Sunday, August 14, 2011

Propellent Depots the Pricey NASA Way

According to the August 10th, 2011 article on the NASA website titled "NASA interest in an interplanetary highway supported by Propellant Depots" the National Aeronautics and Space Administration (NASA) is actively working on a potential solution to what some would describe as one of the central problems for space missions.
The Centaur 2A upper stage of an Atlas IIA.

In essence, NASA wants to remove the need to launch self sufficient rockets containing all the fuel required to complete the mission.

To this end, the NASA Human Architecture Team (HAT) is actively working "on a road map towards evolvable demonstrations of propellant depots – with a potential goal of setting up an “interplanetary highway” to enable low cost exploration."

The theory is that the use of space-based propellant depots will allow launch vehicles to lift more hardware into space and travel further in much the same way that the family automobile can travel longer distances after it stops at the local gas station to refuel.

The only problem is that orbital propellant depots already exist. They're commonly called the "upper stages" of existing rockets.

The article even quotes "industry sources" as pointing to the small "Centaur derived depot" as an initial leading candidate for further study in this area.

According to Wikipedia, the Centaur rocket was the world's first high-energy upper stage, burning liquid hydrogen (LH2) and liquid oxygen (LOX). The first successful Centaur flight took place in 1965 as part of a larger Atlas rocket and the Centaur has performed hundreds of successful launches ever since.

These days, the Centaur-3 derivative of the original Centaur continues to be used as the upper stage of the Atlas V EELV rocket, and Centaur manufacturer United Launch Alliance (ULA) continues to explore new configurations and contracts for the venerable rocket.

Space-X Falcon-9. Not cryogenic?
Like propellant depots.

All you'd really need to do to configure a Centaur as a propellent depot would be to attach a spigot to the side so that the fuel could be transferred.

This suggests to me that most of the preliminary R&D for this specific type of propellent depot has already been completed. That would indeed make the overall cost of the project cheaper, but low cost does not seem to be the primary driver for this project.

For example, the current NASA interest is specifically related the so called high energy or "cryogenic" propellents, which NASA favors because of the higher specific impulse of the rocket exhaust. This makes for a higher change in momentum for each amount of propellant used and supposedly makes the rocket go faster and further.

But cryogenic propulsion tanks also take a lot of work to prepare and maintain in a state of readiness both on the ground and in space.

And cryogenic propulsion is not the only or even the most suitable rocket fuel for this sort of activity.

The LOX and kerosene fueled Rocketdyne H-1.
For example, rocket company Space Exploration Technologies (Space-X) uses a combination of kerosene (RP-1) and liquid oxygen (LOX) for fuel instead of liquid hydrogen and LOX because hydrogen (the lightest element) is difficult to cool and store as a liquid and not terribly dense even then, so the fuel tank has to be quite large and complex. Kerosene is denser and much easier to store since it can be stored at room temperature.

In essence, most of the storage and cooling problems go away with kerosene, which provides substantial cost savings.

LOX and kerosene are used in the lower stages of most Russian and Chinese boosters plus the first stages of the Saturn V and the Atlas V. LOX and liquid hydrogen are used in the upper stages of the Atlas V and Saturn V, the newer Delta IV rocket, the H-IIA rocket, and most stages of the European Ariane rockets. 

But NASA generally prefers the more expensive configuration for it's propellant depots. NASA also seems to think that much research and development needs to be done before this 50 year old, already in existence and continuously in-use technology is ready to be used again.

There are sensible arguments both for and against the extra boost provided by cryogenic fuels and perhaps NASA is right and this little bit of extra boost is needed to explore the solar system. 

Unfortunately, they just got to be aware by now that money no longer grows on trees for NASA. It no longer even flows freely out of the mouths of their political representatives. 

This could possibly be the more important problem for NASA. They might need to form a committee to study this very, very closely before moving forward.

Comments (1):

From: Clark Lindsey (

Hi Chuck,

I enjoy your blog and your commentary about commercial space issues, especially with regard to Canada. Thanks also for the link to my blog.

I've been meaning to comment on your recent post about fuel depots but didn't get around to it till tonight. However, either you don't allow comments or I'm failing to see how to do it. So I thought I'd just drop you an email.

I think people like Dallas Beinhoff at Boeing and Frank Zegler at United Launch Alliance (ULA) would find it ironic that you are criticizing NASA for not realizing that the Centaur already puts cryo fuel depot capability well within reach. Those guys have been pushing NASA for years to back cryo fuel depot development on that very basis - the Centaur has already solved many of the problems and it is not a huge leap to a working depot.

However, fuel depots at NASA have been stuck in the so-called "TRL valley of death". Their technical readiness has reach a level where an orbital demonstration or two are needed to get to the next levels that allow them to be part of a exploration mission architecture. However, orbital demos require a bump up in funding and they haven't gotten the funding because no mission architecture has included depots since they are not at sufficiently high enough TRL.

CRYOTE in launch configuration. Photo c/o ULA.
With the new emphasis at NASA on technology development, they may finally get across this valley. Yes, it will require some funding but not huge amounts. The CRYOTE (Cryogenic Orbital Testbed), for example, is a secondary payload that works with a Centaur to test cryogenic fluid handling hardware and techniques. See the presentations at

Long term LOX storage is basically in hand but work also needs to be done to prove long term LH2 storage. However, again, this doesn't require a huge jump. There are a number of techniques, e.g. advanced insulation, sun shades, etc. that make it feasible now. It just needs a demo to prove it.

Advantages of LOX/LH2 besides the high performance include the fact that it can be produced from in-situ water such as on the Moon and asteroids.

It's a shame that fuel depots got so little attention in the debate the past year and half over the NASA policy changes. The potential savings of depots are tremendous. For example, no super heavy lifter would be needed for deep space exploration. Of course, much of the Congressional emphasis is on the jobs that the HLV would produce and not on the most cost-effective ways to do space exploration and development.

Over time, though, the advantages of depots are too great to ignore. E.g. a lot of people were surprised that Doug Stanley, one of the guys who designed the Constellation architecture with Griffin, was a co-author of this paper: "Near Term Space Exploration with Commercial Launch Vehicles Plus Propellant Depot."

For more info, see these presentations given at the Space Access conferences:

p.s. Space-X does, in fact, have a LOX/LH2 engine project on the drawing board called the Raptor. It would be used in a new high power upper stage -

Editors Note: Clark's right and I still can't get comments to post properly so I'm putting them up manually. Send your questions, queries, concerns and comments to

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