Author Topic: Depots vs. Inflight Refueling  (Read 9072 times)

Offline Danny Dot

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Depots vs. Inflight Refueling
« on: 02/04/2011 04:20 PM »
I would like to open a thread on depots vs. inflight refueling.  We started this discussion in a thread on the Space Launch System, but I don't think that thread is the place for a discussion on depots and inflight refueling.

I am a big fan of inflight refueling for massive lift requirements.  Rather than build a 130 ton launcher, build a 100 ton booster and fuel it after it gets to orbit.  In this case the tankers will feed propellant dirrectly to the rocket that needs it.  In the depot case, the fuel is tranfered to a depot, then the rocket that ultimately needs the prollent gets it from the tanker.

Two big draw backs of the depot are the expense of designing and developing the depot (billions of dollars) and the depot may not be in the needed orbit for the mission.

And I don't see the need for depots or inflight refueling for any mission that a current booster can lift the fully fueled rocket. 

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Offline mmeijeri

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Re: Depots vs. Inflight Refueling
« Reply #1 on: 02/04/2011 04:21 PM »
If you're going to use refueling, why build a huge booster at all?
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Offline rklaehn

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Re: Depots vs. Inflight Refueling
« Reply #2 on: 02/04/2011 05:09 PM »
I would like to open a thread on depots vs. inflight refueling.  We started this discussion in a thread on the Space Launch System, but I don't think that thread is the place for a discussion on depots and inflight refueling.

I am a big fan of inflight refueling for massive lift requirements.  Rather than build a 130 ton launcher, build a 100 ton booster and fuel it after it gets to orbit.  In this case the tankers will feed propellant dirrectly to the rocket that needs it.  In the depot case, the fuel is tranfered to a depot, then the rocket that ultimately needs the prollent gets it from the tanker.

I don't see too much difference in both scenarios wrt the required technology development. For both refueling from tankers and propellant depots, you have to develop the following technologies:
1. transfer propellants from one tank to another
2. build a lightweight tanker spacecraft (or integrate the tanker with the upper stage as ULA proposes)
3. store propellant for extended durations (assuming you want multiple tankers to provide the propellant for one mission)

Once you have developed these technologies, having a depot is a way to decouple the propellant launcher from the mission. In the tanker refueling case, your spacecraft must use an integral multiple of the tanker capacity. Any excess (margin or performance improvements in the launcher) is lost. With propellant depots, you can transfer residual propellants to the depot for the next mission.

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Two big draw backs of the depot are the expense of designing and developing the depot (billions of dollars) and the depot may not be in the needed orbit for the mission.

Pretty much all beyond-LEO missions can be effectively staged from EML1/EML2. If you have a depot there, like the ULA affordable exploration paper proposed, it can be used for moon, mars or NEO missions.

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And I don't see the need for depots or inflight refueling for any mission that a current booster can lift the fully fueled rocket. 

For manned missions there is really not that much you can do with current boosters without some kind of refueling or orbital assembly.

And it might be cheaper to for example launch the spacecraft on an Atlas V 401 and buy the propellant at the depot than to buy a Delta IV heavy to launch the fully fueled spacecraft.
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Offline mmeijeri

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Re: Depots vs. Inflight Refueling
« Reply #3 on: 02/04/2011 05:15 PM »
I don't see too much difference in both scenarios wrt the required technology development.

The difference is more that you don't need a separate vehicle development program for the depot straight away.

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Once you have developed these technologies, having a depot is a way to decouple the propellant launcher from the mission. In the tanker refueling case, your spacecraft must use an integral multiple of the tanker capacity. Any excess (margin or performance improvements in the launcher) is lost. With propellant depots, you can transfer residual propellants to the depot for the next mission.

There can be more than one tanker size and unless you are talking about using HLVs, the tanker capacity would likely be much smaller than the propellant requirements of the spacecraft + EDS. There would likely be very little inefficiency. Dedicated depots could come later as traffic warranted, and could be left safely (even preferably) to the market.

Quote
Pretty much all beyond-LEO missions can be effectively staged from EML1/EML2. If you have a depot there, like the ULA affordable exploration paper proposed, it can be used for moon, mars or NEO missions.

Not only that, using EML1/2 as a staging point is actually far superior for a long list of reasons.
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Offline rklaehn

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Re: Depots vs. Inflight Refueling
« Reply #4 on: 02/04/2011 05:28 PM »
I don't see too much difference in both scenarios wrt the required technology development.

The difference is more that you don't need a separate vehicle development program for the depot straight away.

Quote
Once you have developed these technologies, having a depot is a way to decouple the propellant launcher from the mission. In the tanker refueling case, your spacecraft must use an integral multiple of the tanker capacity. Any excess (margin or performance improvements in the launcher) is lost. With propellant depots, you can transfer residual propellants to the depot for the next mission.

There can be more than one tanker size and unless you are talking about using HLVs, the tanker capacity would likely be much smaller than the propellant requirements of the spacecraft + EDS.

I am not completely sure about what concept danny is proposing. But if you want multiple tanker flights to refuel one EDS, then the EDS has to be able to store the propellant for an extended duration. There also has to be the hardware for precise attitude control and to do cryogenic fluid transfer on the EDS.

You do not need a separate vehicle development program for the depot, because an EDS that is capable of accepting and storing propellant from multiple tankers is a depot.

Quote
There would likely be very little inefficiency. Dedicated depots could come later as traffic warranted, and could be left safely (even preferably) to the market.

So what do you do if your EDS uses 2.5 times the tanker capacity as propellant? You have to throw away the remaining 0.5 tankers full.

Quote
Quote
Pretty much all beyond-LEO missions can be effectively staged from EML1/EML2. If you have a depot there, like the ULA affordable exploration paper proposed, it can be used for moon, mars or NEO missions.

Not only that, using EML1/2 as a staging point is actually far superior for a long list of reasons.

Well, at least that we do agree on.
« Last Edit: 02/04/2011 05:31 PM by rklaehn »
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Offline mmeijeri

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Re: Depots vs. Inflight Refueling
« Reply #5 on: 02/04/2011 05:36 PM »
I am not completely sure about what concept danny is proposing. But if you want multiple tanker flights to refuel one EDS, then the EDS has to be able to store the propellant for an extended duration. There also has to be the hardware for precise attitude control and to do cryogenic fluid transfer on the EDS.

Correct, except perhaps for the ability to transfer fluids both ways. (Or if you want to start with hypergolics as I do, but let's leave that aside for now as most of the argument applies to cryogenics too and I don't want to focus on a tangential and controversial point here.)

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You do not need a separate vehicle development program for the depot, because an EDS that is capable of accepting and storing propellant from multiple tankers is a depot.

That's more or less what I thought Danny was talking about. The same applies to a refuelable lander that can do double duty as a makeshift depot or its own EDS.

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So what do you do if your EDS uses 2.5 times the tanker capacity as propellant? You have to throw away the remaining 0.5 tankers full.

That would be bad, but I think you could expect to use a spacecraft / EDS that took on >= 10-20 times the content of a tanker.
« Last Edit: 02/04/2011 07:39 PM by mmeijeri »
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Offline Cherokee43v6

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Re: Depots vs. Inflight Refueling
« Reply #6 on: 02/04/2011 05:52 PM »
If you're going to use refueling, why build a huge booster at all?

Actually fuel depots are one of the areas that could justify an HLV.  In this case launching a huge mass of fuel to a depot in a single throw would be more cost effective than a bunch of smaller rockets.

After-all, gas stations don't take their deliveries from car fuel tanks, but from large fuel tank trucks.

My question is what will constitute 'fuel' at an orbital depot?

Will it be water?  In which case the depot 'refines' the water into hydrogen and oxygen?

What about Nobel Gasses like Xenon for 'ion' drives?

Storable fuels like Methane?

Nitrogen tetraoxide?

So many different fuels available and used.  It's not like you can pull up in your rocket, grab the hose and punch 'premium'.
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Offline rklaehn

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Re: Depots vs. Inflight Refueling
« Reply #7 on: 02/04/2011 06:04 PM »
If you're going to use refueling, why build a huge booster at all?

Actually fuel depots are one of the areas that could justify an HLV.  In this case launching a huge mass of fuel to a depot in a single throw would be more cost effective than a bunch of smaller rockets.

After-all, gas stations don't take their deliveries from car fuel tanks, but from large fuel tank trucks.

My question is what will constitute 'fuel' at an orbital depot?

Will it be water?  In which case the depot 'refines' the water into hydrogen and oxygen?

What about Nobel Gasses like Xenon for 'ion' drives?

Storable fuels like Methane?

Nitrogen tetraoxide?

So many different fuels available and used.  It's not like you can pull up in your rocket, grab the hose and punch 'premium'.

The options that have been seriously considered are LOX/LH2 and NTO/MMH. NTO/MMH has lower isp (~330s), but is storable and hypergolic. LOX/LH2 has a higher isp, but is more difficult to store, mostly due to LH2 being extremely cryogenic.

Here are a few papers from ULA about near term LOX/LH2 propellant depot concepts:

http://spirit.as.utexas.edu/~fiso/telecon/Kutter_11-10-10/Kutter_11-10-10.pdf
http://www.ulalaunch.com/site/docs/publications/DepotBasedTransportationArchitecture2010.pdf
http://ulalaunch.com/site/docs/publications/PropellantDepots2009.pdf

I am probably not the right person do advocate NTO/MMH depots, but maybe mmeijeri can help.

In the long term, a propellant depot will probably contain several different propellant combinations: NTO/MMH, LOX/LH2 and Argon/Xenon for ion thrusters. NTO/MMH=Diesel, LOX/LH2=gasoline, Argon/Xenon=super  :)
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Offline mmeijeri

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Re: Depots vs. Inflight Refueling
« Reply #8 on: 02/04/2011 06:11 PM »
Actually fuel depots are one of the areas that could justify an HLV.  In this case launching a huge mass of fuel to a depot in a single throw would be more cost effective than a bunch of smaller rockets.

That has been claimed often, but I think the case is very weak. If we want to achieve a breakthrough in launch prices, then it is more likely to come from RLVs than from HLVs. And RLVs need high flight rates in order to be economical. That's a large part of the reason why those who want to see commercial development of space want to see very small RLVs (~2.5mT, maybe even less), so you could have very high flight rates for multiple competing commercial RLVs, even with only a modest exploration program. And a reduction in launch prices by a factor of ten is nothing to sneeze at.

Needing to spend money on development of separate pieces of infrastructure such as full-blown dedicated depots might get in the way of achieving that goal. Starting with in-flight refueling of a refuelable spacecraft and/or EDS would be an easier, incremental step in the right direction.
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Offline Cherokee43v6

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Re: Depots vs. Inflight Refueling
« Reply #9 on: 02/04/2011 06:22 PM »
Actually fuel depots are one of the areas that could justify an HLV.  In this case launching a huge mass of fuel to a depot in a single throw would be more cost effective than a bunch of smaller rockets.

That has been claimed often, but I think the case is very weak. If we want to achieve a breakthrough in launch prices, then it is more likely to come from RLVs than from HLVs. And RLVs need high flight rates in order to be economical. That's a large part of the reason why those who want to see commercial development of space want to see very small RLVs (~2.5mT, maybe even less), so you could have very high flight rates for multiple competing commercial RLVs, even with only a modest exploration program. And a reduction in launch prices by a factor of ten is nothing to sneeze at.

Needing to spend money on development of separate pieces of infrastructure such as full-blown dedicated depots might get in the way of achieving that goal. Starting with in-flight refueling of a refuelable spacecraft and/or EDS would be an easier, incremental step in the right direction.

*nods*  I can see your point.  I think we might be looking at the same justification.  Flight rate is the holy-grail of price reduction on any launch vehicle.  My thoughts are more long-term, since it makes no sense to build a big depot that only needs refueled once ever few years due to low utilization. 

However, the existence of a fuel depot would imply a much higher flight rate for exploration missions than we are currently experiencing.  The question is, at what point do the scales tip to not only justify fuel depots, but the kind of fuel depots we are envisioning.
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Offline muomega0

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Re: Depots vs. Inflight Refueling
« Reply #10 on: 02/04/2011 06:35 PM »
Actually fuel depots are one of the areas that could justify an HLV.  In this case launching a huge mass of fuel to a depot in a single throw would be more cost effective than a bunch of smaller rockets.

That has been claimed often, but I think the case is very weak. If we want to achieve a breakthrough in launch prices, then it is more likely to come from RLVs than from HLVs. And RLVs need high flight rates in order to be economical. That's a large part of the reason why those who want to see commercial development of space want to see very small RLVs (~2.5mT, maybe even less), so you could have very high flight rates for multiple competing commercial RLVs, even with only a modest exploration program. And a reduction in launch prices by a factor of ten is nothing to sneeze at.

Needing to spend money on development of separate pieces of infrastructure such as full-blown dedicated depots might get in the way of achieving that goal. Starting with in-flight refueling of a refuelable spacecraft and/or EDS would be an easier, incremental step in the right direction.
I am a big fan of inflight refueling for massive lift requirements.  Rather than build a 130 ton launcher, build a 100 ton booster and fuel it after it gets to orbit.  In this case the tankers will feed propellant dirrectly to the rocket that needs it.  In the depot case, the fuel is tranfered to a depot, then the rocket that ultimately needs the prollent gets it from the tanker.

Two big draw backs of the depot are the expense of designing and developing the depot (billions of dollars) and the depot may not be in the needed orbit for the mission.

And I don't see the need for depots or inflight refueling for any mission that a current booster can lift the fully fueled rocket. 

Danny Deger
Mars, BEO, Lunar require more mass than even the 130 ton LV, so multiple launches are required.

Some say an upper stage is $5B to develop, so a depot would be what, half of that.  ULA and other state it can evolve to include zero-boiloff and other technologies to reduce the station keeping.

Ares V demonstrated that up to 70 tons of propellant boiled-off is "allowable" during its 400 day loiter for Mars Missions.  Ares V Utilization In Support of A Human Mission to Mars  NASA TM 2010 216450 Nov 2010

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the ability to transfer propellants on-orbit can significantly reduce the sensitivity of the mission architecture to key design variables

Innovation #1:  Determine Annual Launch Mass, divide by 10 = size of LV
Innovation #2:  Don't boil away the propellant
Innovation #3:  Use multiple LVs, (International?) to reduce launch center times from 60 to 45 to 30 to  x days

At $10,000/kg for mass-produced satellites, 350 metric tons is 3.5B/year.  Some say its closer to $100,000/kg for payloads.
350 metric tons divided by 10 is 35 metric ton sized LV.

Multiple LVs combined with depot dramatically reduce the costs of on-orbit fuel.  Bigger LVs do not.

Augustine:
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The mass  that must  be launched  to  low-Earth  orbit  in the current NASA plan, without its fuel on board, is in the range of 25 to 40 mt, setting a notional lower limit on the size of the super heavy-lift  launch vehicle if refueling is available.

Since 70% of the mass if fuel, with inherently no value, LV sizing tends to the smaller higher risk, lower mass to orbit RLV as the most economical.

And based on your work, its better to avoid having strap-ons, especially large diameter strap-ons, or LAS mass is dramatically altered.

So this data suggests that the smaller the LV the cheaper, but requires a 3B(?) dollar propellant depot than can be evolved to include ZBO ($1B?) and launch centers can be reduced by using a fleet of LVs.






Offline mmeijeri

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Re: Depots vs. Inflight Refueling
« Reply #11 on: 02/04/2011 06:43 PM »
However, the existence of a fuel depot would imply a much higher flight rate for exploration missions than we are currently experiencing.  The question is, at what point do the scales tip to not only justify fuel depots, but the kind of fuel depots we are envisioning.

Well, the larger the depots, the more justification they need. It may be a very long time before we'll want anything larger than a 50-100mT cryogenic depot. That's one reason to advocate starting with no more than in-flight refueling of a spacecraft and leaving it to the market to decide when to develop full depots of whatever size investors think is appropriate.
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Offline Cherokee43v6

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Re: Depots vs. Inflight Refueling
« Reply #12 on: 02/04/2011 06:55 PM »
However, the existence of a fuel depot would imply a much higher flight rate for exploration missions than we are currently experiencing.  The question is, at what point do the scales tip to not only justify fuel depots, but the kind of fuel depots we are envisioning.

Well, the larger the depots, the more justification they need. It may be a very long time before we'll want anything larger than a 50-100mT cryogenic depot. That's one reason to advocate starting with no more than in-flight refueling of a spacecraft and leaving it to the market to decide when to develop full depots of whatever size investors think is appropriate.

That makes good sense.  Fuel depots would probably be most cost effective with space resource utilization.  Assuming we are smart enough to work out how to build machines to handle hard-rock mining in an untended environment.  (Volatiles from C-class NEOs).  After-all, that would eliminate the majority of the delta-v mass loss in getting fuel to the depot. (and de-justify the big boosters too)
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Offline Danny Dot

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Re: Depots vs. Inflight Refueling
« Reply #13 on: 02/04/2011 07:32 PM »
If you're going to use refueling, why build a huge booster at all?

Good point.  With inflight refueling the line of current boosters might be adaquate.

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Offline Danny Dot

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Re: Depots vs. Inflight Refueling
« Reply #14 on: 02/04/2011 07:37 PM »
snip
You do not need a separate vehicle development program for the depot, because an EDS that is capable of accepting and storing propellant from multiple tankers is a depot.

snip

I disagree with this.  An Earth Departure Stage that can be fueled is MUCH different than a depot.  Mainly its fueling system doesn't have to have a two way transfer of fuel -- just one way.

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Re: Depots vs. Inflight Refueling
« Reply #15 on: 02/04/2011 07:44 PM »
snip
Some say an upper stage is $5B to develop, so a depot would be what, half of that.  ULA and other state it can evolve to include zero-boiloff and other technologies to reduce the station keeping.

snip

I disagree.  We have and know how to build upper stages.  We have NEVER built a depot.  The development cost of a depot will be much more that developing a new upper stage.  Maybe NASA should do some techology development for depots/refueling.

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Offline rklaehn

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Re: Depots vs. Inflight Refueling
« Reply #16 on: 02/04/2011 07:52 PM »
snip
You do not need a separate vehicle development program for the depot, because an EDS that is capable of accepting and storing propellant from multiple tankers is a depot.

snip

I disagree with this.  An Earth Departure Stage that can be fueled is MUCH different than a depot.  Mainly its fueling system doesn't have to have a two way transfer of fuel -- just one way.

I have no idea if two-way propellant transfer is that difficult. I always thought that the main challenges with a depot are

1) Building a tanker with a decent payload fraction that is nevertheless able to connect to the "customer" (depot or EDS)
2) connecting the two spacecraft and establishing a tight connection for propellant transfer (I don't say docking since this might look completely different to what we're used to when we think of docking)
3) propellant management (using propellant settling or something more exotic such as surface tension devices or even magnetic fields)
4) (in the case of cryogenic propellants) reducing boiloff to an acceptable level (zero boiloff is nice, but not necessary)

All of these are the same for fueling an EDS and for fueling a depot, so maybe you can see where I'm coming from. ULA seem to think that it is manageable, but I am really not an expert.

Just one idea: couldn't you just install the exact same system that you have on the tanker on the EDS to make the EDS a tanker itself? You would have to build an additional system, but the development would already be done for the tanker.

Edit: I just reread the ULA propellant depot 2010 paper Evolving to a Depot-Based Space Transportation Architecture. Not only does it make a great case for cryogenic propellant depots, it also proposes an incremental development approach:

As shown in Figure 6, this evolution plan begins with a testbed, flies this multiple times to mature that hardware, proceeds in increments to a basic depot, and then finally to a depot suited to full-scale lunar and Mars exploration. While this baby-step approach may appear timid, it in fact delivers immediate, valuable, and new apabilities at each step. It allows the maturation of hardware and operational concepts while providing the theoretical underpinnings. It gets our hands dirty early. If our technological noses are to be bloodied we get that experience early on and inexpensively.

This is exactly the right approach. If you only have time to read one of these papers, read this one.
« Last Edit: 02/04/2011 08:50 PM by rklaehn »
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Offline Cherokee43v6

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Re: Depots vs. Inflight Refueling
« Reply #17 on: 02/04/2011 08:10 PM »
Okay, here's a question regarding fuel transfer.

How do the Russians do it?  The Progress freighter has the ability to transfer fuel from its tanks to the space station's thruster tanks.  How do they achieve this?  Is it done under thrust?  Positive pressurization?
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Offline mmeijeri

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Re: Depots vs. Inflight Refueling
« Reply #18 on: 02/04/2011 08:41 PM »
I believe they use metallic diaphragms.
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Re: Depots vs. Inflight Refueling
« Reply #19 on: 02/04/2011 11:23 PM »
For one way transfer the receiving vehicle doesn't need diaphrams or capilary screens to flow propellant out.  Also operations cost for one way transfer will be less.  There will need to be a well staffed control room to manage prop transfer.

I still vote for tankers as the first step to reduce the need for huge launch vehicles.  Maybe a depot if we have a large number of flights that can't be launched fueled. 

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