Author Topic: Propellant Depots - General Discussion  (Read 398737 times)

Offline Robotbeat

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Re: Propellant Depots - General Discussion
« Reply #680 on: 04/26/2010 06:42 pm »
I would definitely leave the lander in LMO. Otherwise, you have to bring it through 3km/s Plus whatever extra velocity you use to shorten delta-t for the crew plus at least another 3km/s just to put it back in LMO for the next time. You're likely looking at at least 10km/s for that piece of hardware (although not all at the same time, which makes things far, far easier). If you leave it in LMO, you can use it for the next mission or for backup.

EDIT:And even if you brought it back to EML1/2, it's still at a Lagrange point and we only have infrastructure in LEO (besides, the radiation picture is probably actually worse at EML2 than at LMO). If you're reusing the lander but have to refurbish it extensively before reuse, it may well be cheaper just to launch a new one. Otherwise, you could refurbish it at LMO, possibly. Reuse without refurbish is doubly good because you don't have to haul it anywhere and you don't need to fund a refurbish mission.
« Last Edit: 04/26/2010 06:59 pm by Robotbeat »
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Offline mmeijeri

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Re: Propellant Depots - General Discussion
« Reply #681 on: 04/26/2010 07:01 pm »
OK, that's another option. I was originally thinking of a journey like this:

ground -> ISS on a commercial crew taxi
ISS to L1/L2 by Orion + EELV upper stage
L1/L2 to SEL-2 by lander + Orion
SEL-2 -> SML-1 by lander + Orion + MTV
SML-1 -> LMO/Phobos by lander (+Orion?)
LMO -> surface of Mars in just the lander

On the way back you'd do that roughly in reverse, only you wouldn't return the Orion to LEO but to the Earth itself.

NSF poster vanilla pointed out that the scheduling flexibility of L1/L2 would probably be better than an extra stop at SEL-2. In that case you wouldn't have a two week journey from L1/L2 to SEL-2 which is what I wanted the lander for. Two weeks in a capsule in a nominal case seemed a bit much. If you skip SEL-2, you would still need a stage for the TMI, and I thought it would be useful to have the lander double as that stage. But you could also use something like the crasher stage we were discussing in another thread. Leaving the lander in LMO would be more mass-efficient, bringing it along could be safer, although it may not give you all that much more safety.
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Offline mmeijeri

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Re: Propellant Depots - General Discussion
« Reply #682 on: 04/26/2010 07:12 pm »
EDIT:And even if you brought it back to EML1/2, it's still at a Lagrange point and we only have infrastructure in LEO (besides, the radiation picture is probably actually worse at EML2 than at LMO). If you're reusing the lander but have to refurbish it extensively before reuse, it may well be cheaper just to launch a new one. Otherwise, you could refurbish it at LMO, possibly. Reuse without refurbish is doubly good because you don't have to haul it anywhere and you don't need to fund a refurbish mission.

Well, the MTV had better be good enough to spend a long time in plus it would only be man-tended once back at a Lagrange point. But I'm not looking at saving costs, I expect that to be a wash, at least as long as launch costs are still high. I'd be looking to transform the nature of the cost structure.

I'd like to go from

fixed costs1 + IMLEO1 * cost/kg + expendable hardware

to

fixed costs2 + IMLEO2 * cost/kg

As long as total costs were similar I'd choose the reusable variant as it seems more likely to get into a virtuous cycle of ever more propellant launches and ever lower launch costs. I believe that's a strategically important consideration. Of course as launch costs go down, the case for reuse actually strengthens. If for whatever reason we couldn't have that, an expendable design could also be fine, but I'd rather not give up on it easily.
« Last Edit: 04/26/2010 07:19 pm by mmeijeri »
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Offline Robotbeat

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Re: Propellant Depots - General Discussion
« Reply #683 on: 04/26/2010 07:41 pm »
To be honest, I don't see what the Sun-Earth Lagrange points get you from a HSF perspective. The Earth-Moon ones are just about as good, far easier to get to, and far closer to Earth in case Something Bad Happens, besides fitting in almost perfectly with Moon missions.

Sun-Mars Lagrange points might be different, I don't know.
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Offline mmeijeri

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Re: Propellant Depots - General Discussion
« Reply #684 on: 04/26/2010 07:49 pm »
According to Farquhar it saves you more delta-v (MTV only has to move between SEL-2 and SML-1, perigee lowering burn is cheaper, moon flyby is supposedly easier), but I suspect vanilla's argument is more important.

For prepositioning propellant the Earth and Mars Lagrange points could still be relevant. As long as we don't have van Allen crossing tugs you'd have to do the first leg by chemical propulsion and in that case moon and Earth Lagrange points are equally expensive, but the Earth Lagrange points are "closer to Mars" and basing your tugs there could be cheaper. But all that would be unmanned. Apart from a "first" and the occasional servicing mission there probably isn't much point in frequent manned missions to the Earth Lagrange points, unless Farquhar's argument turns out to be weightier than vanilla's.
« Last Edit: 04/26/2010 07:50 pm by mmeijeri »
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Offline Robotbeat

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Re: Propellant Depots - General Discussion
« Reply #685 on: 04/26/2010 07:56 pm »
If only, if only, there's water (or oil) on Phobos/Deimos...
« Last Edit: 04/26/2010 08:01 pm by Robotbeat »
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Offline mmeijeri

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Re: Propellant Depots - General Discussion
« Reply #686 on: 04/26/2010 08:10 pm »
Isn't the presence of CHON on Phobos and Deimos a given? It might not be, but that's what I vaguely recall. But it could be a while before we were able to extract those. Brute force oxygen from regolith with nuclear power might be a good first step.

And Ceres is a colossal reservoir of water. Imagine bringing propellant from the edge of the inner solar system back inwards to Phobos... That of course lies far in the future.
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Offline Robotbeat

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Re: Propellant Depots - General Discussion
« Reply #687 on: 04/26/2010 08:21 pm »
Isn't the presence of CHON on Phobos and Deimos a given? It might not be, but that's what I vaguely recall. But it could be a while before we were able to extract those. Brute force oxygen from regolith with nuclear power might be a good first step.

And Ceres is a colossal reservoir of water. Imagine bringing propellant from the edge of the inner solar system back inwards to Phobos... That of course lies far in the future.
Off-topic a little bit, but man, I can't wait until Dawn gets to Ceres! There could be practically anything there... ANYTHING! And probably with an ocean to explore far closer than Jupiter or Saturn (and without the radiation of Jupiter), I'm quite excited!

We need a standardized SEP bus for sending prop depots and landers to (and from!) places like these. That would be a good "game-changing" technology.
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Offline MP99

Re: Propellant Depots - General Discussion
« Reply #688 on: 04/26/2010 08:56 pm »
AIUI, the CEV is carried passively until it undocks from the MTV and performs Earth re-entry. Therefore, the SM is discarded before CEV docks with the MTV.

That has never been the plan. SM stays on the CM until after MTV undocking, both to protect the TPS and to provide maneuvering capability post-undocking. The plan you describe would require the MTV to be on a entry trajectory prior to CM undocking, then the MTV would have to maneuver to avoid re-entry.

I went back and checked DRA 5.0 again, and you're quite right.

I've found the source of my confusion:-

Quote
It was not within the scope of the DRA 5.0 activity to recommend specific design upgrades for the Orion vehicle or to develop an upgrade strategy. Instead, a mass estimate of 10 t was used for the vehicle CM to size propulsion stages. An additional 4 t was book-kept for a service module that may be needed to perform an Earth-targeting burn.
(My highlight of the "may be" text that I'd ignored).

Note that the MTV mass budget only accounts for the Orion (10.6mT incl crew) - there is no accommodation for the SM. I had assumed that the internal CEV propulsion performed re-entry positioning, but not if 300m/s is required !

And yes, I had wondered how the heatshield would be protected during the transits & Mars orbit.



http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090012109_2009010520.pdf

cheers, Martin

Offline Robotbeat

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Re: Propellant Depots - General Discussion
« Reply #689 on: 04/26/2010 09:02 pm »
...
I haven't looked into the plane changes enough to make a reasonable estimate. The difference in inclination is small enough (would lead to ~500m/s delta-v IIRC), but it's the RAAN I know nothing about just yet. I have a link to a table of perturbed orbital elements for the planets somewhere, but I haven't got round to it yet.
...

BTW, the Dawn mission used a Mars flyby for the necessary plane change to visit Vesta:
http://dawn.jpl.nasa.gov/mission/popups/faq_qa_M4.asp
http://dawn.jpl.nasa.gov/mission/faqs.asp
It saved about 2.3km/s for the plane change, 2.6km/s total assist.

Would Phobos and Deimos be any use at all for gravity assists or the Oberth effect?
« Last Edit: 04/26/2010 09:03 pm by Robotbeat »
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Offline clongton

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Re: Propellant Depots - General Discussion
« Reply #690 on: 04/26/2010 11:31 pm »
Why not have it brake into L1/L2/SEL-2 propulsively?

My personal thoughts are for a SEP or NEP powered mission departing from and returning to EML-2, under power all the way, both ways. By the time the MTV approaches EML-2 on the return trip, propulsively breaking most of the way in, enough delta-V will have been shed to make capture no more dramatic than a docking event at ISS.
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Offline Robotbeat

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Re: Propellant Depots - General Discussion
« Reply #691 on: 04/26/2010 11:44 pm »
Why not have it brake into L1/L2/SEL-2 propulsively?

My personal thoughts are for a SEP or NEP powered mission departing from and returning to EML-2, under power all the way, both ways. By the time the MTV approaches EML-2 on the return trip, propulsively breaking most of the way in, enough delta-V will have been shed to make capture no more dramatic than a docking event at ISS.
However, that increases the time the crew spends on the way back versus just accelerating and having the crew leave in a capsule and the MTV enter a solar orbit.

However, with SEP, delta-v is pretty cheap, and this sort of thing can be afforded (especially since now you are close to the Sun and have twice as much power available as when you are at Mars).

I would really like more work on electric propulsion and solar power, as well (seems to me that nuclear power can't beat it in the inner solar system, since a solar array could eventually be made as thin as a solar sail, while solar power also has a definite edge when it comes to near-term solutions in the inner solar system). The specific power of the ion engine and power conditioning equipment becomes quite important, as well (the DS-1's total system not counting the Xenon tanks and solar arrays had a specific power of about 71W/kg... Xenon tanks had an impressively low mass that was less than 10% the mass of the Xenon propellant stored... the power conditioning system weighed more than the engine itself).
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Offline A_M_Swallow

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Re: Propellant Depots - General Discussion
« Reply #692 on: 04/27/2010 01:51 am »
I suspect that Van Allen crossing SEP counts as a new (low TRL) technology.  Fortunately it is scalable.  If the mini SEP tug masses less than 1 mT it can be launched on a Falcon 1e (or rival) for $10.9 million.  If the full size SEP needs an EELV at $200 million then:

Launch costs for 3 test missions and the real launch are 3 * $10.9M + $200M
= $232.7 million
Development costs and part will be extra.

The tug could use a VASIMR or hall effect thruster.

Offline clongton

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Re: Propellant Depots - General Discussion
« Reply #693 on: 04/27/2010 12:13 pm »
I would not either advocate NEP or SEP crew between LEO and EML-2 - too slow.
Crew would need to be Rapid Transit.
But all the supplies, indeed even the MTV, could be prositioned at EML-2 via electric propulsion once the propellant tanks were filled at the depot.
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Offline mmeijeri

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Re: Propellant Depots - General Discussion
« Reply #694 on: 04/30/2010 02:38 pm »
Continuing in a more appropriate thread (I think):

That does not apply to propellant supply flights and you know it

It does apply.  A rendezvous and a transfer is a rendezvous and a transfer, no matter what the cargo.

If you lose a propellant flight it's not a big deal. If you lose a crew or a big and expensive EDS or a lander, it is. The 1-(1-p)^n argument only applies if no individual flight is allowed to fail, but propellants flights are expendable. With Constellation, DIRECT or an EELV-based architecture n=3, for Apollo n=2 and if they had gone with direct ascent with a Nova überbooster they would have had n=1.

Note that n>=4 if you used libs0n's idea of using single stick EELV-class launchers without propellant transfer. Still an interesting reference point and libs0n wasn't arguing against propellant transfer, just detailing what could be done without it.
« Last Edit: 04/30/2010 02:44 pm by mmeijeri »
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Offline sdsds

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Re: Propellant Depots - General Discussion
« Reply #695 on: 05/01/2010 06:21 pm »
Has there been any analysis of crew-tended depots?  I ask because of the concern about the low level of confidence in uncrewed rendezvous and docking operations.

Consider an architecture where a crew is launched to the depot, inhabits a small (maybe Harmony-sized) node, and awaits the arrival of propellant transfer vehicles from any number of suppliers -- as many as can get their vehicle to the depot during the crew's window of availability.  In a two-week stay, a crew could oversee half a dozen dockings, transfers and undockings.

I'm imagining the habitable node's power needs could be supported by small solar arrays and large battery arrays, since it might be inhabited only 10% of the time.

Of course similarly "tended" operations could be used for on-orbit exploration vehicle assembly too....
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Offline mmeijeri

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Re: Propellant Depots - General Discussion
« Reply #696 on: 05/01/2010 06:50 pm »
One consideration that has been mentioned is explosion risk. How easy it would be to shield against that? Wouldn't the shrapnel be launched at low velocities compared to orbital velocities? If so it might help to enclose a depot in an unpressurised inflatable shelter capable of absorbing most of the shrapnel.
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Offline A_M_Swallow

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Re: Propellant Depots - General Discussion
« Reply #697 on: 05/01/2010 11:11 pm »
One consideration that has been mentioned is explosion risk. How easy it would be to shield against that? Wouldn't the shrapnel be launched at low velocities compared to orbital velocities? If so it might help to enclose a depot in an unpressurised inflatable shelter capable of absorbing most of the shrapnel.

If the depot is connected to the control cabin by a tether then most of the shielding could be an armoured disk on the tether.

The operators could pilot the docking tug, possibly by remote control.  I assume that the same tug can be used for both propellant deliveries and customer rockets.

Offline pathfinder_01

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Re: Propellant Depots - General Discussion
« Reply #698 on: 05/01/2010 11:17 pm »
Has there been any analysis of crew-tended depots?  I ask because of the concern about the low level of confidence in uncrewed rendezvous and docking operations.

Consider an architecture where a crew is launched to the depot, inhabits a small (maybe Harmony-sized) node, and awaits the arrival of propellant transfer vehicles from any number of suppliers -- as many as can get their vehicle to the depot during the crew's window of availability.  In a two-week stay, a crew could oversee half a dozen dockings, transfers and undockings.

I'm imagining the habitable node's power needs could be supported by small solar arrays and large battery arrays, since it might be inhabited only 10% of the time.

Of course similarly "tended" operations could be used for on-orbit exploration vehicle assembly too....


I think one of the useful prerequisites to propellant depots will be automated docking. Having crew will increase expenses and expose him to the danger of both explosion and leak. It might be fine for an exploration craft to dock, but having someone on hand constantly could be a problem.  Having automated ones means you could set up a depot anywhere in the solar system and still have it be useful. Needing it manned limits you to LEO for the for seeable future.

Offline sdsds

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Re: Propellant Depots - General Discussion
« Reply #699 on: 05/02/2010 08:25 pm »
I think one of the useful prerequisites to propellant depots will be automated docking.

I think one of the prerequisites to useful propellant depots will be automated docking.  Is that what you meant?

A "useful propellant depot" in that sense is one which makes missions headed beyond LEO cost less than similar mission designs using heavy-lift or rendezvous (no prop transfer) architectures.

You are right that crew-tended operation would increase costs and create the possibility of missions failing with loss of crew.  What then are the advantages?

First, the "technology readiness level" of automated docking and propellant transfer is low.  (Is "Orbital Express" the only mission where this has been demonstrated?)  Somehow this capability needs to be demonstrated on a larger scale if an automated depot architecture is to gain much traction.  I would love to see a robotic mission "top off" its cryogenic upper stage in LEO before e.g. heading off to land on the Moon.  ULA would dearly love to demonstrate that, too! (http://www.ulalaunch.com/docs/publications/SettledCryogenicPropellantTransfer.pdf)

But here's the thing:  for whatever reason there's funding for crewed missions even when there isn't funding available for robotic missions.  I'm suggesting using NASA human spaceflight funding to support "flexible path" crewed missions to propellant depots.

Quote
Needing it manned limits you to LEO for the for seeable future.

Why do you feel a crewed mission to a depot at an Earth-Moon Lagrange point is beyond the foreseeable future?  I think a crew could get there and back with a dual-EELV launch architecture.
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