Author Topic: Propellant depot strategy & tactics pow-wow  (Read 50426 times)

Online TOG

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Re: Propellant depot strategy & tactics pow-wow
« Reply #80 on: 08/12/2009 11:36 pm »
Quote from: TOG link=topic=18116.msg458099#msg458099
IIRC - If  you keep the propellant in a closed system (where the vapor is contained instead of released, and you have an exterior shield to prevent tank wall heating, (based on standard chemistry) won't our propellant reach an equilibrium point where the act of evaporation will keep the remainder of the propellant cool?  And at the same time, with the increase in pressure due to the "vapor active" fluids, won't there be a degree of condensation internal to the tank?

I don't see the need.  There are commercial providers with cryo experience (think instrument cooling- hint, hint) whom could build a closed loop system to return vapor to a tank in liquid form.  Once you get that, you get infinite loiter as long as your depot stays powered.

Quote
The big IFs here is the method we are using to prevent additional heat from being applied to the system and whether we can contain the vaporous propellant (to keep a "closed" system) until we start to transfer the propellant to the active spacecraft.

Again, no need. put the solar arrays on the sun side of the tankage, and the reprocessing facilities and radiators in the shadow.  The arrays (and possibly additional MLI or other insulation) help keep the tankage cool, and supply power for the system to cool propellant and  reprocess boiloff.

Jim- Not a bad direction, but to take it a step further, if, instead of standard solar panels you use heat exchange units (similar to a heat pump or more commonly called "air conditioner") to both power the unit and keep the propellant cool.  Two birds with existing technology - probably cheaper than sending the "normal" solar panels.

What do you think of that idea?
M's Laws of Aerodynamics:                                    On Physics Exam:
1) if you push anything hard enough it will fly          Q)The allegory of Schrödinger's cat shows what?
2) if you stop pushing it stops flying                        A)That Shrödinger was a sadistic cat hater

Offline gin455res

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Re: Propellant depot strategy & tactics pow-wow
« Reply #81 on: 08/13/2009 12:58 am »
I've seen  suggestions for a lox only depot, is there any rationale for an N2O4 only depot?

For example, what is the isp of N2O4/H2? if it is higher than hydrazine, then maybe it is possible to get the operational convenience of a storeable depot combined with a better isp, one that would make a storeable depot in leo worthwhile?

Offline gin455res

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Re: Propellant depot strategy & tactics pow-wow
« Reply #82 on: 08/13/2009 01:54 am »
Just tried to find isp  of n2O4/hydrogen using
http://rocketworkbench.sourceforge.net/equil.phtml
Used frozen flows,  chamber pressure 40bar. exit 0.1 bar
7 mols h2, 1 mol n2o4
- no idea if this is a sensible set-up but got output below
not sure if it is 363 or 378 that is the isp figure

Results

Propellant composition
Code  Name                                mol    Mass (g)  Composition
618   N2O4 (NTO NISC)                     1.0000 92.0111   2N  4O 
457   HYDROGEN (CRYOGENIC)                7.0000 14.1112   2H 
Density :  0.408 g/cm^3
3 different elements
N  O  H 
Total mass:  106.122241 g
Enthalpy  : -779.71 kJ/kg

29 possible gazeous species
2 possible condensed species

                       CHAMBER      THROAT        EXIT
Pressure (atm)   :      40.000      22.394       0.100
Temperature (K)  :    2941.315    2648.263     858.604
H (kJ/kg)        :    -779.714   -1804.553   -7386.048
U (kJ/kg)        :   -2640.413   -3479.865   -7929.208
G (kJ/kg)        :  -52013.330  -47933.613  -22341.737
S (kJ/(kg)(K)    :      17.419      17.419      17.419
M (g/mol)        :      13.143      13.143      13.143
(dLnV/dLnP)t     :    -1.00000    -1.00000    -1.00000
(dLnV/dLnT)p     :     1.00000     1.00000     1.00000
Cp (kJ/(kg)(K))  :     3.52882     3.46336     2.63710
Cv (kJ/(kg)(K))  :     2.89622     2.83076     2.00449
Cp/Cv            :     1.21843     1.22348     1.31560
Gamma            :     1.21843     1.22348     1.31560
Vson (m/s)       :  1483.35544  1431.67916   813.51089

Ae/At            :                 1.00000    28.59611
A/dotm (m/s/atm) :                52.25471  1494.28144
C* (m/s)         :              2090.18852  2090.18852
Cf               :                 0.68495     1.73904
Ivac (m/s)       :              2601.85206  3784.35164
Isp (m/s)        :              1431.67916  3634.92349
Isp/g (s)        :               145.99065   370.65904

Molar fractions

H                     1.2547e-02  1.2547e-02  1.2547e-02
HNO                   5.1643e-07  5.1643e-07  5.1643e-07
HNO2                  1.3835e-08  1.3835e-08  1.3835e-08
HO2                   4.5824e-07  4.5824e-07  4.5824e-07
H2                    3.6840e-01  3.6840e-01  3.6840e-01
H2O                   4.8960e-01  4.8960e-01  4.8960e-01
H2O2                  3.7989e-07  3.7989e-07  3.7989e-07
N                     5.2246e-07  5.2246e-07  5.2246e-07
NH                    1.0871e-06  1.0871e-06  1.0871e-06
NH2                   3.3825e-06  3.3825e-06  3.3825e-06
NH3                   2.2785e-05  2.2785e-05  2.2785e-05
NO                    2.9311e-04  2.9311e-04  2.9311e-04
NO2                   1.8924e-08  1.8924e-08  1.8924e-08
N2                    1.2369e-01  1.2369e-01  1.2369e-01
N2O                   4.1857e-08  4.1857e-08  4.1857e-08
O                     1.1054e-04  1.1054e-04  1.1054e-04
OH                    5.2720e-03  5.2720e-03  5.2720e-03
O2                    5.8312e-05  5.8312e-05  5.8312e-05




Offline jongoff

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Re: Propellant depot strategy & tactics pow-wow
« Reply #83 on: 08/13/2009 02:19 am »
One question I haven't seen asked.

If propellant depots are actually developed. Could they have commercial role? I mean, could ULA and/or Energia and Krunichev finally build and launch tugs that stay in space and transfer comsats and weatherbirds to GEO and other non LEO orbits?


Certainly.  And hopefully that will be/would be the goal.

I'd say it's a definitely maybe.  The technical capability of doing so would definitely be there.  It would more be a case of making the business numbers work.  I *think* there are some markets there, and possibly some big ones.  But only time will tell.

How was that for a complete non-answer?

~Jon

Offline lewis886

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Re: Propellant depot strategy & tactics pow-wow
« Reply #84 on: 08/13/2009 02:48 am »
I'm sorry if this is a stupid question, but i've been wondering this for a while.

When all of you are talking about propellant depots, how are you assuming that they operate?  Specifically,  do they have a large permanent tank(s) in which is stored the propellant that is brought up in smaller tanks and then transferred into the depot (as shown on the left part of my crude illustration below)

OR

Is a depot imagined as being only a central core with the plumbing/pumping/cooling features, into which the propellant delivery tanks are "plugged in", and then those tanks are discarded when empty? (as shown on the right side of my crude drawing)


I can see benefits and drawbacks to both ways.  First, the large tank takes much less plumbing/complexity inside the depot itself, plus the delivery tanks can be somewhat simpler. 

On the other hand, the plug-in tanks only have to last for a limited amount of time anyway, and you have to send the propellant up in something, so why transfer it until you need to transfer it into the EDS/spacecraft that needs it?  Just have a standard pump adapter on the top of all deliveries, plug them in to separate sockets, and transfer the propellant through the depot when a spacecraft docks at the specified fueling attachment.   Plus, you wouldn't have to worry about MMOD problems with the large permanent tank if you are always jettisoning and replacing these smaller tanks instead.

I'm really not sure.  As, I said, it could be a stupid question, but I'm still curious.
« Last Edit: 08/13/2009 04:23 am by lewis886 »

Offline mmeijeri

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Re: Propellant depot strategy & tactics pow-wow
« Reply #85 on: 08/13/2009 09:33 am »
I've seen  suggestions for a lox only depot, is there any rationale for an N2O4 only depot?

If that requires new engines then it makes the concept a lot less interesting. I think the main argument in favour of using a hypergolics depot for exploration is that you can do it safely and soon because it is a proven technology with a more than 30 years operational history.

A secondary argument is that it would be good for refueling reusable tugs. Those would be necessary for efficient resupply of both depots and space stations. An existing launch vehicle could drop off its cargo near the station or depot and the tug would pick it up and bring it to its destination. This saves you an expensive and heavy SM every time, which makes a lot of difference. There is a whole list of planned or existing vehicles that could serve as interim tugs (Orion, ATV, HTV, Cygnus, Dragon) and all use hypergolics. In the longer term you might see tugs with nontoxic RCS emerge.

Hypergolics would also be good for deep space missions because of reliable ignition, no need for turbopumps etc. Density would be good too if you're basing things on commercial launchers only, but methane also has good density and better Isp and better ISRU potential as well.

If you don't want hypergolics, you could also consider a kerosene depot with the EDS bringing its own LOX.
« Last Edit: 08/13/2009 09:51 am by mmeijeri »
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Offline mmeijeri

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Re: Propellant depot strategy & tactics pow-wow
« Reply #86 on: 08/13/2009 09:46 am »
On the shape of depots:

I'd be in favour of a universal jack of all trades vehicle. A fully reusable hypergolic lunar lander could contain 80-100mT of propellant and have a delta-v of 5-6 km/s. This is enough for a round trip L1-moon without refueling, a one way trip from LEO to L1 or back with sizeable cargo, a one way trip from EML1 to Mars L1 or back and Mars all propulsive landing or ascent.

This vehicle could be the basis of a depot, mini space station, cargo transfer stage for extremely valuable cargo from LEO to L1, a Mars transfer vehicle, a moon lander, an all propulsive Mars lander and makeshift surface hab. Propellant and uncrewed cargo could be prepositioned to L1 by cryogenic propulsion and efficient trajectories and from L1 to Mars by SEP tug, giving high effective Isp. It would be the Space Shuttle of the new exploration age and that would be a good thing.

Orion, this universal vehicle and EELV Phase 1 are all we need. The rest (cryogenic depots, SEP, aerobraking, ISRU etc maybe even HLV) would be valuable later additions but could be put on the technology development track, safely off the critical path. If you're willing to beef up future commercial crew capsules you could have them dock with your universal vehicle at L1 and such a smaller capsule would only require a modified Centaur, which can be launched fully fueled on existing EELVs. The Centaur would still need modifications, so you might as well go ahead with a new upper stage
« Last Edit: 08/13/2009 01:08 pm by mmeijeri »
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Offline grdja

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Re: Propellant depot strategy & tactics pow-wow
« Reply #87 on: 08/13/2009 04:24 pm »
I'd say it's a definitely maybe.  The technical capability of doing so would definitely be there.  It would more be a case of making the business numbers work.  I *think* there are some markets there, and possibly some big ones.  But only time will tell.

How was that for a complete non-answer?

~Jon

Thank you very much. "Definitely maybe" is much better than "wont ever work economically, only usable for big crewed spaceships", that I feared will hear.

Offline jongoff

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Re: Propellant depot strategy & tactics pow-wow
« Reply #88 on: 08/14/2009 03:04 am »
I'm sorry if this is a stupid question, but i've been wondering this for a while.

When all of you are talking about propellant depots, how are you assuming that they operate?  Specifically,  do they have a large permanent tank(s) in which is stored the propellant that is brought up in smaller tanks and then transferred into the depot (as shown on the left part of my crude illustration below)

OR

Is a depot imagined as being only a central core with the plumbing/pumping/cooling features, into which the propellant delivery tanks are "plugged in", and then those tanks are discarded when empty? (as shown on the right side of my crude drawing)


I can see benefits and drawbacks to both ways.  First, the large tank takes much less plumbing/complexity inside the depot itself, plus the delivery tanks can be somewhat simpler. 

On the other hand, the plug-in tanks only have to last for a limited amount of time anyway, and you have to send the propellant up in something, so why transfer it until you need to transfer it into the EDS/spacecraft that needs it?  Just have a standard pump adapter on the top of all deliveries, plug them in to separate sockets, and transfer the propellant through the depot when a spacecraft docks at the specified fueling attachment.   Plus, you wouldn't have to worry about MMOD problems with the large permanent tank if you are always jettisoning and replacing these smaller tanks instead.

I'm really not sure.  As, I said, it could be a stupid question, but I'm still curious.

Give me a month.  There's a slew of depot related papers that will be presented at Space 2009.  And several of them go into this question.

~Jon

Offline MP99

Re: Propellant depot strategy & tactics pow-wow
« Reply #89 on: 08/14/2009 07:19 am »
Another question...

For H2/O2 in a passively cooled depot, is the boiloff rate affected by the propellant load? IE is the boiloff rate the same regardless of whether the depot is 1% full or 99% full? First principles suggest there is an incoming heat load, therefore a fixed rate of boiloff is required to keep the depot cold.

I remember getting an answer about this at one point, but I can't remember the details.  I'll have to get back with you later.  Send me an email to remind me.

~Jon


Jon,

did you manage to find up the previous answer to this question?

Cheers, Martin

Offline Xplor

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Re: Propellant depot strategy & tactics pow-wow
« Reply #90 on: 08/22/2009 10:41 pm »
This is somewhat off topic but does anyone know why the ULA slides include the Scorpius Launch Vehicles? (center bottom page 3)

Seems a rather odd choice.

While Atlas and Delta are givens in a ULA slide it makes sense for the entire launch industry to participate in launching propellant.  Falcon is an obvious additional rocket.  So are the various foreign rockets.

Who else would you suggest?

Offline adamsmith

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Re: Propellant depot strategy & tactics pow-wow
« Reply #91 on: 08/24/2009 03:20 am »
On the shape of depots:

I'd be in favour of a universal jack of all trades vehicle. A fully reusable hypergolic lunar lander could contain 80-100mT of propellant and have a delta-v of 5-6 km/s. This is enough for a round trip L1-moon without refueling, a one way trip from LEO to L1 or back with sizeable cargo, a one way trip from EML1 to Mars L1 or back and Mars all propulsive landing or ascent.

This vehicle could be the basis of a depot, mini space station, cargo transfer stage for extremely valuable cargo from LEO to L1, a Mars transfer vehicle, a moon lander, an all propulsive Mars lander and makeshift surface hab. Propellant and uncrewed cargo could be prepositioned to L1 by cryogenic propulsion and efficient trajectories and from L1 to Mars by SEP tug, giving high effective Isp. It would be the Space Shuttle of the new exploration age and that would be a good thing.

Orion, this universal vehicle and EELV Phase 1 are all we need. The rest (cryogenic depots, SEP, aerobraking, ISRU etc maybe even HLV) would be valuable later additions but could be put on the technology development track, safely off the critical path. If you're willing to beef up future commercial crew capsules you could have them dock with your universal vehicle at L1 and such a smaller capsule would only require a modified Centaur, which can be launched fully fueled on existing EELVs. The Centaur would still need modifications, so you might as well go ahead with a new upper stage

Please see the following page as an advance Solar electric Mars system using Hypergolics that presumes a depot.


http://www.lpi.usra.edu/publications/reports/CB-1106/wash01.pdf

Stanley

Offline robertross

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Re: Propellant depot strategy & tactics pow-wow
« Reply #92 on: 08/24/2009 10:44 pm »
Another question...

For H2/O2 in a passively cooled depot, is the boiloff rate affected by the propellant load? IE is the boiloff rate the same regardless of whether the depot is 1% full or 99% full? First principles suggest there is an incoming heat load, therefore a fixed rate of boiloff is required to keep the depot cold.

cheers, Martin

As an aside, something which was discussed well in the past was the use of Propane for engine fuel. In tandem with LOX, you would get a good balance of storage temperatures, not have to worry of Kerosene, and steer WAY clear of LH2.

I believe the Japanese are working on a propane-powered engine. This might be something to consider in the future. Of course Methane would be best for an ascent stage, but considering the volumes in play simply for ascent-only, long-term cryo storage has major benefits (especially for breathing oxygen/nitrogen).

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Re: Propellant depot strategy & tactics pow-wow
« Reply #93 on: 08/25/2009 07:35 pm »
The viability of propellant depots depends very much on how much propellant can be launched per supply flight. There have been extremely different assumptions on this topic.

On the one hand, there are people that assume that a significant part of a propellant depot flight will be lost for a spacecraft that is handling the approach and proximity operations close to the depot. For an example, see this recent space review article.

On the other hand we have optimists that assume that almost the entire payload of a propellant depot supply flight will be actual propellant and that the costs of propellant delivery will be dominated by the launch costs and not by the costs of the propellant delivery spacecraft.

So which is closer to the truth?

I think that since most upper stages are basically complete spacecraft with RCS and all, there would be a significant benefit when delivering the same propellant as used from the upper stage.

In this case you would have to increase the size of the propellant tanks of the upper stage. For an atlas 552 propellant launch you would have to build a centaur with larger tanks and then use the centaur to fly close to the propellant depot. You would not need a payload fairing or payload adapter since the payload is the propellant remaining in the centaur tanks after reaching LEO. All proximity operations would be done by the depot itself, with the centaur acting as a passive target.

I realize that this is not easy to do. But is there anything fundamentally wrong with this approach?

Offline Downix

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Re: Propellant depot strategy & tactics pow-wow
« Reply #94 on: 08/26/2009 02:37 am »
A thought on a depot storage, we launch a perfect storage container every time we launch a shuttle... the fuel tank.  There is no reason why a shuttle launch using a specially modified tank could not carry the tank into an orbital location, or even a modified tank (using a Jupiter rocket-base w/ a single motor mounted) designed to move the tank into the proper orbit.  It's an incomplete idea, would need work and modification to the tank itself, but it seems doable.

**edit** Ok, random thought for a modification, if the tank could be placed into orbit, and a way to have the shuttle re-attach to it once in orbit, if the shuttle had the ability to have a full fuel tank waiting for it in orbit, how far up could it go?
« Last Edit: 08/26/2009 03:12 am by Downix »
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Offline jongoff

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Re: Propellant depot strategy & tactics pow-wow
« Reply #95 on: 08/26/2009 02:42 am »
The viability of propellant depots depends very much on how much propellant can be launched per supply flight. There have been extremely different assumptions on this topic.

On the one hand, there are people that assume that a significant part of a propellant depot flight will be lost for a spacecraft that is handling the approach and proximity operations close to the depot. For an example, see this recent space review article.

On the other hand we have optimists that assume that almost the entire payload of a propellant depot supply flight will be actual propellant and that the costs of propellant delivery will be dominated by the launch costs and not by the costs of the propellant delivery spacecraft.

So which is closer to the truth?

I think that since most upper stages are basically complete spacecraft with RCS and all, there would be a significant benefit when delivering the same propellant as used from the upper stage.

In this case you would have to increase the size of the propellant tanks of the upper stage. For an atlas 552 propellant launch you would have to build a centaur with larger tanks and then use the centaur to fly close to the propellant depot. You would not need a payload fairing or payload adapter since the payload is the propellant remaining in the centaur tanks after reaching LEO. All proximity operations would be done by the depot itself, with the centaur acting as a passive target.

I realize that this is not easy to do. But is there anything fundamentally wrong with this approach?

This is fairly close to the approach ULA has suggested.  Stretching the Centaur tank axially is a fairly minor change (they've done it many times in the past), and adding an extra hydrazine bottle if necessary is also relatively easy (they've already got locations on the Centaur aft end that are sized right for attaching another tank).  Supposedly the Centaur avionics are capable of at least rendezvous.  If you combined the system with some sort of "boom rendezvous and docking" approach, it might make sense.

Me personally, I'd prefer using a tug system instead, since it makes it easier for different launch providers to compete (which should help keep costs lower).  But the idea of having the depot delivery tank be the same tank as the stage isn't bad.

~Jon

Offline A_M_Swallow

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Re: Propellant depot strategy & tactics pow-wow
« Reply #96 on: 08/26/2009 02:47 am »
Given a standard interface many ways of delivering and storing the fuel can be used.
« Last Edit: 08/26/2009 02:48 am by A_M_Swallow »

Offline Arthur

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Re: Propellant depot strategy & tactics pow-wow
« Reply #97 on: 08/26/2009 05:13 pm »
With all the discussion recently, let me take a moment to try to clarify the Depot architecture decisions which we have made for DIRECT.

There is much debate about using an all-EELV-class approach.   What this would require, is approximately 9 launches for each mission.   Assuming a combination of 20mT and 25mT vehicles the following approach is hypothetically possible (though only if you choose to completely ignore the volume/diameter issues entirely):

1 Orion (fueled) -- Heavy
2 Lander Ascent Stage (fueled) -- Intermediate
3 Lander Descent Stage (dry) -- Heavy
4 EDS (mostly dry) -- Intermediate
5 Fuel for Descent Stage -- Intermediate
6 Fuel for EDS -- Intermediate
7 Fuel for EDS -- Intermediate
8 Fuel for EDS -- Intermediate
9 Fuel for EDS -- Intermediate

This architecture certainly requires the use of Propellant Transfer technologies and would almost-certainly require a full Depot to be deployed as part of the baseline Critical Path to success.

Of these 9 launches, the first 4 in that list are all mission critical and the loss of any one would result in an LOM situation.   The latter 5 launches are somewhat "interchangable" so there is "Partial Redundancy" possible there.   It's not too bad, but the logistics and the necessity to coordinate the launch of 4 of those vehicles perfectly in support of each mission, plus the constant fuel deliveries as well, makes it a very demanding logistical nightmare.



Comparatively, DIRECT chooses a three-step approach to getting to the final arrangement.

Firstly we deploy Jupiter-130 in order to preserve jobs & experience to secure the political backing we need in Congress.   This provides an incredibly capable system all by itself, but still isn't quite enough for Lunar use (although with a Delta Upper Stage, the Flyby missions are quite possible).

Step 2 is the deployment of the Jupiter-24x and the Altair lander, which opens up the Lunar capabilities without requiring any Propellant Transfer technologies at all.   This is an interim step designed to begin our new exploration efforts and to allow NASA to start the Exploration efforts in earnest while other important technologies continue to be developed -- without those technologies every appearing on the "Critical Path".

Step 3 is the ultimate goal though.   Here, every Jupiter launch represents a complete mission, supported by a constant stream of fuel deliveries going to a (one or more) Depot.   The mission Hardware all launches upon a single launcher which then rendezvous with the Depot, fills up all the tanks it needs to with whatever fuel load is required for that mission and then departs upon its mission without ever requiring any other docking events.

The purpose of this approach is to maximize the number of units in production, not just for the costly launch vehicles, but also for the even more expensive spacecraft as well.

This architecture opens the door not to just 2 Lunar-class missions per year, but to a possible 8 (or more) every year.   More importantly, this approach also enables all of the NEO and Mars missions as well without further investment in the basic infrastructure.   This approach is quite capable of sending hundreds of tons of useful payload material towards Mars -- or even Jupiter if required.

I am including the costs for the launches below.

Ross.

I wanted to move this post and my question to a topic more "DEPOT" related. 

Does anyone know where similar data for an EELV can be found?

 I would like to compare the cost per kg to orbit for several different "market conditions" to generate a comparison of cost vs supply curves. Are DEPOTS cheaper for all annual demands? Is Jupiter cheaper for all annual demands? Is there a volume where the curves cross?

Offline rklaehn

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Re: Propellant depot strategy & tactics pow-wow
« Reply #98 on: 08/26/2009 06:09 pm »
The viability of propellant depots depends very much on how much propellant can be launched per supply flight. There have been extremely different assumptions on this topic.

On the one hand, there are people that assume that a significant part of a propellant depot flight will be lost for a spacecraft that is handling the approach and proximity operations close to the depot. For an example, see this recent space review article.

On the other hand we have optimists that assume that almost the entire payload of a propellant depot supply flight will be actual propellant and that the costs of propellant delivery will be dominated by the launch costs and not by the costs of the propellant delivery spacecraft.

So which is closer to the truth?

I think that since most upper stages are basically complete spacecraft with RCS and all, there would be a significant benefit when delivering the same propellant as used from the upper stage.

In this case you would have to increase the size of the propellant tanks of the upper stage. For an atlas 552 propellant launch you would have to build a centaur with larger tanks and then use the centaur to fly close to the propellant depot. You would not need a payload fairing or payload adapter since the payload is the propellant remaining in the centaur tanks after reaching LEO. All proximity operations would be done by the depot itself, with the centaur acting as a passive target.

I realize that this is not easy to do. But is there anything fundamentally wrong with this approach?

This is fairly close to the approach ULA has suggested.  Stretching the Centaur tank axially is a fairly minor change (they've done it many times in the past), and adding an extra hydrazine bottle if necessary is also relatively easy (they've already got locations on the Centaur aft end that are sized right for attaching another tank).  Supposedly the Centaur avionics are capable of at least rendezvous.  If you combined the system with some sort of "boom rendezvous and docking" approach, it might make sense.

Carrying the payload in the upper stage would eliminate two failure modes from the launch: fairing separation (caused two launch failures this year!) and spacecraft separation. You would also save the expense for the fairing and spacecraft separation system.

Quote
Me personally, I'd prefer using a tug system instead, since it makes it easier for different launch providers to compete (which should help keep costs lower).  But the idea of having the depot delivery tank be the same tank as the stage isn't bad.

A tug would be the best solution for a large depot. But for a small depot you could just combine the functions of depot and tug.

By the way: One very nice thing about depots is that it decouples launcher capacity and payload mass:

If your GTO or TLI launch requires topping off at a depot, any performance improvement or large propellant margin of the launcher immediately translates to cost savings since you don't need as much propellant.

Likewise, if you have some weight growth of your spacecraft there is little risk of the spacecraft getting too heavy for the launcher. Instead you just have to buy a bit more propellant at the depot.

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Re: Propellant depot strategy & tactics pow-wow
« Reply #99 on: 08/27/2009 04:33 pm »
This could be a very different story, had a propellant depot been available at L1 or LLO:

http://www.space.com/missionlaunches/090826-lcross-anomaly.html

Tog
M's Laws of Aerodynamics:                                    On Physics Exam:
1) if you push anything hard enough it will fly          Q)The allegory of Schrödinger's cat shows what?
2) if you stop pushing it stops flying                        A)That Shrödinger was a sadistic cat hater

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