Author Topic: Depots vs. Inflight Refueling  (Read 9068 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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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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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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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.

Quote
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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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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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

Quote
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.






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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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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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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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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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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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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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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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Re: Depots vs. Inflight Refueling
« Reply #20 on: 02/04/2011 11:27 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. 

Danny Deger
For two-way transfer, you don't need diaphragms or capillary screens either. All you need is a very tiny push by thrusters or maybe slight rotation. There are still yet more options.
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Re: Depots vs. Inflight Refueling
« Reply #21 on: 02/04/2011 11:39 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.

A restartable upper stage like an EDS will need some way to settle propellant prior to reignition of the main engines. I think centaur uses gaseous hydrogen venting to settle the propellant. The propellant outlet for a depot would be the engine inlet for the EDS. Just have valves that control whether the propellant goes to the engines or to the receiving spacecraft.

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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. 

I still prefer depots, but anything is better than this launch vehicle focused approach NASA seems to have now. There is a huge list of problems we have to overcome to become a spacefaring species, and not a single one of them is being addressed by building another HLV.
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Offline A_M_Swallow

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Re: Depots vs. Inflight Refueling
« Reply #22 on: 02/05/2011 01:11 AM »
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.

Danny Deger

There is one big difference between an EDS and a depot.  A depot is a satellite so its thrusters need to be designed for station keeping.  That is the thrusters must to survive many restarts, each with a relatively low delta-V.

A depot's inlet and outlet could be different pipes and connectors.  This would allow everything to be one way.

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Re: Depots vs. Inflight Refueling
« Reply #23 on: 02/05/2011 02:07 AM »
I've always thought that the best way to do depots is to standardize on a range of tank sizes and interconnects, then instead of trying to transfer the propellant, simply disconnect the empty tank and connect in the full one. The empties can be collected by an automated space tug and recycled as radiation shielding mass, or potentially habitable volume if the tank size was large enough.  I should think the problems with connecting and disconnecting a tank would be much easier to solve than pumping cryogenic fluids around in zero G.

Of course, trying to get manufacturers to standardize might be difficult, but it has worked in other industries where vendors realize that interoperability is an important accelerator for growth.

Offline rklaehn

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Re: Depots vs. Inflight Refueling
« Reply #24 on: 02/05/2011 09:23 AM »
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. 

Another thing:

Depots have some large advantages when it comes to staging. For example, in the ULA architecture the spacecraft refuel from an EML2 depot before going on their exploration missions.

The EML2 depot is refueled the following way: 2 out of 3 tanker flights go to the LEO depot, deposit their propellant, and are expended. The third tanker flight fills up at the LEO depot and continues onward to EML2. So only 1 out of 3 tankers have to go all the way to EML2.

If you wanted to refuel a spacecraft stationed at EML2 using propellant tankers but without depot, every propellant tanker would have to fly all the way to EML2. So you would have three times the dead mass to EML2, and correspondingly less propellant.

You might say that you can also do staging using tankers: launch a tanker to LEO, refuel it from 2 additional tankers, and then fly that to EML2. But that would require a tanker that can loiter in LEO until the two additional tanker flights are ready, and that can both accept and donate propellant. So basically a depot  :)

With a tanker you always have a tradeoff: add more insulation, and you reduce boiloff but increase empty mass and reduce payload faction. With a depot you can basically add as much insulation as you want because it does not have to go anywhere.

But in any case it is not necessary to fight about depots vs. refueling:

-Both need boiloff reduction (I assume that you intend to use cryogenic propellants)
-Both need establishing a connection and propellant transfer
-Both need a lightweight tanker stage

So maybe we can get these technologies to TRL 9 by getting NASA to fund a few CRYOTE demonstrations, and then continue discussing refueling from tankers vs. depots.
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Re: Depots vs. Inflight Refueling
« Reply #25 on: 02/05/2011 10:15 AM »
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.

Danny Deger

There is one big difference between an EDS and a depot.  A depot is a satellite so its thrusters need to be designed for station keeping.  That is the thrusters must to survive many restarts, each with a relatively low delta-V.

A propellant tanker will also need a precise attitude control system so it can connect to the "customer" (depot/EDS). But you are right that the attitude and orbit control system for the depot is special. It should use the same propellants the depot is storing. Otherwise the depot will be useless once it runs out of AOCS propellants, even if it is full of propellant.

ULA plans to use the boiloff from the depot for a gaseous O2/H2 AOCS. They also plan to upgrade the centaur so that it does not use any fluids other than LH2/LOX.

Even if you are not ULA, gaseous O2/H2 AOCS are a commercial off the shelf item: orion propulsion developed a gaseous H2 AOCS for bigelow.
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Re: Depots vs. Inflight Refueling
« Reply #26 on: 02/05/2011 10:48 AM »
I've always thought that the best way to do depots is to standardize on a range of tank sizes and interconnects, then instead of trying to transfer the propellant, simply disconnect the empty tank and connect in the full one. The empties can be collected by an automated space tug and recycled as radiation shielding mass, or potentially habitable volume if the tank size was large enough.  I should think the problems with connecting and disconnecting a tank would be much easier to solve than pumping cryogenic fluids around in zero G.

Of course, trying to get manufacturers to standardize might be difficult, but it has worked in other industries where vendors realize that interoperability is an important accelerator for growth.

That would work, but it has some downsides. Stages and spacecraft often use the tank as the structural backbone of the entire spacecraft. The typical upper stage is just a large tank with some components (engines etc.) attached to it. You wouldn't be able to do that if you wanted to exchange the tank. So you would never get as good a mass fraction as for example centaur with an exchangeable tank.
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Offline DarkenedOne

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Re: Depots vs. Inflight Refueling
« Reply #27 on: 02/06/2011 12:51 AM »
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. 

Danny Deger

First of all I question the notion that a fuel depot would cost billions to design.  As I understand it from what I have read.  Practically all of the technological hurtles have already been overcome. 

In particular the autonomous rendezvous and fuel transfer was accomplished by DARPA with ASTRO and NextSat. 

Essentially a primitive fuel depot would be no different than a large satellite with rendezvous and fuel transfer capability. 

Of course it depends on the fuel being stored.  Hydrazine would be a hell of a lot easier than cryogenic fuels.  Hydrogen in particular is very difficult to store due to its low density and boiling point. 

Offline DarkenedOne

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Re: Depots vs. Inflight Refueling
« Reply #28 on: 02/06/2011 01:13 AM »
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. 

Another thing:

Depots have some large advantages when it comes to staging. For example, in the ULA architecture the spacecraft refuel from an EML2 depot before going on their exploration missions.

The EML2 depot is refueled the following way: 2 out of 3 tanker flights go to the LEO depot, deposit their propellant, and are expended. The third tanker flight fills up at the LEO depot and continues onward to EML2. So only 1 out of 3 tankers have to go all the way to EML2.

If you wanted to refuel a spacecraft stationed at EML2 using propellant tankers but without depot, every propellant tanker would have to fly all the way to EML2. So you would have three times the dead mass to EML2, and correspondingly less propellant.

You might say that you can also do staging using tankers: launch a tanker to LEO, refuel it from 2 additional tankers, and then fly that to EML2. But that would require a tanker that can loiter in LEO until the two additional tanker flights are ready, and that can both accept and donate propellant. So basically a depot  :)

With a tanker you always have a tradeoff: add more insulation, and you reduce boiloff but increase empty mass and reduce payload faction. With a depot you can basically add as much insulation as you want because it does not have to go anywhere.

But in any case it is not necessary to fight about depots vs. refueling:

-Both need boiloff reduction (I assume that you intend to use cryogenic propellants)
-Both need establishing a connection and propellant transfer
-Both need a lightweight tanker stage

So maybe we can get these technologies to TRL 9 by getting NASA to fund a few CRYOTE demonstrations, and then continue discussing refueling from tankers vs. depots.

I think you are missing one of the largest benefit to in space fuel storage. 

If look at things here on Earth we use different types of transport for different items.  For example people are transported between the continents by plane whereas fuel is transported by oil tanker.  Now it is not that we cannot transport fuel by plane because it would be far more expensive and carry no benefit.

Now I bring this up because we now have proven alternatives to chemical rockets for space transport.  For even the most efficient chemical thrusters it would cost you several kg of propellant for every kg of propellant in lunar orbit.  Same goes for GEO. 

Ion and plasma thrusters like those used in on the DAWN and Smart-1 spacecraft can deliver the fuel for a small fraction of the cost.  The problem with regards to manned spaceflight is that they would take too long at least for short distances.  NASA would not want its astronauts to be exposed to radiation for several months while fuel sipping ion engines go from LEO to high orbit, but for fuel this is just fine. 

Honestly the biggest advance in decreasing cost and increasing payload is to ship fuel and supplies using ion propulsion rather than chemical.  It will take longer, but cost less. 

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Re: Depots vs. Inflight Refueling
« Reply #29 on: 02/06/2011 10:47 PM »
 


Another option is solar thermal. While most efficient using hydrogen, other propellants including aluminum may be used with higher thrust per given solar concentrator size but lower Isp. Depending on the specific power of the concentrator, 1 ft./sec. acceleration or better is possible.
 There are already millions pounds of aluminum in GEO that could be used.
I'm just sayin...

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Re: Depots vs. Inflight Refueling
« Reply #30 on: 02/07/2011 03:28 PM »



Another option is solar thermal. While most efficient using hydrogen, other propellants including aluminum may be used with higher thrust per given solar concentrator size but lower Isp. Depending on the specific power of the concentrator, 1 ft./sec. acceleration or better is possible.
 There are already millions pounds of aluminum in GEO that could be used.
I'm just sayin...

Yeah any high ISP, low cost method could be used to deliver the fuel. 

Also fuel depots are a critical part of the infrastructure for future ISRU.


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Re: Depots vs. Inflight Refueling
« Reply #31 on: 02/07/2011 03:58 PM »
I think you are missing one of the largest benefit to in space fuel storage. 

If look at things here on Earth we use different types of transport for different items.  For example people are transported between the continents by plane whereas fuel is transported by oil tanker.  Now it is not that we cannot transport fuel by plane because it would be far more expensive and carry no benefit.

Now I bring this up because we now have proven alternatives to chemical rockets for space transport.  For even the most efficient chemical thrusters it would cost you several kg of propellant for every kg of propellant in lunar orbit.  Same goes for GEO. 

Ion and plasma thrusters like those used in on the DAWN and Smart-1 spacecraft can deliver the fuel for a small fraction of the cost.  The problem with regards to manned spaceflight is that they would take too long at least for short distances.  NASA would not want its astronauts to be exposed to radiation for several months while fuel sipping ion engines go from LEO to high orbit, but for fuel this is just fine. 

Honestly the biggest advance in decreasing cost and increasing payload is to ship fuel and supplies using ion propulsion rather than chemical.  It will take longer, but cost less. 

You don't have to sell me the idea of propellant depots.

But I think you underestimate how much development work it would be to build solar electric tugs capable of supplying for example a propellant depot for manned exploration in EML2. All SEP spacecraft flown so far have been less than a ton, and have benefited from chemical propulsion.

DAWN was sent on an interplanetary trajectory by a chemical stage, and requires several years of almost continuous thrusting for its mission delta-v.

Smart-1 started up in GTO. And even so, after reaching LLO its solar arrays were seriously degraded from repeated passages through the van allen belts.

Besides, if you need a year or two to get your propellant to the depot, that pretty much eliminates cryogenic propellant unless you have active refrigeration.

I think in the near term chemical propulsion is more realistic, and it's perfectly capable of supporting large scale manned missions to anywhere in the inner solar system.
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Re: Depots vs. Inflight Refueling
« Reply #32 on: 02/07/2011 04:14 PM »
Another option is solar thermal. While most efficient using hydrogen, other propellants including aluminum may be used with higher thrust per given solar concentrator size but lower Isp. Depending on the specific power of the concentrator, 1 ft./sec. acceleration or better is possible.
 There are already millions pounds of aluminum in GEO that could be used.
I'm just sayin...

Solar thermal is great. It offers relatively high thrust. And you should be able to use almost unprocessed ISRU volatiles such as water and ammonia.

But aluminium? As fuel? For a solar thermal engine? Never heard of that!
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Offline muomega0

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Re: Depots vs. Inflight Refueling
« Reply #33 on: 02/07/2011 05:38 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.

Danny Deger

A technology development can take R&D or just development (spend cash-concept exists).

You state that a depot has NEVER been built, but is it because of lack of funding, or is there some physical constraint or concept that is not in place?

The Constellation Program deemed it allowable to boil-off 70 tons of propellant during the 400+ day loiter for a MARS DRM 5.0 mission.  This was accomplished by adding passive insulation and other techniques to  a typical upper stage to reduce boil-off from say 2% to 0.1 % per day.

So to make a depot with lower boil-off rate, what is needed?  three fundamental things in addition to passive:

A sunshield - deployable one for Webb, no real show stopper here.

A cryocooler - many flown in space.  The astronauts retrofitted Hubble with a cryocooler on-orbit.  no real show stopper here.

Fluid transfer: Ullage motors show that only milli-gee accelerations are needed to move the main engine liquid propellants to the bottom of their tanks, so they can be pumped into the engine plumbing to avoid excessive gas intake, while Ullage gas floats to the top, away from the engine inlets.  Once concept is that you simply rotate the depot.

Tanks vs Depots:  so is it simply the two-way transfer versus one-way transfer of propellant that makes you state it will take billions to develop depots?  If so, why is the rotation concept to provide mill-gee acceleration inadequate?  what would take Billions to develop?

Offline DarkenedOne

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Re: Depots vs. Inflight Refueling
« Reply #34 on: 02/07/2011 07:18 PM »
I think you are missing one of the largest benefit to in space fuel storage. 

If look at things here on Earth we use different types of transport for different items.  For example people are transported between the continents by plane whereas fuel is transported by oil tanker.  Now it is not that we cannot transport fuel by plane because it would be far more expensive and carry no benefit.

Now I bring this up because we now have proven alternatives to chemical rockets for space transport.  For even the most efficient chemical thrusters it would cost you several kg of propellant for every kg of propellant in lunar orbit.  Same goes for GEO. 

Ion and plasma thrusters like those used in on the DAWN and Smart-1 spacecraft can deliver the fuel for a small fraction of the cost.  The problem with regards to manned spaceflight is that they would take too long at least for short distances.  NASA would not want its astronauts to be exposed to radiation for several months while fuel sipping ion engines go from LEO to high orbit, but for fuel this is just fine. 

Honestly the biggest advance in decreasing cost and increasing payload is to ship fuel and supplies using ion propulsion rather than chemical.  It will take longer, but cost less. 

You don't have to sell me the idea of propellant depots.

But I think you underestimate how much development work it would be to build solar electric tugs capable of supplying for example a propellant depot for manned exploration in EML2. All SEP spacecraft flown so far have been less than a ton, and have benefited from chemical propulsion.

First of all there are many satellites greater than 1 ton that use SEP for station keeping and some even use it for orbital topping as I understand.  The recent AEHF Satellite used it to carry itself to the correct orbit when its main chemical thruster failed.  That one had a mass of over 6000 kg.  DAWN was over a ton, and was capable of a delta-v of 10 km/s.

Second of all there are multiple SEP thrusters that would be capable of moving heavy loads already in development and nearing completion.  You have NASA's NEXT thruster.  You have VASIMR.

Lastly with NEXT and VASIMR being perfect examples, development of SEP does not cost that much.  In reality I think you are over estimating the development work.     

Quote
DAWN was sent on an interplanetary trajectory by a chemical stage, and requires several years of almost continuous thrusting for its mission delta-v.

Smart-1 started up in GTO. And even so, after reaching LLO its solar arrays were seriously degraded from repeated passages through the van allen belts.

Every single probe starts out with a boost from the chemical rocket that puts it into space.  The important take away point is that DAWN's SEP drive is able to get 1250 kg up to 10 km/s with only 425 kg of fuel, and Smart-1's SEP was able to get 367 kg up to 2.7 km/s with only 60 kg of fuel. 

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Besides, if you need a year or two to get your propellant to the depot, that pretty much eliminates cryogenic propellant unless you have active refrigeration.

Well it depends on the crogenic fuel being used.  If you use liquid hydrogen than the boil of will be much greater than LOX/Kerosene.  In any case the benefits of using 5000 ISP ion drive are far superior to the 100 ISP increase that LH2/LO2 rocket give you. 

Quote
I think in the near term chemical propulsion is more realistic, and it's perfectly capable of supporting large scale manned missions to anywhere in the inner solar system.

Given the current fiscal environment chemical propulsion is not going to get manned spaceflight anywhere anytime soon.  The problem is that the mass to be lifted into orbit can only be done by extremely expensive super heavy lift vehicles.  These vehicles are extremely costly to develop.  They take many years to develop.  Once they are developed they are expensive to maintain and have a very low fly rate.  Then they have no use beyond human space flight.  No country in the world currently has such rockets and it does not appear like there will be any time soon.

Using ion propulsion combined with fuel depots drasically reduces the mass need to conduct human spaceflight.  It is a technology well within short term capability.  Ion propulsion in particular is a very mature technology used on many satellites and now interplanetary space probes. 
« Last Edit: 02/07/2011 07:21 PM by DarkenedOne »

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Re: Depots vs. Inflight Refueling
« Reply #35 on: 02/07/2011 07:52 PM »
Quote
DAWN was sent on an interplanetary trajectory by a chemical stage, and requires several years of almost continuous thrusting for its mission delta-v.

Smart-1 started up in GTO. And even so, after reaching LLO its solar arrays were seriously degraded from repeated passages through the van allen belts.

Every single probe starts out with a boost from the chemical rocket that puts it into space.  The important take away point is that DAWN's SEP drive is able to get 1250 kg up to 10 km/s with only 425 kg of fuel, and Smart-1's SEP was able to get 367 kg up to 2.7 km/s with only 60 kg of fuel. 

A fuel transporter would have to go from LEO to somewhere interesting (EML2 or LLO), not just circularize an elliptical orbit. So it will require many passes through the van allen belts.

I still haven't heard how you are going to deal with the resulting degradation of the solar arrays. For a one-way mission you will just make the solar array a bit bigger to account for the degradation. But for a reusable tanker? Or is the plan to throw away the tanker after he has delivered the propellant?

Quote
Quote
Besides, if you need a year or two to get your propellant to the depot, that pretty much eliminates cryogenic propellant unless you have active refrigeration.

Well it depends on the crogenic fuel being used.  If you use liquid hydrogen than the boil of will be much greater than LOX/Kerosene.  In any case the benefits of using 5000 ISP ion drive are far superior to the 100 ISP increase that LH2/LO2 rocket give you. 

Depends on what you want to do. If you want to land on the moon or mars that 5000s ISP won't do you any good. And for an interplanetary injection the low thrust engine will not let you use a hohmann transfer, let alone the oberth effect.

Quote
Quote
I think in the near term chemical propulsion is more realistic, and it's perfectly capable of supporting large scale manned missions to anywhere in the inner solar system.

Given the current fiscal environment chemical propulsion is not going to get manned spaceflight anywhere anytime soon.  The problem is that the mass to be lifted into orbit can only be done by extremely expensive super heavy lift vehicles.

The main point of propellant depots is that you don't have to build a heavy lifter for ambitious missions. Most of the mass that has to be lifted to orbit is propellant, which is almost infinitely divisible and will thus be lifted on the launcher that is cheapest per pound, regardless of payload. The high flight rate required will reduce the cost per flight and create a market for more ambitious reusable launchers.

It's all in the ULA papers. They are proposing a modest upgrade of the venerable centaur upper stage, but if you don't have money for that you can also do a lot with the standard centaur or the large delta IV upper stage used in the delta IV heavy.

An existing and mature delta IV heavy upper stage, refueled at LEO and EML2, could send an 80000kg spacecraft to TMI.

Quote
Using ion propulsion combined with fuel depots drasically reduces the mass need to conduct human spaceflight.  It is a technology well within short term capability.  Ion propulsion in particular is a very mature technology used on many satellites and now interplanetary space probes. 

Ion propulsion has not yet been used to get a spacecraft out of LEO. And there is a reason for that.

But the topic of this thread is depots vs. inflight refueling not "chemical versus SEP". So let's stop the off-topic chemical vs. SEP discussion or at least take it to another thread.
« Last Edit: 02/07/2011 08:15 PM by rklaehn »
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Re: Depots vs. Inflight Refueling
« Reply #36 on: 02/07/2011 07:54 PM »
Given the current fiscal environment chemical propulsion is not going to get manned spaceflight anywhere anytime soon. The problem is that the mass to be lifted into orbit can only be done by extremely expensive super heavy lift vehicles. These vehicles are extremely costly to develop. They take many years to develop. Once they are developed they are expensive to maintain and have a very low fly rate. Then they have no use beyond human space flight. No country in the world currently has such rockets and it does not appear like there will be any time soon.
There is an underlying assumption here that needs to be questioned; Why is it "assumed" that an HLV is required for propellant flight? ULA has shown that current Atlas-V, and Delta-IV (non-"heavy") boosters are quite sufficent for propellant lift to depots. And doing so would raise the flight rate which in and of itself should bring launch costs down somewhat.

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Re: Depots vs. Inflight Refueling
« Reply #37 on: 02/07/2011 08:12 PM »
Given the current fiscal environment chemical propulsion is not going to get manned spaceflight anywhere anytime soon. The problem is that the mass to be lifted into orbit can only be done by extremely expensive super heavy lift vehicles. These vehicles are extremely costly to develop. They take many years to develop. Once they are developed they are expensive to maintain and have a very low fly rate. Then they have no use beyond human space flight. No country in the world currently has such rockets and it does not appear like there will be any time soon.
There is an underlying assumption here that needs to be questioned; Why is it "assumed" that an HLV is required for propellant flight? ULA has shown that current Atlas-V, and Delta-IV (non-"heavy") boosters are quite sufficent for propellant lift to depots. And doing so would raise the flight rate which in and of itself should bring launch costs down somewhat.

Exactly. For the most frequent LEO depot flights, ULA would use the Atlas V with 5 SRM, which has 20t to LEO and would have 26 with the proposed new ACES upper stage / propellant tanker.

ULA has vast overcapacity, so the increased flight rate would bring down prices.

ULA also forsee that propellant would be launched on other domestic vehicles (falcon 9, taurus II) or international partners (ariane V, gslv, hII-a, long march 5?)
« Last Edit: 02/07/2011 08:24 PM by rklaehn »
Try the ISS 3D visualization at http://www.heavens-above.com/ISS_3D.aspx

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