Propellant Depots

Over at Aviation Week, Frank Morring says the NASA studies continue:

Michael Gazarik, NASA’s space technology program director, says that CPST and the Space Launch System (SLS) heavy-lift rocket currently under development are complementary technologies. “To explore deep space we need a heavy-lift vehicle — SLS — and we need this technology. We need to be able to demonstrate how to handle cryogenic fluids in space.”

He has to say that. It’s literally politically incorrect to say anything else, and will be until SLS dies. But the reality is that propellant storage on orbit is essential to spacefaring. Heavy lift is not.

[Update a while later]

And…the empire strikes back. A piece defending SLS/BMR by Mike Griffin and Scott Pace, over at Space News. Will I have a response? You bet. Stay tuned.

[Update a while later]

Here is one point (though there are others) that I will really pound on:

The challenge for fuel depots is simply that the marginal specific cost of payload to orbit is generally lower for larger launch vehicles. There may be exceptions, but the trend is clear.

There are at least two avenues of attack. What mine will be is left as an exercise to the students. Oh, and initial link fixed. Sorry.

[Late evening update]

Clark Lindsey has started to rebut, and it’s a good start. But there are a lot more fish in that barrel…

35 thoughts on “Propellant Depots”

  1. But the reality is that propellant storage on orbit is essential to spacefaring. Heavy lift is not.

    Be that as it may, it’s interesting to note that when SpaceX wanted to compete in the geosynchronous market they opted for heavy lift (Falcon Heavy) over a propellant depot in low earth orbit.

      1. Designing a GEO comm bird to rendezvous with a depot and fuel up in space is going to increase its cost. Unless there is an impressive economy of scale in delivering the fuel and oxidizer (usually MMH/NTO or N2H4/NTO) to orbit, they won’t make it back on launch costs, conservative disposition or not.

    1. Plans for FH are only a recent development, earlier they were talking about F9 Heavy, which is heavy lift only in the sense that Ariane and the EELV Heavies (I like that name) are. Their manufacturers call them heavy lift, but that’s not how HLV fanboys generally use the term. And even FH (like EELV Heavy) is only a smallish EELV, and one that scales down.

  2. “NASA Still Studying Space-Based Fuel Depots” – i.e. they obtained Zarya/FGB blueprints from Roscosmos and cant figure the cyrillic out ?

  3. The Griffin piece is a gold mine. 😀

    If we always use the largest vehicle for long distance transportation, what happened to supertankers and why do Harley’s haul more people across the country than railroads do? Why is the 787 so much smaller than the 747? Has he looked at the cost of a Shuttle payload versus a commercial one?

    We tend to use larger vehicles for delivering cargo on Earth for several reasons, many of which don’t apply in space.

    For one, we use crewed vehicles, and crews cost money. An ocean going cargo ship has a relatively fixed crew size, regardless of tonnage. Yet when people want to go out an explore on the water, they use small boats, not barges. Regarding trains, larger ones require no more crew than small ones, and a few large trains are much easier to schedule on congested tracks than a multitude of smaller ones. Space doesn’t run on congested tracks.

    Second, ships gain speed and efficiency with size because of drag, especially wave drag. On the ocean, increasing the size of the ship allows for less fuel per ton delivered. In rocketry, such scaling laws don’t apply outside of the lower atmosphere.

    He’s trying to apply scaling laws caused by aero-hydrodynamics to vehicles operating in a vacuum.

    1. when we hauling millions of tons in space, we will have large rockets [and small rockets], the trick is getting to that future.

      I think we need a large rocket to get to Mars quickly, but the large rocket needed in in space, not on the launch pad from earth.
      I think our current hvy lifts are big enough to get that large rocket in space, but maybe a larger earth lift could make that easier.
      But we aren’t going to Mars at the moment, and there isn’t any value to large lift [130 ton] to go to the Moon. And to lift a fully fueled large rocket in space, requires something like Sea dragon rather than some puny 130 ton lift.
      So to get to Mars with large spacecraft we need in space fueling [or assembly].
      Need in space refueling now, might need heavy lift in 10 to 20 years and should able to make it a few years similar to how Saturn V was built.
      And since Musk is planning on building 70 ton lift- don’t need NASA building another one.
      And bigger 70ish ton lift could be “evolved” from either or both of EELV vehicles for the redundant/competitive aspect.

      What NASA needs to do now is something like Spudis & Lavoie

      1. Heavy lift in space is a nice term. And yes, we will probably want to push 100mT through TMI at a time, but we only need (and should) do that from a high energy staging orbit, most likely a Lagrange point. One or two Titan second stage equivalents should be enough for that. And those fit easily inside an EELV fairing and are light enough to fit on a F9 or maybe even a Taurus II.

  4. Whether fuel depots make sense in the near term depends upon what question we are trying to answer. If the question is, “What kind of space architecture will generate a high traffic model for private space firms without having to pay for missions that actually go beyond LEO?” then fuel depots are an attractive concept. But if the question is instead, “How can we efficiently create the strategic space transportation capabilities to enable humans to explore beyond LEO?” then they are not.

    Classic. A high traffic model is exactly what is needed to reduce the cost of launch. That’s precisely what NASA should be aiming to achieve. Cheap launch and propellant depots is the “strategic space transportation capabilities to enable humans to explore beyond LEO”.

    Whereas a billion-dollar-per-launch heavy lift vehicle is no better than Apollo and will achieve nothing more.

    1. Well Griffin is right that depots need sufficient traffic, but the same goes for HLVs and HLVs need much higher traffic than depots do, so that’s not an argument for HLVs, much less a sole-source multi-decade contract for an HLV that will only be used by NASA if at all.

      Also, as you point out, you shouldn’t only compare costs (a comparison HLV would lose), but also benefits such as achieving cheap lift for crying out loud. A NASA administrator who doesn’t realise how crucial that is, is either corrupt or grossly incompetent. To first order it’s the only thing that matters. And the fact that Griffin and other HLV shills never seriously address the arguments of their opponents suggests it’s dishonesty not just incompetence.

      Still, I think it’s going to be a while before dedicated depots can be justified by the levels of traffic. Fortunately, we can make do with refuelable storable spacecraft in the mean time and reap nearly all of the benefits of dedicated cryogenic depots. Unfortunately, NASA appears to mainly studying depots and propellant transfer in order to discredit them while pretending to like them for eventual use at some unspecified time in the future.

      We agree on starting with storable propellant transfer, but I don’t think I’ve ever seen you offer an opinion on preferring to start with in-flight refueling instead of full, dedicated depots.

    2. Interestingly enough, NASA had a mere 30 years to figure out how to turn a high traffic model with a heavy lifter (shuttle stack). They couldn’t and didn’t. Time for someone (anyone ? everyone?) else to take the lead.

      Doing the same thing time after time and expecting different results is the definition of insanity. Cheers –

  5. For fuel and other consumables which have a low dollar value then a really big, cheap, less reliable booster is the best choice. The SLS does not fit that bill; but if you go back in time to Bob Truax’s ideas for really big dumb boosters, those are the perfect complement to fuel depots.

        1. For expendable vehicles, where the vehicle is thrown away after use, you may save by skimping on reliability. But if you hope to recover and reuse the vehicle low reliability can be expensive.

        2. Perhaps, but what I’d really like to see is safe and dependable cheap lift in the 3-5mT class, enough for launching people. Because that is what we need if we want to become a spacefaring civilisation. This is much more important than going beyond LEO, although doing that would be an excellent way to provide funding for such launchers.

        3. If you are willing to give up reliability then, all else being equal, costs will be less.

          It’s worth noting that a cheap big rocket with high launch frequency is likely to be more reliable in the long run than an expensive big rocket with low launch frequency. This assumes a rational operator who bothers to learn from experience and monitors their rockets in operation.

    1. For fuel and other consumables which have a low dollar value then a really big, cheap, less reliable booster is the best choice.

      How did you come to a conclusion that the best choice has to be big ? I’m betting that a moderately sized or smallish “minimum cost design” throwaway rocket would work out to be more economical for such application for several reasons, but i have not done the analysis.

      Others have, on and off, see Microcosm Scorpius.

      A few of the more obvious reasons why smaller sized rocket could work out to be more economical are pure handling and transportation issues. Falcon I size rocket was transported on a truck, could be erected pretty much anywhere and launched by a very small crew.
      Plus, if you are seriously planning to mass build these, automated factory line setup, automated QA processes, all get ridiculously difficult beyond a certain size.

  6. I think Griffin’s fundamental problem is that he can’t imagine space launch in the absence of a standing army of support workers. Stuck in the rubric of maintaining the standing army, he can only imagine architectures which require a standing army. He simply can’t imagine that increasing the flight rate and reducing the standing army provide comparable if not better returns on the margin than a giant rocket.

      1. There is less evidence Mike Griffin is an honest man than there is for Paul Krugman. Their academic credentials make it very unlikely they are stupid enough not to realise the enormity of their pronouncements. I do think that Griffin simply doesn’t understand business and economics, suffering from the focus on technical processes and specifications instead of cost effectiveness that many techies have. That would still make him unfit for his former job of course.

  7. Large vehicles save on some economies of scale. But unless there is enough payload being carried in big enough chunks they lose on economies of scale producing the vehicles themselves, or in keeping the vehicle in productive use.

    The DC-3 was larger than most preceding aircraft, making better use of irreducible flight and ground crews. But it was also small enough that a great many of the aircraft were kept busy during the years they were in use.

    The near term launch market isn’t expected to support enough lift to use enough heavy lift rockets to justify development of the monsters some want.

  8. Large vehicles save on some economies of scale.

    This is the gospel, but it has not ever been demonstrated with launch vehicles.

    I’ll say that frequently flying vehicles save on economies of scale instead, and for a fixed tonnage of payloads to orbit available, small vehicles win.

    1. In addition, to the degree it is true the market will find that out. It may well be that a partially reusable FH will have an economic edge because of its size, but it could also be that the current crop of suborbital companies will eventually be cheaper. We should let the market decide, not Mike Griffin and his cronies.

      The big point is that you cannot expect a reduction in commercial launch prices by at least an order of magnitude, or more generally to have a cost-effective and affordable space transportation infrastructure without letting the market run the show. History has amply demonstrated there is no way a government monopoly can be cost-effective. Indeed, the real reason for most government activity seems to be to work inefficiently as a means to redistribute wealth.

      1. I absolutely agree that market should decide, and for all i care the most economical solution will end up being shooting carrots to orbit from a cannon or whatever.

        I just have a problem with people repeating the established mantras ( i.e. big launch vehicles have economies of scale ) without really thinking about it or considering alternatives, or even just lacking any sort of reference for such claims.

        1. Well, Peterh wasn’t saying there were overall economies of scale, just that some aspects scale well with scale while others don’t. There are some real cube-square considerations, but the real figure of merit is economical (cost/kg), not technical (max payload, mass fractions).

          1. There are also some scaling laws working against larger vehicles. A pressure vessel’s mass scales with internal pressure, not volume, so the taller you build it the more hydrostatic pressure you have to contend with at its base (under x G’s of acceleration), which means a lower mass fraction for fuel. Similarly, the taller the vehicle the more mass that must be devoted to columnar loading.

            But as has been mentioned, I think the big kicker is huge vehicles launched infrequently require vastly more infrastructure overhead and make assembly line methods problematic. Instead of churning them out like sausages, each one is like a giant home-built project with government contractor custom-item pricing.

  9. The Pork Launch System supposedly will fly once per year. What are the odds that each time it flies, all of the potential payloads will need to go to the same place? What are the odds that all of the potential payloads will be physically compatible with one another?

    And, echoing Peterh, what will the specialized, trained ground crews be doing for the rest of the year?

    1. And, what are the odds that all of the payloads will be ready to go at the same time? And that none of them will have a prelaunch glitch that requires fixing?

  10. Imagine the savings we could have had if, instead of the Interstate Highway System, the United States had built one big train to carry all of the travelers and commerce between all of our cities!

  11. Korolev thought it was good enough to have a rocket with half the capacity of Saturn V to make a lunar launch using assembly in earth orbit. He had a much lower budget at his disposal so this made more sense to him. It still makes more sense now. Since then computers and automated docking have evolved. Rockets and engines do not explode as much. So why the obsession with Saturn V like levels (or superior) of weight capacity?

    Something with half the capacity of Saturn V and many launches would be good enough.

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