Propellant Depots - General Discussion

[clongton]

Over on the "Griffin Blames Atlas for Ares Dissent" thread the discussion of propellant depots has started to take on a life of its own, so I started this thread so that topic can be pursued and the original thread get back to Mike Griffin. I am going to ask Chris to grab all the relavant posts and move them here, then I'll post a link to here from that thread in case anybody goes looking for them.

Here's the original topic location:
http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=11786#M258022

[Eerie]

I don`t understand something. Someone still have to send all the fuel to the depot.

And if you send the fuel up to the depot by several launches, why not to send them straight to your empty spacecraft to fuel it? It will amount to the same number of dockings, only you dock with the spacecraft, and not with the depot.

[jongoff]

Eerie,

I don`t understand something. Someone still have to send all the fuel to the depot.

And if you send the fuel up to the depot by several launches, why not to send them straight to your empty spacecraft to fuel it? It will amount to the same number of dockings, only you dock with the spacecraft, and not with the depot.

That's a good question.  Here's the reasons I can think of right now:

1. The depot, since it doesn't need to move constantly back and forth from LEO to GEO/L*/LLO/etc, doesn't have anywhere near as much mass constraints.  Therefore you can add more insulation, you can add zero boiloff systems, in order to reduce the boiloff further than you could with a typical flight stage.

2. The depot can have more facilities (including tugs or robot arms) to make receiving and transferring propellants easier.  If you have to actually "dock" stages together for transfer, that's going to have a lot higher parasitic mass on your outbound vehicle than if you could be berthed to a station using robotic arms and such.

3. Related to two, the station can be built more robustly, such that a berthing mishap is less likely to damage your probably much more fragile outbound stage.  Now your outbound stage has only *one* docking event, even if your depot has tons of them--this makes it easier to take propellants in whatever sized package ends up being most economical.

There are probably other reasons, but those are a few.

The key thing to remember is that propellant depots don't have to be $20B ISS-sized monstrosities.  It may be cheaper to have an actual depot than to try and do without one.  If you do it right.  YMMV, etc.

~Jon

[clongton]

Eerie - 18/3/2008  2:09 PM

I don`t understand something. Someone still have to send all the fuel to the depot.

And if you send the fuel up to the depot by several launches, why not to send them straight to your empty spacecraft to fuel it? It will amount to the same number of dockings, only you dock with the spacecraft, and not with the depot.

It's a gas station. The depot owner keeps it full, just like the gas station owner keeps his tanks full. The expectation is that traffic will eventually require a steady stream of delivery from multiple propellant sources to fill the demand for propellant supply.

There will be lots of small spacecraft that take on 10-15mT of propellant. But there will also be the large mission spacecraft that take on 100mT at a time. That kind of supply can only be made available on-orbit from a depot that carries that much supply. Right now, nobody can deliver that in one shot; but lots of smaller launchers could do that, and a lot of people would make a lot of money doing it.

[meiza]

It's practically useless to pretend a gas station model as there is only one customer, NASA. Nobody else has enough bucks to buy any significant amount of propellants in LEO every year.

[Eerie]

clongton - 18/3/2008  1:38 PM

Eerie - 18/3/2008  2:09 PM

I don`t understand something. Someone still have to send all the fuel to the depot.

And if you send the fuel up to the depot by several launches, why not to send them straight to your empty spacecraft to fuel it? It will amount to the same number of dockings, only you dock with the spacecraft, and not with the depot.

It's a gas station. The depot owner keeps it full, just like the gas station owner keeps his tanks full. The expectation is that traffic will eventually require a steady stream of delivery from multiple propellant sources to fill the demand for propellant supply.

There will be lots of small spacecraft that take on 10-15mT of propellant. But there will also be the large mission spacecraft that take on 100mT at a time. That kind of supply can only be made available on-orbit from a depot that carries that much supply. Right now, nobody can deliver that in one shot; but lots of smaller launchers could do that, and a lot of people would make a lot of money doing it.

But by the same logic, you could just keep a full "depot" of rockets with fuel tanks on earth, and launch them when necessary. A big hangar with a hundred stored Falcon-9. When you need, you take 1\5\20, fill them with fuel and send one by one.

[Eerie]

meiza - 18/3/2008  1:47 PM

It's practically useless to pretend a gas station model as there is only one customer, NASA. Nobody else has enough bucks to buy any significant amount of propellants in LEO every year.

Well, it could be International Space Depot...

[clongton]

meiza - 18/3/2008  2:47 PM

It's practically useless to pretend a gas station model as there is only one customer, NASA. Nobody else has enough bucks to buy any significant amount of propellants in LEO every year.

There are three customers: NASA, ESA and Russia. All would start making use of it once it is available. I would not be surprised to see India and Japan show interest, and once China gets its feet a little wetter, it will also begin to take advantage of it. Once the spacefaring nations begin to grasp what the availability of this technology means to their efforts, they will most assuredly wish to avail themselves of it.

What we are seeing here is the potential, very real potential, for a paradigm shift in how nations view their space efforts, and how smaller nations begin to view their “potential” participation.

The depot can be kept full by any combination of these plus other NGOs like LM, Boeing, and potentially SpaceX if they become successful. This is an enabling technology for everybody’s use.

Yes, it will begin slowly. But that will change. There is simply far too much advantage to making use of the depot to ignore it, especially if other nations are using it.

[clongton]

Eerie - 18/3/2008  2:56 PM

Well, it could be International Space Depot...

As good a name as any.

[meiza]

How much and how often are you envisioning for this international space depot (good name!)?

I'm afraid it doesn't work out. We are talking some tens of tonnes per year at most. Existing satellite launchers can put that up in one piece, no depot is needed.
ESA and RSA are drafting some lunar co-operation already which includes no heavy lift, but the missions are intermittent and small. These space agencies simply don't have enough money. NASA would have, but it has decided to put it on heavy lift so no depot there either.

Also, Europe has been burned before when it has relied on mission critical US hardware to achieve something in space, although there are many successes too.

[Eerie]

Meiza, if Depot makes space flight cheaper, use rate may rise.

There must be some point where investment in space will start to pay off, after all.

Problem is, we don`t know...

[meiza]

What do you technical people view as minimum gauge issues in propellant depots? The docking hardware and sensors and maneuvering capability for the tankers costs mass, and the depot of course has to be insulated, possibly actively cooled and probably have even more docking hardware etc.

The major space powers have existing launchers that can put about 20 - 25 t to LEO (Ariane 5, Delta IV heavy, the future CZ-5). About 10 t of that could be propellant if you try to cram everything into one launch. ESA has looked at 23 t mini EDS stages, launched by Ariane and Angara rockets. Chain two and you can send a reasonable payload towards the moon.

So, I'd hunch that these things mean that a propellant depot is
-certainly only a useless complication if it holds only 10 t of propellants
-quite useless if it holds only 20 t
-breakeven and possibly redundant perhaps with 40 t of propellants (2 Ariane class launches)

So, if we assume that the players could afford an architecture using 40 t of LEO propellant once a year (the budgets would probably have to increase from current), what would the savings be in using a propellant depot instead of chained mini-EDS stages? You could perhaps get partners to launch propellant with small rockets 7 t or 10 t at a time if we want to up flight rates, meaning 4-6 propellant launches, or 8-12 flights a year. That's nice, probably a significant percentage in LEO equivalent mass compared to the total worldwide commercial launch market.

A Soyuz class 8 ton RLV would gain perhaps 12 flights per year from this.

I don't think it's very comparable to the 100-200 ton NASA market that would have transformed space launch.

[A_M_Swallow]

Lets try a mission that will be hard without a space depot - sending 100 metric tons to Low Lunar Orbit in one go.
ISP of RL-10-2 burning LOX & LH2 is 462
delta-v LEO to LLO = 4.04 km/s

m0 = m1 exp(delta-v/(ISP * g)) = 100 * exp( 4040 / (462 * 9.81)) = 244 mT
Mass of fuel = 244 - 100 = 144 mT

This option would require 3 launches in a short timescale without a fuel depot.
I am not certain that KSC is set up for machining gunning off Ares-V/J-232 rockets.

[clongton]

No one pretends that this will be an easy transition or without its false starts. The United States, Russia, ESA, China, India and Japan will all eventually make use of the capability, whether by using International depots or native ones, but most likely by a combination of the two. It will be a time-consuming process, not a reportable “event”.

Missions are sized what they are today because that’s what the available launch vehicles will support. The prime driving force behind mission limitation is the requirement to carry the mission propellant up the earths gravity well together with the mission spacecraft, a mass that typically accounts for anywhere from 50% to 80% of the total mission mass. Eliminating the necessity to do that will transform the way mission planners design their spacecraft. Making that transformation will take time. This will not happen overnight.

A prime example of how this depot architecture could be of near term value is the Mars Sample Return mission. The mission planners are having a very difficult time of it trying to match everything they need to make the mission a minimum success with the lift capacity of the available launch vehicles. They will pull this off, but the mission capabilities will be diminished by the necessity of carrying their mission propellant up with the spacecraft. This problem would be totally eliminated if a depot were available for the spacecraft to fly to before departing for Mars. The mission planners would be able to concentrate more on creating capability than on down-sizing to the launch vehicle.

That’s just a single example that pops to mind. I’m sure if I thought about it there would be others as well. But as time moves forward, and new missions are planned (a long and time-consuming process), the availability of a depot will begin to take on more and more significance in how missions are put together. Possessing the ability to use the launch vehicle to loft the spacecraft instead of spacecraft and propellant will change the face of exploration forever. Purchasing the propellant on-orbit will always be more expensive than purchasing the propellant on the ground, but the tradeoff is the ability to create spacecraft that are freed from the necessity of including their mission propellant in their liftoff weight. Spaceflight will never be the same again.

Edit: Just did a quick calculation on a particular launch system, no names being mentioned because I do NOT want to change the topic, but as a second example ONLY, if the current 2-launch solution to the moon were to have a propellant depot available, the mission could be done with a single launch. Not only that, but the size of the mission works out thus:
18.6mT EDS
20.2mT CEV
182.0mT LSAM gross
231.0mT depot propellant
All pushed thru TLI at a dV of 3150 meters per second
That puts an approximate 60mT LSAM on the lunar surface, 3x NASA's desired target for Ares-V mission.

Please note that these numbers are "ballpark" numbers, not exact, but they are not far off what "exact" numbers would produce.

That's an example of how a depot can completely change the dynamics of space exploration. Any launch vehicle can see these types of efficiencies of scale if they make use of a depot architecture.

[A_M_Swallow]

clongton - 18/3/2008  9:03 PM

A prime example of how this depot architecture could be of near term value is the Mars Sample Return mission. The mission planners are having a very difficult time of it trying to match everything they need to make the mission a minimum success with the lift capacity of the available launch vehicles. They will pull this off, but the mission capabilities will be diminished by the necessity of carrying their mission propellant up with the spacecraft. This problem would be totally eliminated if a depot were available for the spacecraft to fly to before departing for Mars. The mission planners would be able to concentrate more on creating capability than on down-sizing to the launch vehicle.

I wonder if a firm could charge the Mars Sample Return say 90% of the cost of building the fuel depot?  Then they would only need a second sale to complete paying for the construction and to make a profit.

[Eerie]

Clongton, again, if your LV is too small, why not launch two? It will be the same thing exactly, only you won`t need to create the fuel depot first.

[clongton]

Eerie - 18/3/2008  4:44 PM

Clongton, again, if your LV is too small, why not launch two? It will be the same thing exactly, only you won`t need to create the fuel depot first.

Eerie - see the edit to my last post.

[Eerie]

I still don`t get it.

At some point, you send rockets with tanks, to fuel the depot, right? You can`t avoid it.

Why don`t you just leave those tanks in a special orbit? You will get lots of small depots instead of a big one, and spare building a dedicated spacestation. Your spacecraft then will suck fuel from them in turn, until it is fueled, and the empty tanks will be deorbited, as they will anyway, even if the depot exists.

Of course, you could just keep all those rockets on Earth and launch them when necessary. This way, you store the fuel on Earth, which is much easier (because you can just create it on the spot ).

Problem is, both approaches needs MORE LAUNCHES. Once you will have MORE LAUNCHES, you can build depots, shmepost and whatever. It is not about depot at all, but about economy of scale...

[clongton]

Eerie - 18/3/2008  5:14 PM

I still don`t get it.

At some point, you send rocket to fuel the depot, right? You can`t avoid it.

Why don`t you just leave those tanks in a special orbit? You will get lots of small depots instead of a big one, and spare building a dedicated spacestation.

Of course, you could just keep all those rockets on Earth and launch them when necessary...

When you go to a gas station with your car, do you just fill your tank from the pump, or do you go all around the property picking up 1 liter containers, one at a time and dumping them into your tank and then going to find the next one and dumping it into the tank, and the next one and dumping it into the tank, and the next one and dumping it into the tank, and the next one and dumping it into the tank, and the next one and dumping it into the tank and so on etc, etc, etc?

It's a gas station, not a warehouse. And it is not a spacestation; it is unmanned and fully automated.

[Eerie]

clongton - 18/3/2008  4:19 PM
When you go to a gas station with your car, do you just fill your tank from the pump, or do you go all around the property picking up 1 liter containers, one at a time and dumping them into your tank and then going to find the next one and dumping it into the tank, and the next one and dumping it into the tank, and the next one and dumping it into the tank, and the next one and dumping it into the tank, and the next one and dumping it into the tank and so on etc, etc, etc?

It's a gas station, not a warehouse. And it is not a spacestation; it is unmanned and fully automated.

Entirely wrong analogy. Gas station is fueled by huge trucks or by pipes. In translation to space, by Super Heavy LV or by Space Elevator.

Space depot will be fueled by "1 liter containers", because fueling it by Super Heavy LV is pointless - with it you can just lift your spacecraft already fueled.

So it is actually easier to fuel directly by "1 liter containers", without bringing them through the Depot first. "Cut the middleman", you know?