Author Topic: Hypergolic depots versus Cryogenic depots  (Read 65023 times)

Offline jongoff

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Re: Hypergolic depots versus Cryogenic depots
« Reply #140 on: 05/10/2013 06:32 pm »
Hmmmm....  I detect another reason that SpaceX is interested in methane as a fuel.

A quick bit of googling shows (that with proper orientation of shading and radiation coolers of the sun and the earth) that temperatures capable of keeping both O2 and CH4 liquid in LEO.

Not true for H2: you'd always have gas boiling off in LEO: keeping things at 20K is hard to do....

While it's true that you must have active cooling to get zero LH2 boiloff in LEO...it's also somewhat beside the point. Unless you're using something propellantless for stationkeeping (like say an electrodynamic tether), you're going to be consuming propellant to keep the depot boosted to the right altitude. LH2 has the nice property that you can use it to soak up heat and then eject it as a warm gas for stationkeeping with Isp's almost as good as hypergols, IIRC.

Ie if you compare the consumables rate for a hypergolic depot and a LOX/LH2 depot, they might not be as different as you would think (unless you have a propellantless or high-Isp stationkeeping system, both of which require almost as much power as zero boiloff systems).

You can't just look at one subcomponent of the depot--you need to look at the whole system.

~Jon

Online mmeijeri

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Re: Hypergolic depots versus Cryogenic depots
« Reply #141 on: 05/10/2013 06:42 pm »
It depends on the altitude of the depot. A 500 km altitude depot wouldn't need a lot of reboosting. That said, the choice between hypergolics and LOX/LH2 won't be made on the basis of reboost requirements, but more likely on the basis of Isp and TRL. It does seem likely the boil-off vs reboost argument could influence the optimal altitude for a LOX/LH2 LEO depot once you decided to build one. I suspect the optimal altitude for a LOX/LH2 depot would be lower than that for a hypergolics depot. Then again, I'm not convinced LEO hypergolics depots would be all that useful, while LOX/LH2 ones certainly would be.
« Last Edit: 05/10/2013 07:47 pm by mmeijeri »
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Offline jg

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Re: Hypergolic depots versus Cryogenic depots
« Reply #142 on: 05/10/2013 07:09 pm »
It depends on the altitude of the depot. A 500 km altitude depot wouldn't need a lot of reboosting. That said, the choice between hypergolics and LOX/LH2 won't be made on the basis of reboost requirements, but more likely on the basis of Isp and TRL. It does seem likely the boil-off vs reboost argument could influence the optimal altitude for a LOX/LH2 LEO depot once you decided to build one. I suspect the optimal altitude for a LOX/LH2 would be lower than that for a hypergolics depot. Then again, I'm not convinced LEO hypergolics depots would be all that useful, while LOX/LH2 ones certainly would be.

Remember that such a depot will be (relatively) dense; it's drag to mass ratio will be lower than for many/most satellites I suspect, and orbits decay more slowly.  So I don't think station keeping losses are going to be very high. But figuring out how low one can go is beyond my current knowledge.

Certainly LOX/LH2 or LOX/CH4 depots are likely to be very useful, particularly once one can supply them from somewhere other than the surface of our planet...

Offline jongoff

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Re: Hypergolic depots versus Cryogenic depots
« Reply #143 on: 05/10/2013 07:52 pm »
It depends on the altitude of the depot. A 500 km altitude depot wouldn't need a lot of reboosting. That said, the choice between hypergolics and LOX/LH2 won't be made on the basis of reboost requirements, but more likely on the basis of Isp and TRL. It does seem likely the boil-off vs reboost argument could influence the optimal altitude for a LOX/LH2 LEO depot once you decided to build one. I suspect the optimal altitude for a LOX/LH2 would be lower than that for a hypergolics depot. Then again, I'm not convinced LEO hypergolics depots would be all that useful, while LOX/LH2 ones certainly would be.

One problem with higher depot altitudes is that they impose a higher delta-V penalty on the depot architecture. Raising apogees (so long as the line of apsides is oriented right) doesn't really cost you anything on net, but raising the perigee (to circularize at a higher altitude for the depot) does have a net cost delta-V wise. It's not a ginormous penalty for most high-performance expendable launch vehicle, but it is a non-negligible one that needs to be factored in.

~Jon

Offline jongoff

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Re: Hypergolic depots versus Cryogenic depots
« Reply #144 on: 05/10/2013 07:58 pm »
Certainly LOX/LH2 or LOX/CH4 depots are likely to be very useful, particularly once one can supply them from somewhere other than the surface of our planet...

Definitely agreed, but I think that unless you're talking about small depots specifically focused on existing, unmanned customers (ie satellite refueling), that LOX/LH2 or LOX/CH4 depots make sense in the near-term as well. Going with a human spaceflight-sized hypergolic depot is going to involve designing and qualifying all new large hypergolic transfer stages and landers, where with LOX/LH2 stages that are close to what you need are either in operations (Centaur, DCSS) or in development (ACES, Startolaunch upper stage). I'd rather see money spent on getting depot/tanker technology matured enough to use existing high performance stages as transfer stages or as part of landers, than to see money spent on stages that are really only an interim step, as nature has an unfortunate habit of "freezing" "accidents".

~Jon

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Re: Hypergolic depots versus Cryogenic depots
« Reply #145 on: 05/10/2013 08:11 pm »
One problem with higher depot altitudes is that they impose a higher delta-V penalty on the depot architecture. Raising apogees (so long as the line of apsides is oriented right) doesn't really cost you anything on net, but raising the perigee (to circularize at a higher altitude for the depot) does have a net cost delta-V wise. It's not a ginormous penalty for most high-performance expendable launch vehicle, but it is a non-negligible one that needs to be factored in.

I've never been convinced by that argument and the ULA people seem to have abandoned it. Unless you have inevitable boil-off, the higher reboost requirements of low orbits are a disadvantage, and if you do have it, that boil-off is still a bad thing. Delta-v differences are a secondary correction to that. It's more accurate to say that partially offsetting boil-off losses by slightly lowering delta-v is a clever trick, rather than high orbits being a problem.
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Offline JohnFornaro

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Re: Hypergolic depots versus Cryogenic depots
« Reply #146 on: 05/11/2013 01:58 pm »
Does anybody know what are the actual numbers comparing the Isp of warm gas H2 and hypergols?
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Offline deltaV

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Re: Hypergolic depots versus Cryogenic depots
« Reply #147 on: 05/11/2013 11:14 pm »
Does anybody know what are the actual numbers comparing the Isp of warm gas H2 and hypergols?

Download the Rocket Propulsion Analysis Lite tool and you can generate numbers like this to your heart's content: http://www.propulsion-analysis.com/downloads.htm . Its default thermodynamic database seems to only include H2 (gas) down to 200K, so you either need to extend that database yourself or limit the expansion ratio to keep exhaust temperature above that.
« Last Edit: 05/11/2013 11:19 pm by deltaV »

Offline Robert Thompson

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Re: Hypergolic depots versus Cryogenic depots
« Reply #148 on: 05/12/2013 09:52 am »
http://spacefellowship.com/news/art33618/engineers-focus-on-super-cold-propellants.html
Engineers Focus on Super-cold Propellants Klaus Schmidt Fri May 10

Offline JohnFornaro

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Re: Hypergolic depots versus Cryogenic depots
« Reply #149 on: 05/12/2013 03:42 pm »
Does anybody know what are the actual numbers comparing the Isp of warm gas H2 and hypergols?

Download the Rocket Propulsion Analysis Lite tool and you can generate numbers like this to your heart's content: http://www.propulsion-analysis.com/downloads.htm . Its default thermodynamic database seems to only include H2 (gas) down to 200K, so you either need to extend that database yourself or limit the expansion ratio to keep exhaust temperature above that.

Thanks.  I put it in my "favorites" folder for later review.
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Offline SpunkyEnigma

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Re: Hypergolic depots versus Cryogenic depots
« Reply #150 on: 05/14/2013 05:19 am »
What about a BYOH (Bring Your Own Hydrogen) LOX only depot?

Long duration with minimal or no boil off in LEO.  Methane and hydrogen are significantly lighter and could be more easily included in the IMLEO mass of the main mission. 

And heck, if we get cheap oxygen off of the moon or an asteroid we got an infrastructure to handle it already.  Oxygen is easier to get to liquid phase using relatively low energy systems.

It's the heaviest part of the fuel regime.  Optimize for that problem. 

Might as well haul hypergolics up as well to the same depot.  There are tons of mission scenarios that could benefit greatly from having a ready supply of one or both of these two materials in LEO, L2 or even in Mars orbit.... 

Just remember to BYOH.

Offline JohnFornaro

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Re: Hypergolic depots versus Cryogenic depots
« Reply #151 on: 05/14/2013 01:46 pm »
What about a BYOH (Bring Your Own Hydrogen) LOX only depot?

Long duration with minimal or no boil off in LEO.  Methane and hydrogen are significantly lighter and could be more easily included in the IMLEO mass of the main mission. 

And heck, if we get cheap oxygen off of the moon or an asteroid we got an infrastructure to handle it already.  Oxygen is easier to get to liquid phase using relatively low energy systems.

It's the heaviest part of the fuel regime.  Optimize for that problem. 

Might as well haul hypergolics up as well to the same depot.  There are tons of mission scenarios that could benefit greatly from having a ready supply of one or both of these two materials in LEO, L2 or even in Mars orbit.... 

Just remember to BYOH.


Of course, there will always be a need for hypergolics in the cis-lunar arena.  Kinda like there will always be a need for lawn mowers in the cis-yard arena.

The main problem with BYOH, assuming you mean to bring it from Earth, is that the tanks have to be so physically large, and once you bring 'em up to your O2 Depot, you still have to deal with boil-off.  I'm not sure what you think the savings are.
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Offline SpunkyEnigma

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Re: Hypergolic depots versus Cryogenic depots
« Reply #152 on: 05/14/2013 05:13 pm »
What about a BYOH (Bring Your Own Hydrogen) LOX only depot?

Long duration with minimal or no boil off in LEO.  Methane and hydrogen are significantly lighter and could be more easily included in the IMLEO mass of the main mission. 

And heck, if we get cheap oxygen off of the moon or an asteroid we got an infrastructure to handle it already.  Oxygen is easier to get to liquid phase using relatively low energy systems.

It's the heaviest part of the fuel regime.  Optimize for that problem. 

Might as well haul hypergolics up as well to the same depot.  There are tons of mission scenarios that could benefit greatly from having a ready supply of one or both of these two materials in LEO, L2 or even in Mars orbit.... 

Just remember to BYOH.


Of course, there will always be a need for hypergolics in the cis-lunar arena.  Kinda like there will always be a need for lawn mowers in the cis-yard arena.

The main problem with BYOH, assuming you mean to bring it from Earth, is that the tanks have to be so physically large, and once you bring 'em up to your O2 Depot, you still have to deal with boil-off.  I'm not sure what you think the savings are.

I'm thinking that since LOX is the heavier item and it's much more storable than hydrogen, that it would make more sense and is doable today as opposed to waiting for ZBO long term hydrogen storage.  For HSF you need oxygen for life support as well.  We also don't have to choose between methane or hydgrogen for the fuel side of the depot.  Heck if you overload with RP-1 you could top off a F9 2nd stage as well :)

The trick of course is getting quick rendezvous and quick loading of LOX into the spacecraft to limit H2 boiloff.

Offline JohnFornaro

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Re: Hypergolic depots versus Cryogenic depots
« Reply #153 on: 05/17/2013 01:21 pm »
Quote from: Spunky
The trick of course is getting quick rendezvous and quick loading of LOX into the spacecraft to limit H2 boiloff.

That's fine, but still, that approach would crimp your mission style for re-fueling.

I have a five gallon can of gas for my garden tractors.  I fill it up, and over the course of a month or two, I can till the garden, mow the grass, haul away brush, and many other "missions" here at the Fornaro Compound.  I keep the cap on, so I don't have to worry about "boiloff".   What you're suggesting, in my analogy, is that as soon as I fill up the five gallon can, I need to fill up the tractors and use it.

One of the big benefits of having some sort of prop depot up there is to decouple the exquisite timing of the mission from the act of re-fueling.  This allows scheduling flexibility, which is a very desirable trait.

The way I see it, using the H2 supply as you suggest, is a limiting factor which works against a depot.
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Online mmeijeri

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Re: Hypergolic depots versus Cryogenic depots
« Reply #154 on: 05/17/2013 01:30 pm »
Bring your own hydrogen can be made to work if necessary, and could be part of early cryo depots. And because LH2 is such an excellent coolant, it might even be useful for the LOX in a kerolox depot. Then again, I keep wondering about cryocoolers, which have flown in space. I can understand why you wouldn't want to put one on your transfer stage, but not why you would want to keep it off a depot. And they're available today.
« Last Edit: 05/23/2013 05:02 pm by mmeijeri »
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Offline A_M_Swallow

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Re: Hypergolic depots versus Cryogenic depots
« Reply #155 on: 05/23/2013 06:55 pm »
Bring your own hydrogen can be made to work if necessary, and could be part of early cryo depots. And because LH2 is such an excellent coolant, it might even be useful for the LOX in a kerolox depot. Then again, I keep wondering about cryocoolers, which have flown in space. I can understand why you wouldn't want to put one on your transfer stage, but not why you would want to keep it off a depot. And they're available today.

Small cryocoolers are certainly available.  I suspect that a bigger one is needed to cool the hydrogen tank of a Centaur.  3.05 metre diameter by ~12.68/2 metres length.

Offline paulthew

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Re: Hypergolic depots versus Cryogenic depots
« Reply #156 on: 07/28/2013 05:17 am »

 My main issue is starting with existing technology, not hypergolics per se.

And we already have, with regard to ISS. (Sorry, I'm repeating myself.) Maybe the thread title should be "Hypergolic Depots and Cryogenic Depots."  :)


Amen, brother!

Offline Nilof

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Re: Hypergolic depots versus Cryogenic depots
« Reply #157 on: 08/01/2013 03:02 pm »
If your propellant depot is at the edge of Earth's gravity well, you have the option of refueling but you'll need more propellant for the same trip using chemical because you can't use the Oberth effect.

I would suggest a focusing on an oxidizer depot in LEO. If you're using LH2/LOX the bulk of the mass to be launched is going to be the LOX, while the LH2 could be launched in vast quantities in a single launch. Hydrocarbons are also typically much lighter than the oxidizer they're burned with, though apart from methane they are not cryogenic and fairly easy to store.

One possible option for orbital storage is a LOX/N2O mixture. Or more generally, mixing LOX with another oxidizer so you can store it at a higher temperature. You'll likely get a hit to your Isp, but from the numbers in my link you could get a better performance than with hypergolics.

EDIT: and of course someone else mentioned bring your own hydrogen right before my post. Serves me right for not reading through the entire thread before posting.
« Last Edit: 08/01/2013 03:03 pm by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Online mmeijeri

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Re: Hypergolic depots versus Cryogenic depots
« Reply #158 on: 08/01/2013 04:12 pm »
If your propellant depot is at the edge of Earth's gravity well, you have the option of refueling but you'll need more propellant for the same trip using chemical because you can't use the Oberth effect.

You can still use the Oberth effect if you do a powered flyby of Earth. This can be done especially efficiently if you also do a flyby of the moon. You would have to cross the van Allens two times more, but that is not a problem for a Mars Transfer Vehicle which needs a radiation storm shelter anyway.

Quote
I would suggest a focusing on an oxidizer depot in LEO. If you're using LH2/LOX the bulk of the mass to be launched is going to be the LOX, while the LH2 could be launched in vast quantities in a single launch. Hydrocarbons are also typically much lighter than the oxidizer they're burned with, though apart from methane they are not cryogenic and fairly easy to store.

LOX depots are good, but they are not the easiest place to start. Similarly, LEO is not the easiest place to start.
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Offline JohnFornaro

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Re: Hypergolic depots versus Cryogenic depots
« Reply #159 on: 08/01/2013 04:47 pm »
If your propellant depot is at the edge of Earth's gravity well, you have the option of refueling but you'll need more propellant for the same trip using chemical because you can't use the Oberth effect.

I would suggest a focusing on an oxidizer depot in LEO. If you're using LH2/LOX the bulk of the mass to be launched is going to be the LOX, while the LH2 could be launched in vast quantities in a single launch. Hydrocarbons are also typically much lighter than the oxidizer they're burned with, though apart from methane they are not cryogenic and fairly easy to store. ...

The LH2 requires the most volume.  The trade is not exactly equal.  True, O2 could be used with several different fuels, kerosene and H2 being the two most promising.  I ould see the possibility of a four tank system up there.

Hypergols, H2, Kero, O2.

But that would be a future refinement, which would wait untill all three propellant compounds could be made available, not to mention space tugs designed for one or another propellant.
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