Propellant Depots - General Discussion

[MichaelF]

Unattended depots? Maintenance/PMCS/Troubleshooting comes from where?

This would be a giant concern if someone was betting their $Billion+ probe or mission on their being fuel ready to go....

Secondly, where are these fuel-intensive missions headed?  There are none (nor any requirement for such) in the forseeable future.

[Bill White]

Wouldn't micro-gravity transfers of cryogenic fuels (H2 in particular) be a real bleep to accomplish?

Dry, rocket engines are rather light. An RL-10B-2 only masses ~664 pound (dry).

Rather than pump fuel as the Air Force does from a tanker to an F-15 (non-cryo fuel, inside the atmosphere, with gravity) why not stockpile Earth-built Centaur stages (or something similar) at the depot and simply swap out the entire propulsion module?

If it is a crewed depot, the engines that are turned in are cleaned and refurbished and mated to another filled fuel tank (filled at the depot's leisure rather than in "underway replenishment" mode) prior to the arrival of a vessel needing to tank up on fuel.

This way tanks and engines could be thoroughly inspected between uses.

= = =

What types of spacecraft do you foresee using a depot? Since on orbit fuel transfer has yet to be invented perhaps it would be easier to invent modular spacecraft that permit rapid swapping out and replacement of filled fuel tanks  (like glass milk bottles) or filled fuel tanks pre-mated to fully inspected and refurbished engines.

[A_M_Swallow]

MichaelF - 19/3/2008  4:08 AM

Secondly, where are these fuel-intensive missions headed?  There are none (nor any requirement for such) in the forseeable future.

The missions are headed to the Moon and Mars mostly.  Human transfer vehicles, manned landers and cargo landers are the vehicles that need to use chemical fuel.

As for the shortage of missions that is one of the public embarrassments we are trying to fix.

[tankmodeler]

MichaelF - 18/3/2008  11:08 PM

Unattended depots? Maintenance/PMCS/Troubleshooting comes from where?

From the ground, like they do now for ISS. You use the RMS and an SPDM to swap out ORUs of any of the systems, including things like docking collars, tanks, cooling systems, RMS, whatever.

We showed we could do this for the Hubble Servicng Mission using SPDM to remove instruments that were never designed to be opened on orbit. Surely we could plan to do it for everything on the spacecraft if given the chance right from the start. Once the business case closes for a depot, you would design it to be as robust as possible with all the systems being capable of being replaced on orbit. Spares would be delivered by the unpressurised equivalent of an HTV or ATV.

Secondly, where are these fuel-intensive missions headed?  There are none (nor any requirement for such) in the forseeable future.

The point is all missions are fuel intensive if only because you've been forced to buy a Delta IV instead of a Delta II because you are lugging your GEO fuel up with the spacecraft. If you are bringing your fuel up with the spacecraft, then you are buying a larger launcher than you need to if the fuel is on orbit already.

[tankmodeler]

One thing that we haven't touched on is the use of the truly low cost launcher to pitch fuel at the depot.

We've been talking Atlas, Jupiter, Delta as if this design paradigm is the only way.

If there was a market for fuel in orbit, I can see a smart group looking at the whole launcher business & thinking "there has to be a better way." Maybe there is. It's been mentioned elsewhere before, but I'm going to do it again here. Modern launchers have grown up in the realm of really expensive payloads and the need for ultimate reliability to prevent these payloads, be they breathing or not, from being spread over a greater part of the earth's surface. If you are launching fuel and nothing but fuel, do you really care if you pitch some of it in the ocean if the launcher costs are cheap? I mean really cheap. No, cheaper than that. I'm talking cheap as dirt. So cheap that you can afford to lose one in 20 and still sell fuel at a profit.

Think a slightly modified V2 with commercial grade electronincs, a GPS based nav system, orbital control from the ground, pressure fed engines and a 2 tonnes of fuel to orbit payload. The Germans managed to launch roughly 2500 V2s in 8-9 months with about a 60% success rate in the middle of losing a war. If you had a stable economy and business behind you and were tooled up to make 100 per year & launch them from a TEL with a ground crew of 100 (like a V2 launch battery) how cheap could you really make them if you were willing to lose 1 in 20? Launch 100, lose 5, get 190 mT of go juice to orbit per year.

If a nameless 100mT launcher is estimated at about $4500/kg in orbit, then I have to be able to launch a 2 tonne payload for something like $8.55 million, including launcher, development ammortisation and profit and assuming a loss rate of 1 in 20.

Anyone want to design the "Cessna 172 of space" around those numbers? It's really tight, in fact, obscenely tight in aerospace terms. But might it be able to be done?

If you are buying propellant at $7500/kg you get to play with $14.25 million per launch.

Anyone? Anyone? Beuler? Anyone?


Paul

[tankmodeler]

OK, Rick, I had a think on your depot and herby submit one of my own.

- I've separated the docking & control centre from the tankage for safety. Don't want a slighly missed docking attempt to take out the depot.
- There's an RMS to berth the vehicles to the depot.
- An OMS and RCC at the back for stationkeeping Mounting the RCC at the control Centre may be better from a maintenance POV. Time for a trade study on that one.
- The docking port is universal, propellants in and out through the same mechanism. This doesn't mean the same lines are used, just that they all congregate at one port. Slide up, whip out the credit card (first, please, don't want any "dine & dash" customers) and then take your fill of whatever is on tap.
- There are PDGFs all over the depot to allow the RMS to roam around & do maintenance as required.
- All of the depot systems are in ORUs mounted at the control centre. They can be swapped in or out as needed. This includes things like the radiators & solar panels.
- All of the berthing hardware is on the depot. The coarse work is done by the vehicle but the depot does the final little bit, including berthing the vehicle to the depot.

Whattaya think?

Paul

[Eerie]

clongton - 18/3/2008  7:58 PM

It's just like buying gasoline at a filling station. I could care less how many tankers, pickup trucks or station wagons it takes for the station owner to fill his storage tanks. I just buy the gas. He operates on completely different economies of scale than I do so the cost of the gas to me is very different than if I were dragging a 500 gallon tank of gasoline around with me hitched to my ball hitch trailer hitch.

Clongton, even if station owner (NASA+?) wil operate on "completely different economies of scale" - and he won`t, because there is no market - there is still the question of why to keep the fuel in orbit and not on Earth. With "completely different economies of scale" you can just store a surplus of rockets.

[jeff.findley]

Don't forget that if NASA actually goes back to the moon that it has to "invent" something for the international partners to do.  The fuel depot provides a handy way to do this.  You get Russia, ESA, and JSA to provide "free" fuel for the depot in exchange for their astronauts getting to participate in lunar missions.  One of the unwritten rules for this sort of thing is that money ought not cross international borders when doing this sort of thing.  This is why ESA and JSA provide their own labs for ISS and will be providing their own supply vehicles (ATV and HTV).  This isn't the most *globally* cost efficient way of doing things since now you have three unmanned resupply vehicles for ISS (four if you think NASA COTS will work), but it works politically.

Also, this keeps the international partners off the "critical path" since you could still use the most cost efficient US launch vehicles to provide fuel for the depot, if needed.  US launch vehicles for this task could be Jupiter, Atlas, Delta, Falcon, or whatever is cheapest and/or available.  NASA (and the US in general) tends to be a bit of a control freak when it comes to internaional "cooperation", so they won't want anyone but themselves on "the critical path".

[Eerie]

Did any international partner approach NASA to participate in the Constellation project?

Does NASA wants any international partners?

[MichaelF]

jeff.findley - 19/3/2008  9:52 AM

Don't forget that if NASA actually goes back to the moon that it has to "invent" something for the international partners to do.  .

No, it doesn't.

[meiza]

Okay, this thread title is supposed to be general discussion about propellant depots by the title but has been so far about a single narrow idea: NASA flying a heavy lifter launcher and using a propellant depot on the side. This isn't very sensible.

What propellant depots are really the best at, is decoupling beyond LEO mass from launcher size, thus eliminating the need for a heavy lifter altogether. The three different spacecraft of ESAS origin, the Orion capsule, the Artemis lander and the EDS stage all weigh less than 25 t empty, which can be lifted by launchers that exist today and have flown many times.

You could achieve similar things by chaining up mini-EDS stages, but that has some problems, like boiloff, bad mass ratio and reliability regarding so many stagings and ignitions. Not insurmountable. A propellant depot can also avoid boiloff by being well insulated, having a sunshield or even being actively cooled: it can afford this mass since it is not going to the moon itself.

Also, with numerous redundant launchers available for sending tankers to the depot, the approach is inherently improvable. You get high flight rate which means low costs. You can enter better launchers that are cheaper or more reliable for example. This is because the mass launched at one time is a reasonable chunk like ten or twenty tonnes, that the launchers can still have other viable missions as well. Ultimately, a propellant depot could be a market for a reusable launch vehicle that could drop the cost of space travel substantially. The biggest hurdle for RLV economics has been lack of market and thus a low flight rate. A depot could change all that. Only NASA has enough money to buy so much propellant on orbit that an RLV becomes feasible.

If NASA builds a heavy lifter, all this becomes moot. Most of the propellant is lifted by the heavy lifter, and thus it makes no sense to make a propellant depot for just the few intermittent refueling tonnes, it is much cheaper to launch another heavy lifter. A purpose built heavy lifter will have big fixed costs and low launch rate, meaning adding more launches will not add much more cost anymore, the savings were missed when the thing was built in the first place.
Also, a purpose built NASA heavy lifter is not an agile system which could easily be improved and it is in practice an irreplaceable monolith. Griffin himself has said how the launchers that are built now will stick for decades. (Well, that has happened with the exception of Saturn.)

Most of the arguments have been hashed before, for example in this thread about an alternative for ESAS replacing heavy lift with a liquid oxygen propellant depot:
http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=4047

To put the effects of high fixed costs and low flight rate in nine words:
No savings from depot unless you scrap heavy lift.

[MichaelF]

tankmodeler - 19/3/2008  12:27 AM

MichaelF - 18/3/2008  11:08 PM

Unattended depots? Maintenance/PMCS/Troubleshooting comes from where?

From the ground, like they do now for ISS. You use the RMS and an SPDM to swap out ORUs of any of the systems, including things like docking collars, tanks, cooling systems, RMS, whatever.
.

So, a kink (rather, frozen propellant, most likely) in the fuel line will cost you a manned launch (including booster, spacecraft and ground infrastructure) and an EVA (or several).

That's an expensive piece of frozen gas.

It's also a bullet in the head of your business model.  Never mind the expense of your maintenance mission (which is going to gobble up your profit margin, or budget, if your NASA),  you've got an on-orbit customer waiting on fuel (who might have missed his mission window).  He's all frowny, and potential customers take note.

Also, launches, even "emergency" missions, aren't exactly quick.  Months.  If you're NASA (who operate on a budget),  a non crew-fatal emergency could wait years until a launch is scheduled (Hubble, to reference your example).  

[MichaelF]

meiza - 19/3/2008  11:13 AM


What propellant depots are really the best at, is decoupling beyond LEO mass from launcher size, thus eliminating the need for a heavy lifter altogether. .

This assumes that no missions will require the volume that Ares V can provide (fairing diam.).  This is not a safe assumption.

If nothing else, it artificially constrains missions.

I can take many times the mass of my couch across town in my car, given 4-5 trips.  I still can't take my couch across town in my car (which was a pain, the last time I moved.  I swear I'm getting a furnished apartment next time).

[meiza]

Well the LSAM with the hydrogen tanks is the only diameter constrained thing (EDS is just a stage and Orion is 5 m in diameter), if it wants to be wide for a vertical landing with a low climb to the surface. But there are multiple ways to design around that, for example the LM Centaur-style dual axis lander with one big common bulkhead tank (good for reduced boiloff!), or a lander with a spherical tank on each side of the hab, launched sideways.
I've heard that EELV:s can be fitted with quite big fairings if need be, they have good control authority.

Having a monolithic launcher constrains missions actually more, as you can't grow or shrink so easily.

[William Barton]

It seems to non-engineer me, for a fuel depot to really decouple mass in LEO from LV payload, it would need to be part of a permanent manned orbital assembly facility. Pretty much what Von Braun proposed close to sixty years ago.

[Gary]

Lugging fuel from the Earth to LEO for a depot makes no sense. What makes more sense is fuel production on the moon and a lunar fuel depot with regular trips of tanker craft from the moon to the depot keeping it topped up.

[Bill White]

MichaelF - 19/3/2008  11:27 AM

meiza - 19/3/2008  11:13 AM


What propellant depots are really the best at, is decoupling beyond LEO mass from launcher size, thus eliminating the need for a heavy lifter altogether. .

This assumes that no missions will require the volume that Ares V can provide (fairing diam.).  This is not a safe assumption.

If nothing else, it artificially constrains missions.

I can take many times the mass of my couch across town in my car, given 4-5 trips.  I still can't take my couch across town in my car (which was a pain, the last time I moved.  I swear I'm getting a furnished apartment next time).

This also assumes NASA can retain its funding levels once the heavy lift infrastructure and related jobs are scrapped.

Perhaps the fuel depot advocates should focus on non-NASA users and non-NASA funding sources.

[Smatcha]

meiza - 19/3/2008  9:13 AM

What propellant depots are really the best at, is decoupling beyond LEO mass from launcher size, thus eliminating the need for a heavy lifter altogether.

Actually no, that is not what they are good at nor is that debate relevant to the topic of propellant depots.  What propellant depots enable is the staging of the dominate component of mission mass which is cheap propellant.  The extremely expensive dry spacecraft then rendezvous with the depot taking on the amount of propellant required by the mission.

Aerial tankers don’t eliminate the need for large bombers or smaller jet fighters they both have a utility independent of the tanker.  The aerial tanker like the propellant depot is a force or mission multiplier respectively.

As soon as COTS or EELV can beat a $2K/kg variable cost to LEO let me know.

[jongoff]

Michael,

This assumes that no missions will require the volume that Ares V can provide (fairing diam.).  This is not a safe assumption.

In engineering "needs" are very rarely decoupled from what's attainable.   What I mean is, that extra payload volume comes with a *very* large price tag--tens of billions worth of development cost and billions a year worth of operations costs.  Very few missions actually absolutely need a fairing that big.  Some could take advantage of it, if they didn't actually have to pay for the capability themselves.  But when you include the cost of that extra volume into the equation, I bet you most people who say right now that they "need" the extra volume can find much cheaper alternative approaches.

There may be a couple of minor missions that just can't be done without a 10m diameter fairing, but the question becomes, in a world of finite resources, is it better to forgo a few missions in exchange for a much more sustainable, robust, and cost-effective architecture?

If nothing else, it artificially constrains missions.

I can take many times the mass of my couch across town in my car, given 4-5 trips.  I still can't take my couch across town in my car (which was a pain, the last time I moved.  I swear I'm getting a furnished apartment next time).

But that's not the reality of the situation.  Every single component of the ESAS architecture is independently launchable on non-HLV existing boosters.  There's nothing too big to fit in the car.  The reason they want HLVs is because most of the mass involved in moving that couch around is propellant in this case.  Why build a huge new vehicle that is mostly just lifting propellants?

~Jon

[Smatcha]

Gary - 19/3/2008  10:16 AM

Lugging fuel from the Earth to LEO for a depot makes no sense. What makes more sense is fuel production on the moon and a lunar fuel depot with regular trips of tanker craft from the moon to the depot keeping it topped up.

I agree once we have production level of ISRU working on the Moon along with a transportation infrastructure.  At which point one logical position for the propellant depot would be the EML1 and/or EML2 point being serviced by a recycling lunar lander.  The only question (depending on if there is a high enough concentration of ice at the lunar poles) is if we can also get Hydrogen from the Moon or do we need to still send that up from Earth to keep the system cycling away delivering Lunar LOX to the depot.  Of course Hydrogen is not ideal for long duration storage and transfer so LCH4 may be the best choice.  But Carbon is very rare on the Moon as well.

Once you pencil up all the development costs, system start-up mass, and mass required to run this system though the benefits are still their but not as great as you would expect vs. the incremental cost of a few more Jupiter-232 launches.

The incremental cost per additional kg to LEO once you have committed to a heavy launch system is not very high so the high upfront cost for ISRU will have to be traded against a comparatively low cost avoidance.