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#180
by
alexterrell
on 16 May, 2011 15:48
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Reading DRM5 and similar, I see that as in Apollo only thing that returns to Earth is the crew capsule. Six or seven monstrous Ares V launches; over thousand tons IMLEO and you recover and reuse nothing. Ok the hab stays on Mars; if you want your eventual second mission to go to very same place the first one did.
How much would departure vehicle mass IMLEO grow if you wanted to
return entire vehicle to say L1?
I understand there is very extreme and very widespread hatred for anything that can be mocked with "Battlestar" tag; but is it really economic to build thousand ton vehicles that cost tens of billions and then throw away every single part of them during one mission.
It should be possible with aerocapture, but I'm not sure if this has ever been done - though it should work.
My Mars reference plan (and I'm beyond version 5), now has SEP tugs carrying everything to L1, then depart from L1 (chemical) before aerocapture into Mars orbit. Then depart from Mars orbit and Aerocapture in Earth Orbit with rendez-vous again at L1.
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#181
by
Robotbeat
on 18 May, 2011 17:10
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I suppose the empty Mars Transfer Vehicle (in a heliocentric orbit) could be captured by a SEP tug and brought back to EML1. You'd probably want to launch the SEP tug on a rendezvous trajectory before the crew even left the MTV (perhaps even before the crew left Mars).
For a MTV of ~100 tons and with ~4km/s to get it from its heliocentric trajectory to EML1, you'd need a SEP tug with a dry mass of ~10 tons, ~20 tons of fuel, given state-of-the-art solar and electric propulsion technology (and about half a year of thrusting with a 500kW solar array). Probably just as doable with a larger delta-v, it'd just take longer (it's going to take a while for the MTV to circle back to Earth, anyways).
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#182
by
hyper_snyper
on 03 Jun, 2011 21:48
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I've been thinking about Mars mission architectures recently.
If you launch a bunch of cargo landers to Mars how hard would it be to get them to land relatively close to one another (say < 5 km). So astronauts on the surface can get to them. Assume heat shield, parachute, rocket propulsion for entry, descent, and landing.
What kind of navigation system would this entail? INS, of course but what else? Something like a homing beacon (akin to ILS in aviation) maybe?
Is this an open problem? If not, I'd like to read up on what ideas people have about doing precision landings on another world with no nav aids.
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#183
by
JayP
on 04 Jun, 2011 02:40
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I've been thinking about Mars mission architectures recently.
If you launch a bunch of cargo landers to Mars how hard would it be to get them to land relatively close to one another (say < 5 km). So astronauts on the surface can get to them. Assume heat shield, parachute, rocket propulsion for entry, descent, and landing.
What kind of navigation system would this entail? INS, of course but what else? Something like a homing beacon (akin to ILS in aviation) maybe?
Is this an open problem? If not, I'd like to read up on what ideas people have about doing precision landings on another world with no nav aids.
Have enough delta V to place the lander in orbit before PDI instead of a direct entry fromthe transfer orbit and use a lifting reentry like the MSL will. You could also land a pathfinder payload with a homing beacon first and use that as a nav aid.
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#184
by
Jackspace
on 12 Mar, 2012 00:43
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Would the Space X Dragon be a good mars lander, and only a lander.
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#185
by
Dalhousie
on 12 Mar, 2012 04:28
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I ment telephone.
Polar explorers managed without telephones until quite recently, and often spent years away from their families. Submariners manage without telephones, even today.
People on a Mars mission will have email, video messages, voice messages, and probably something like twitter and SMS. Compared with previous explorers they will live in luxury.
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#186
by
kkattula
on 12 Mar, 2012 06:13
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... If not, I'd like to read up on what ideas people have about doing precision landings on another world with no nav aids.
I suggest landing a rover first, preferably tele-operated from orbit (Phobos?), that would survey a landing site, place several radio beacons around the site to allow precision landing, and even clear small rocks.
Give the rover a bulldozer attachment (or land a separate ROV bulldozer) and clear a nearby base area. Give the landers wheels, and the rover could tow each one to the base area. Maybe even deploy some equipment, hook up comms & power cables etc.
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#187
by
baldusi
on 31 Jul, 2012 20:43
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How can I calculate the pressure of the Marsian wind at a given altitude? I'm wondering what sort of strain it would put on a vertical solar panel, and if it would work as a sail. I've seen that most if not all solar powered devices on Mars have used mostly horizontal solar panels. So I'm wondering if wind pressure is an issue, or is simply avoiding to actually point the panel in the Sun's direction.
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#188
by
su_liam
on 22 Aug, 2012 22:18
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How many cycler stations in Hohmann orbits would be required to allow for reliable available at all windows(every 780 days, I think)?
Would it be reasonable to put up cycler stations on more eccentric/faster orbits, such that one would be able to facilitate Earth/Mars transit on similarly or more frequent time windows?
Thanks.
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#189
by
Hop_David
on 28 Aug, 2012 14:17
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How many cycler stations in Hohmann orbits would be required to allow for reliable available at all windows(every 780 days, I think)?
Would it be reasonable to put up cycler stations on more eccentric/faster orbits, such that one would be able to facilitate Earth/Mars transit on similarly or more frequent time windows?
Thanks.
Depends on what cycler you use. There's an Aldrin cycler that flies by earth and Mars each synodic period (roughly 2 1/7 years). But the line of apsides must be substantially rotated each orbit. And it zooms by Mars orbit at a pretty good angle, so the taxis moving between Mars and the cycler would have a steep delta V budget. So in terms of delta V, this cycler's not so good.
More Hohmann like are the Niehoff VISIT 1 and VISIT 2 cyclers. Their periods are 1.25 years and 1.5 years. (Period of an Earth Mars Hohmann ellipse is about 1.4 years). Less delta V for orbit maintenance as well as less for taxi rendezvous. Planetary fly bys more infrequent, though.
With a synodic period of about 2 1/7 years, you can see 7 synodic periods are about 15 years. But 7 periods isn't exactly 15 years, more like 14.95 years, so constant tweaking would be needed.
Venus is much more amenable to cyclers. Earth Venus synodic period is 1.5987 years, quite close to 1 3/5. 5 synodic periods is 7.993 years, very close to an 8 year cycle. What's more is an earth Venus Hohmann ellipse has a period of .7998 years, very close to 4/5 of a year. This makes for a
5 pointed star where the cyclers are traveling nearly Hohmann orbits.
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#190
by
clongton
on 29 Aug, 2012 00:21
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Cyclers would be a great way to supply a base. The transfer points could be EML-2 and Phobos for Mars and EML-2 and HVO for Venus.
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#191
by
guckyfan
on 11 Oct, 2012 14:07
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Cyclers would be a great way to supply a base. The transfer points could be EML-2 and Phobos for Mars and EML-2 and HVO for Venus.
Actually they are not at all suited for cargo. They don't give any delta-v for free. Getting something to the cycler and from there to the destination will require more delta-v than flying direct. It is worth it only if you save on mass which you cannot do with cargo.
What they do is give astronauts a habitat for the transfer so you save the mass of the habitat. Even that may not be enough to justify a cycler unless it has equipment for water and air recycling and maybe even food production which would save a lot of mass in supplys to launch with the astronauts.
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#192
by
guckyfan
on 11 Oct, 2012 14:33
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I am trying to understand different possible mission profiles.
Frequently missions are proposed using L-points as staging points for missions to Mars. It seemed plausible to me. But then I found out about the Oberth-Effect and found some interesting delta-v charts for missions to the Moon, to Mars, and to Venus.
So getting to any L-point requires as much delta-v as direct transfer to Mars on a Homann-Trajectory. It looks to me as if using L-points for staging missions could be worth it only if using both SEP-tugs from LEO to the L-point and lunar fuel assuming quite low cost for both. And even then the advantage would not be very big compared to launching directly from LEO.
Am I missing something here?
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#193
by
mmeijeri
on 11 Oct, 2012 15:57
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I am trying to understand different possible mission profiles.
Frequently missions are proposed using L-points as staging points for missions to Mars. It seemed plausible to me. But then I found out about the Oberth-Effect and found some interesting delta-v charts for missions to the Moon, to Mars, and to Venus.
You can still use the Oberth effect together with Lagrange points. The general idea is to drop from a Lagrange point to LEO altitude (perhaps with a lunar flyby) and then do a powered flyby to Mars.
So getting to any L-point requires as much delta-v as direct transfer to Mars on a Homann-Trajectory.
Considerably less than Mars, 3.2km/s - 3.8km/s depending on how long your trip is allowed to be. In addition, since EML1/2 is close to Earth you can use it as an assembly point, which means you can launch individual modules that will fit on a Centaur or DCSS instead of needing a huge EDS. You couldn't do that in Mars orbit, because the trip to Mars orbit takes far too long for that to be practical.
It looks to me as if using L-points for staging missions could be worth it only if using both SEP-tugs from LEO to the L-point and lunar fuel assuming quite low cost for both. And even then the advantage would not be very big compared to launching directly from LEO.
SEP and ISRU would add to the utility of Lagrange points, which is large to begin with.
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#194
by
guckyfan
on 11 Oct, 2012 16:18
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You can still use the Oberth effect together with Lagrange points. The general idea is to drop from a Lagrange point to LEO altitude (perhaps with a lunar flyby) and then do a powered flyby to Mars.
That would involve two more passes of the VanAllen Belt. I don't think that would be a good idea especially for manned flights. Also it would require a lot of delta-v eating up much of the Oberth-Effect advantage. I can't really calulate that though so I cannot be positive on this.
So getting to any L-point requires as much delta-v as direct transfer to Mars on a Homann-Trajectory.
Considerably less than Mars, 3.2km/s - 3.8km/s depending on how long your trip is allowed to be. In addition, since EML1/2 is close to Earth you can use it as an assenbly point, which means you can launch individual modules that will fit on a Centaur or DCSS instead of needing a huge EDS. You couldn't do that in Mars orbit, because the trip to Mars orbit takes far too long for that to be practical.
The two charts I have seen both showed more delta-v for L-points than for a Homann-trajectory to Mars. Even a geostationary Orbit needs more.
You can assemble the modules in LEO which may be a good idea anyway even if you then lift them with SEP tugs afterwards.
Edited to correct quote nesting
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#195
by
mmeijeri
on 11 Oct, 2012 16:24
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That would involve two more passes of the VanAllen Belt. I don't think that would be a good idea especially for manned flights.
That's not a problem in an MTV, which needs good radiation shielding anyway.
Also it would require a lot of delta-v eating up much of the Oberth-Effect advantage. I can't really calulate that though so I cannot be positive on this.
It wouldn't. Traveling between EML1/2 and high Mars orbit would be on the order of 2.5km/s, which is the same as to the lunar surface. This cuts down enormously on the required size of a transfer stage, as well as on the thrust and Isp requirements.
The two charts I have seen both showed more delta-v for L-points than for a Homann-trajectory to Mars. Even a geostationary Orbit needs more.
You'd need slightly more delta-v, but much smaller launch vehicles and transfer stages, and you could greatly increase effective Isp.
You can assemble the modules in LEO which may be a good idea anyway even if you then lift them with SEP tugs afterwards.
You can, but that's more difficult technologically. And even if you did, there would still be many advantages to using Lagrange point rendez-vous in addition to LEO rendez-vous.
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#196
by
guckyfan
on 11 Oct, 2012 16:57
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That would involve two more passes of the VanAllen Belt. I don't think that would be a good idea especially for manned flights.
That's not a problem in an MTV, which needs good radiation shielding anyway.
The radiation shielding cannot be that good. The radiation in the belt is a lot higher than outside.
Also it would require a lot of delta-v eating up much of the Oberth-Effect advantage. I can't really calulate that though so I cannot be positive on this.
It wouldn't. Traveling between EML1/2 and high Mars orbit would be on the order of 2.5km/s, which is the same as to the lunar surface. This cuts down enormously on the required size of a transfer stage, as well as on the thrust and Isp requirements.
How does increased requirement of acceleration translate into enormously reduced transfer stage?
But you have one point. If you can get very cheap fuel from Moon the mass from LEO becomes less. However the requirement of delta-v from L-points is at least half that from LEO so this becomes an advantage only if fuel from the moon to L-point is much cheaper than from Earth to LEO. I don't see that any time soon.
Edit: OK if you assume SEP-Tug to L-Point the transfer stage can be smaller.
You can assemble the modules in LEO which may be a good idea anyway even if you then lift them with SEP tugs afterwards.
You can, but that's more difficult technologically. And even if you did, there would still be many advantages to using Lagrange point rendez-vous in addition to LEO rendez-vous.
How is assembly in LEO more difficult than in an L-point?
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#197
by
mmeijeri
on 11 Oct, 2012 17:23
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The radiation shielding cannot be that good. The radiation in the belt is a lot higher than outside.
It has to be that good, because it has to shield the crew against solar particle events. The idea is to have a storm shelter where the crew can sit things out for a couple of hours to days. It would be perfect for crossing the van Allens too.
How does increased requirement of acceleration translate into enormously reduced transfer stage?
By the
rocket equation propellant usage is an exponential function of delta-v.
But you have one point.
I have many, not invented by me but by experts in the field.
If you can get very cheap fuel from Moon the mass from LEO becomes less. However the requirement of delta-v from L-points is at least half that from LEO so this becomes an advantage only if fuel from the moon to L-point is much cheaper than from Earth to LEO. I don't see that any time soon.
Your reasoning doesn't make sense to me.
Edit: OK if you assume SEP-Tug to L-Point the transfer stage can be smaller.
Not just if you use a SEP tug. There are two things that can significantly affect the size of the transfer stage: transporting modules individually instead of together and much lower delta-v.
How is assembly in LEO more difficult than in an L-point?
Assembly isn't more difficult in LEO, it may actually be easier, though not by much. Transporting large payloads to Mars orbit or even just L1/L2 by SEP is much more challenging than transporting individual modules by chemical propulsion. You'd need very large SEP tugs.
You seem to be trying to prove Lagrange points are a bad idea instead of trying to learn why they might be a good idea. Why is that?
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#198
by
guckyfan
on 11 Oct, 2012 18:16
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How does increased requirement of acceleration translate into enormously reduced transfer stage?
By the rocket equation propellant usage is an exponential function of delta-v.
So the rocket equation tells you you need less propellant for more acceleration? Somehow I don't believe that.
If you can get very cheap fuel from Moon the mass from LEO becomes less. However the requirement of delta-v from L-points is at least half that from LEO so this becomes an advantage only if fuel from the moon to L-point is much cheaper than from Earth to LEO. I don't see that any time soon.
Your reasoning doesn't make sense to me.
And yours does not to me, see the Rocket equation.
Edit: OK if you assume SEP-Tug to L-Point the transfer stage can be smaller.
Not just if you use a SEP tug. There are two things that can significantly affect the size of the transfer stage: transporting modules individually instead of together and much lower delta-v.
Again your reasoning does not make any sense to me at all. But maybe that is because we are talking about different things. I assumed you were talking about the transferstage that sends the completed stack off to Mars. I read your latest statement however as you are talking about the SEP-tugs that can be smaller if they shift the modules individually.
How is assembly in LEO more difficult than in an L-point?
Assembly isn't more difficult in LEO, it may actually be easier, though not by much.
But you said so in your last post.
Transporting large payloads to Mars orbit or even just L1/L2 by SEP is much more challenging than transporting individual modules by chemical propulsion. You'd need very large SEP tugs.
Agree, I was only talking about SEP-tug LEO L1/L2, to make that point clear. Using chemical propulsion for that part however would get you nothing. For the same or very similar delta-v you can get directly to Mars so why bother with L1/L2.
The delta-v maps I am using give me 3.6km/s from LEO to Hohmann transfer.
They give me 3.8 to L1, that is actually more.
Are they so extremely off?
You seem to be trying to prove Lagrange points are a bad idea instead of trying to learn why they might be a good idea. Why is that?
Not at all. I am trying hard to understand any advantage of L1/L2.
Maybe we should start all over from zero. We may have accumulated misunderstandings due to the nested quotes.
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#199
by
mmeijeri
on 11 Oct, 2012 18:57
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So the rocket equation tells you you need less propellant for more acceleration? Somehow I don't believe that.
No it doesn't tell you that. Let me try to explain it better than I did before. The point is that if you use various staging orbits along the way and refuel there, then you can effectively reset the rocket equation at each node. So instead of having to send everything through TMI and MOI from LEO, say 5.7 km/s, you only have to push individual modules from LEO to L1/L2, say 3.2 km/s and the whole assembly from L1/L2 to a Mars Lagrange point, roughly 2.5km/s. The individual hops are now much smaller, and you need a much smaller transfer stage.
Again your reasoning does not make any sense to me at all. But maybe that is because we are talking about different things. I assumed you were talking about the transferstage that sends the completed stack off to Mars.
I was, but with a Lagrange point you only have to move the completed stack through 2.5km/s instead of the 5.7 you might need for a direct appoach.
I read your latest statement however as you are talking about the SEP-tugs that can be smaller if they shift the modules individually.
Not just the SEP tugs, the same would be true for chemical transfer stages.
But you said so in your last post.
I didn't phrase it as clearly as I might have. I meant that moving the completed stack to L1/L2 with SEP would be much more technologically challenging. It would require enormous scaling up compared to the state of the art.
Agree, I was only talking about SEP-tug LEO L1/L2, to make that point clear. Using chemical propulsion for that part however would get you nothing. For the same or very similar delta-v you can get directly to Mars so why bother with L1/L2.
Many reasons, smaller size of the individual hops, easier phasing, longer launch windows, easier reuse of the MTV, easier to increase effective Isp by using different forms of propulsion for payload / segment combinations where that makes sense, better thermal environment. By contrast, why should we care about a small difference in total delta-v, one that is easily swamped by higher effective Isp?
The delta-v maps I am using give me 3.6km/s from LEO to Hohmann transfer.
They give me 3.8 to L1, that is actually more.
Are they so extremely off?
3.8km/s is correct for a fast transfer to L1. You might want to use that for crew.