

2. While I'd like to put boots on the moons, its not essential. It actually makes more sense to teleoperate a robot on Phobos from the surface, than the other way around.

3. If we're talking about putting more hardware on or near a moon (particularly Phobos) the main bonus for me is protection from radiation. However, is this still true at the L point? There is an argument for putting the small habitat (mentioned above) actually on the Phobos surface (but with rendezvous at the L point).


Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #33 on: 03/28/2017 06:12 AM »
The question I have is what is the equivalent length of time spent transferring from the surface to a circular low Mars orbit? And what are the issues with launch windows when comparing going to a specific low Mars orbit and going to Phobos?

I'm not certain on launch windows but considering they deal with similar problems with low Earth orbit NASA, even from a distance, would ensure astronauts would know when to liftoff.  Given the Phobos ascent I told you about before is in the area of 2.5 hours, an hour or less is a reasonable estimate; and if your life support system can't keep people alive for even an hour NASA wouldn't consider it!  :P

Three basic philosophical points here.

1. I've grown less worried about having lots of fuel, at least in Mars orbit. Musk is right about one thing. Its about economy of scale and while its nice to be mindful of total mass, I don't think that mass of fuel is as big an issue as has been thought.

2. While I'd like to put boots on the moons, its not essential. It actually makes more sense to teleoperate a robot on Phobos from the surface, than the other way around.

3. If we're talking about putting more hardware on or near a moon (particularly Phobos) the main bonus for me is protection from radiation. However, is this still true at the L point? There is an argument for putting the small habitat (mentioned above) actually on the Phobos surface (but with rendezvous at the L point).

1 - Relying on Mars for fuel will solve plenty of problems, but I wouldn't say "I don't think that mass of fuel is as big an issue as has been thought" just yet.

2 - Sending robotic sample return missions should be the easiest task in the solar system but that hasn't happened; I will grant telerobotics is an option but you realize the inverse has been argued by the orbit-first people; ironically they'd land on a moon and let robots explore Mars for the astronauts.

3 - If you press a scientist hard enough they'll answer "not really."  Also the Lagrange points for Phobos/Deimos are within kilometers of the moons.  Plenty of station keeping required although not impossible.
« Last Edit: 03/28/2017 06:15 AM by redliox »
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Offline Rei

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Re: Phobos and Deimos on the return to Earth
« Reply #34 on: 03/28/2017 09:17 AM »
When you're talking saving mass with ion propulsion it can be a tricky question.  What you're doing, to a degree, is trading the immense mass of fuel chemical rocketry burns and exchanging it for a power system; if your power supply is big and heavy it is going to ruin the fuel/mass-saving merits of electric propulsion you wish to achieve.

150W/kg is not "big and heavy". It's the lightest in-use solar power system to date, and not all that far from the estimates for space nuclear power systems.  The next generation, Ultraflex (should be flight tested within a few years), is 200W/kg, and no less repackable. Because the same issues apply: if your solar panels are so fragile that they're likely to be broken by stowing them, then they're too fragile, period.  Solar arrays aren't made lighter today specifically for that reason: fragility is not acceptable.

But yes, you're absolutely right that things have been trending lighter.  Surprisingly quickly. :)  If I remember right, ISS's solar arrays are only 27W/kg.  Ultraflex is already light enough for a 150d (from LEO) VASIMR opposition-class Mars mission using 25% of the spacecraft's mass** - not counting aerobraking.  It's easy to see how with aerobraking and/or and the continuation of this trend (better materials, deployable truss or boom reinforcement on multiple axes, etc) one could get to 50-75d ion transits a decade or two from now. Which, given how little propellant needs to be replaced on each leg, opens up very interesting possibilities.

** - Assuming a constant 200W/m².
« Last Edit: 03/28/2017 11:30 AM by Rei »

Offline Hop_David

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Re: Phobos and Deimos on the return to Earth
« Reply #35 on: 03/28/2017 01:16 PM »
Scenario 3 Breakdown: Deimos with Oberth Effect maneuvering
1: Launch from Mars to Deimos Transfer Orbit: 4.7 km/s
2: Periapsis Raise: 0.7 km/s
3: Deimos Ops: 0.16 km/s
4: Periapsis Brake:  0.64 km/s
5: Trans Earth Injection: 1.68 km/s
Total Delta-V: ~7.9  km/s

You're getting better! One criticism: since you already included the brake to drop to periapsis in step 4, I think you can call TEI 1.04 km/s.

I've eavesdropped conversations between astrogators and I get the impression they like to pad delta V budgets by 10% for safety reasons. Something to think about.

I'm tossing out another scenario. Departing directly from Mars. If you're going to include steps 1 and 2 (getting off Mars and rendezvous with Deimos) in your total delta V budget, why not leave directly from Mars?

I usually regard steps 1 & 2 as a separate delta V budget. Presumably these would be for a light weight Mars Ascent Vehicle (MAV) for getting from Mars to Deimos. Then steps 3 through 5 would be for the more massive interplanetary vehicle that was berthed at Deimos. It's worth noting the spreadsheet is for impulsive burns. That is, chemical rockets, not ion.

Also there is the possibility of ISRU propellent at Deimos. If Deimos is a propellent source, you can start over with a new delta V budget.

Anyway, here's my way of getting delta V for TEI from Mars' surface. Set periapsis at 0 altitude (Mars surface). Since Mars surface is moving 0 km/s with regard to Mars, just use the hyperbolic periapsis velocity. 5.7 km/s. Well, actually Mars equator is moving about .25 km/s wrt to Mars center. Launching from Mars equator can confer a little help, sort of like French Guiana on earth.
« Last Edit: 03/28/2017 01:25 PM by Hop_David »

Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #36 on: 03/28/2017 06:18 PM »
Scenario 3 Breakdown: Deimos with Oberth Effect maneuvering
1: Launch from Mars to Deimos Transfer Orbit: 4.7 km/s
2: Periapsis Raise: 0.7 km/s
3: Deimos Ops: 0.16 km/s
4: Periapsis Brake:  0.64 km/s
5: Trans Earth Injection: 1.68 km/s
Total Delta-V: ~7.9  km/s

You're getting better! One criticism: since you already included the brake to drop to periapsis in step 4, I think you can call TEI 1.04 km/s.

So the end result would be a total delta-v of 7.25 km/s, which saves about 250 m/s versus the 'direct' route away from Deimos.  Alright, so it is good that there is merit for the Oberth Effect at Deimos after all!  :)

I've eavesdropped conversations between astrogators and I get the impression they like to pad delta V budgets by 10% for safety reasons. Something to think about.

Not surprised.  I already have one scenario where a would be Mars-to-Deimos mission plans for a direct escape from Deimos' orbit with 60 mt of leftover methalox and a specific impulse of 382 seconds (modeled after SpaceX Raptor engines).  It would still have 2.2 km/s potentially available which could be enough for both correction maneuvers and even Earth capture (especially if some of your charts are correct about it being easier than we expect).

I'm tossing out another scenario. Departing directly from Mars. If you're going to include steps 1 and 2 (getting off Mars and rendezvous with Deimos) in your total delta V budget, why not leave directly from Mars?

Oh I am well aware a direct departure from Mars is the most efficient than trying the same from Mars orbit; by easily a 2 km/s difference.  I'd even advocate that the crew's primary vehicle, especially if something goes wrong with a pre-positioned Phobos/Deimos vehicle, skip visiting the moon(s) and go directly to Earth.  Visiting Phobos and Deimos are mere side options.

Also there is the possibility of ISRU propellent at Deimos. If Deimos is a propellent source, you can start over with a new delta V budget.

I hope that is the case, but because we've had only flybys of Phobos at best our information on both moons is minimal.  This is one reason I think visiting the moons has merit, so we can find out if they have ISRU potential.  JPL, NASA, etc is heavily focused on Mars itself so it is very unlikely we'll see a decent robotic visit to them (although JAXA is readying something up that alley it appears!  ;D ).

ISRU (for the moons that is) is a possibility, but not in the immediate future is my only argument against it.

Anyway, here's my way of getting delta V for TEI from Mars' surface. Set periapsis at 0 altitude (Mars surface). Since Mars surface is moving 0 km/s with regard to Mars, just use the hyperbolic periapsis velocity. 5.7 km/s. Well, actually Mars equator is moving about .25 km/s wrt to Mars center. Launching from Mars equator can confer a little help, sort of like French Guiana on earth.

It is delightful there are a few tricks available to ease flying around Mars and, as this thread obviously implies, landing on its moons.  Between things like centrifugal force from Mars' rotation at the equator, the Oberth Effect (applicable to Deimos specifically), and the need to keep a reserve of 10% delta-v, I'd presume budgeting a mission designed around a brutish direct-TEI from the moons' orbits would be the first step...plus of course ensuring there is reserve fuel or, if the Phobos/Deimos vehicle has the room on its own launcher, including a TEI stage with it to transfer to the Martian vehicle.
« Last Edit: 03/28/2017 06:32 PM by redliox »
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Offline Rei

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Re: Phobos and Deimos on the return to Earth
« Reply #37 on: 03/28/2017 07:29 PM »
JPL, NASA, etc is heavily focused on Mars itself so it is very unlikely we'll see a decent robotic visit to them (although JAXA is readying something up that alley it appears!  ;D ).

That's one mission I'm really crossing my fingers for  :)  Mineralogical mapping with a focus on hydrated minerals (globally for Phobos, partially for Deimos), followed by two landing site selections and two sample return. If there's anything of interest exposed on the surface of interest, we'll have two samples (each from >2cm deep) of it in the lab to make simulants of. How cool is that?  :)  And even if there's nothing neat right near the surface, they'll be doing gravity mapping to look for possible subsurface ice deposits that a followup mission could check out.

JAXA's budget isn't huge, but they do some neat stuff with it.

Offline Star One

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Re: Phobos and Deimos on the return to Earth
« Reply #38 on: 03/28/2017 08:51 PM »
Quote
Jeff Foust‏ @jeff_foust

Zelenyi: studying a mission called Boomerang to return samples from Phobos in 2020s. Also interested in Mars sample return.

https://mobile.twitter.com/jeff_foust/status/846819996177182720

Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #39 on: 03/29/2017 12:18 AM »
Quote
Jeff Foust‏ @jeff_foust

Zelenyi: studying a mission called Boomerang to return samples from Phobos in 2020s. Also interested in Mars sample return.

https://mobile.twitter.com/jeff_foust/status/846819996177182720

AH!  The secret codename for Phobos-Grunt 2.  Although I don't wish bad news for Russia's robotic space exploration...but they seem to have nothing but it.  :(

JPL, NASA, etc is heavily focused on Mars itself so it is very unlikely we'll see a decent robotic visit to them (although JAXA is readying something up that alley it appears!  ;D ).

That's one mission I'm really crossing my fingers for  :)  Mineralogical mapping with a focus on hydrated minerals (globally for Phobos, partially for Deimos), followed by two landing site selections and two sample return. If there's anything of interest exposed on the surface of interest, we'll have two samples (each from >2cm deep) of it in the lab to make simulants of. How cool is that?  :)  And even if there's nothing neat right near the surface, they'll be doing gravity mapping to look for possible subsurface ice deposits that a followup mission could check out.

JAXA's budget isn't huge, but they do some neat stuff with it.

I'm crossing my fingers too!  Even if the mission is small ANY new data is good since the knowledge on the moons is THAT limited, Mars Express being the best thus far.  The only thing confirmed about the moons' surface is that they're dry, but it could easily be a different story below the surface.

Incidentally Rei, I believe JAXA was pursuing a hybrid solar-electric option for the mission.  Perfectly suited for probes with more time to spare.
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Offline Russel

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Re: Phobos and Deimos on the return to Earth
« Reply #40 on: 03/29/2017 12:00 PM »
There was another option floating around in my head that is worth mentioning.

It involves first having a modest habitat in Mars orbit which happens to have SEP capability. The principle here is that you want to spend as much time as possible in a comfortable and relatively safe environment.

Initially your MAV delivers you to low Mars orbit where you join the habitat. It spirals out to Phobos L1 and loiters there while you use modest means to visit the surface. After you've explored Phobos you spiral out to do the same at Deimos. Finally you use SEP to achieve something closer to a capture orbit. You rendezvous with the Earth return vehicle and it uses chemical thrust to return to Earth.

Reason I'm not that fussed about this is that given sensible parameters its still a couple of months worth of extra mucking around in Mars orbit. Which means more supplies. So its doubtful if it really wins over just using the MAV as a "taxi" for the moons. But worth considering.

Offline Hop_David

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Re: Phobos and Deimos on the return to Earth
« Reply #41 on: 03/29/2017 12:57 PM »
Given an approximately 1000 km tether going up from Phobos and an ~3000 tether going down from Deimos, there exists a ZRVTO (Zero Relative Velocity Transfer Orbit) between Phobos and Deimos.

It'd be possible to travel between the two moons using almost zero reaction mass.

Offline Russel

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Re: Phobos and Deimos on the return to Earth
« Reply #42 on: 03/30/2017 01:10 AM »
Where do you end up if you miss catching the Deimos tether
« Last Edit: 03/30/2017 01:11 AM by Russel »

Offline Hop_David

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Re: Phobos and Deimos on the return to Earth
« Reply #43 on: 03/30/2017 10:03 PM »
Where do you end up if you miss catching the Deimos tether

In some ways the problem is like landing a passenger jet on an runway. For a soft landing you need the vehicle and runway at the same place and time with close to the same velocity vectors.

But it differs in some ways. The approach to a Deimos or Phobos tether would be more gradual than a jet’s approach to a runway. There would be time to adjust the trajectory if the approach is too fast or too slow.

Also the acceleration is different. On earth’s surface, acceleration is 9.8 m/s^2. At the top of 937 km Phobos tether the acceleration is .14 m/s^2. At the bottom of a Deimos tether the acceleration is about .03 m/s^2.

The milder the acceleration, the more forgiving the catch. It’d be like doing an earth catch in slow motion.

At the Phobos tether, the catch would be about 70 times as slow as an earth catch. At the Deimos tether it’d be about 290 times as slow as an earth catch.

Offline Russel

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Re: Phobos and Deimos on the return to Earth
« Reply #44 on: 03/31/2017 01:12 AM »
Don't you also have phasing issues? How often are both Phobos and Deimos in the right points in their orbits to do this?

Offline Hop_David

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Re: Phobos and Deimos on the return to Earth
« Reply #45 on: 04/04/2017 12:51 AM »
Don't you also have phasing issues? How often are both Phobos and Deimos in the right points in their orbits to do this?

Launch windows between Phobos and Deimos tethers would open each 10.25 hours or so. Trip time between moon tethers is about 9 hours.
« Last Edit: 04/04/2017 12:54 AM by Hop_David »

Offline redliox

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Re: Phobos and Deimos on the return to Earth
« Reply #46 on: 04/04/2017 07:39 PM »
Doing a review breakdown on the most fuel-efficient routes from Mars...

So for Phobos, the end result is a delta-v of 7 km/s
1: Launch from Mars to Phobos Transfer Orbit: 4.3 km/s
2: Periapsis Raise: 0.6 km/s
3: Phobos Ops: 0.16 km/s
4: Direct Trans Earth Injection: 1.9 km/s

And for Deimos, including Oberth Effect, the end results is a delta-v of 7.25 km/s
1: Launch from Mars to Deimos Transfer Orbit: 4.7 km/s
2: Periapsis Raise: 0.7 km/s
3: Deimos Ops: 0.16 km/s
4: Periapsis Brake:  0.64 km/s
5: Trans Earth Injection: 1.05 km/s

(Hop_David are these figures accurate including for Oberth?)

There could be several different options for a Mars-Deimos/Phobos-Earth route:
1) Single vehicle (Mars Direct [tweaked to include moon visit])
2) Two vehicles: MAV+Orbital ERV (Mars Semi-Direct route with ERV doing moon visit])
3) Two vehicles: Primary Vehicle+Moon Vehicle
4) Three vehicles: Options 2 or 3 +booster stage

I believe the easiest (or perhaps most efficient being correct wording) would be options 1 and 3.  The fuel would come chiefly from Mars and have the option of returning directly to Earth if the moon visit is ruled out.  In option 3 the Moon Vehicle could specialize in rendezvous plus remove the ~160 m/s budget from the Primary's fuel.  Option 4 is probably the best for safety since you would have the TEI waiting w/o Martian ISRU; the main downside would depend on what kind of booster you use; current solids or hypergolics (and their available engines) wouldn't have enough kick to meet much more than ~500 m/s even with a large stage and the boil-off from liquid hydrogen is concerning (methalox I'll give a 'maybe' to).
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