Benefits of Orbital Propellant Transfer: Adaptability, Capability, Etc.

[Editor’s note: A good friend of mine from Santa Clara, Henry Cate, is starting up a Carnival of Space. I’m usually not a huge fan of blog carnivals, but I think this is a creative idea, and wanted to support him on this, so this is my first “Carnival” post.]

Of the top ten technologies that I discussed previously as being critical for a spacefaring society, one of the technologies that I’ve repeatedly stressed has been orbital propellant transfer and storage. And for good reason. Other than lower cost launch (which I think has been discussed to death already), these two technologies are probably the ones that can have the largest impact on space exploration and development. I’d just like to summarize some of the key benefits I see of using orbital propellant transfer and storage in a space transportation architecture.

Adaptability: Propellant transfer and storage technologies (especially in the form of propellant depots) allow a space transportation system to take advantage of improvements in launch vehicles over time. By separating the launcher from the interorbital transfer stages, landers, and other in-space hardware, it makes it a lot easier to take advantage of upgrades over time.

In a way it’s kind of like the computer I’m writing this post on. This computer started out as a machine I bought on eBay back before my mission (in ’99 I think). Over time as new chips and better hard-drives came out, I was able to incrementally upgrade things without having to fork out all the money for a brand-new machine. By now, the only hardware I have on this computer that was on the original machine is the smaller of the two hard disks. The modularity of a PC architecture has allowed me to inexpensively upgrade things as I had the time and money available, instead of forcing me to buy a whole new system. Now, not everyone does it that way, but the option is there if you want to.

Reusability:Propellant transfer and storage makes it much easier to move towards a more reusable transportation infrastructure. In fact, without the ability to transfer propellants on orbit, there are some segments of a lunar or Martian transportation infrastructure that really can’t be reused. With propellant transfer capabilities (eventually augmented by ISRU capabilities), there really aren’t any parts of the transportation architecture that need to be expendable.

The economics of reusing in-space hardware may actually be even more compelling than reusing orbital launch vehicles (and the case for reusing orbital launch vehicles is pretty darned compelling). Unlike orbital launch vehicles, reuse of on-orbit vehicles doesn’t involve adding much if any hardware that wouldn’t be needed already for just performing the basic mission. Design for reusability does tend to drive you in different directions from design for expendable vehicles (such as pushing you to multi-engine landers with engine-out capability instead of rolling the dice every time you land with a single-engine lander), but in many cases those changes can actually make things less expensive in the end.

But all of that is moot if you can’t refuel the vehicles except on planetary surfaces.

Capability: Orbital propellant transfer and storage can allow for much more capable missions than you could perform without them. Dallas Bienhoff (of Boeing) recently presented a paper at the recent STAIF 2007 conference discussing how much you could increase the lunar surface mass of the planned ESAS architecture if you used orbital propellant transfer and “dry-launch” techniques for the EDS and LSAM (Dry Launch is where you launch the transfer stage and lander empty, and top them up on orbit from a depot or from fuelers). I don’t have the exact numbers handy, but the increase was substantial. It may have been over double the cargo to the lunar surface.

More interestingly to me, these technologies can allow you to get much more capability even if you don’t develop new launch vehicles. Every component of the planner lunar stack is light enough to be launched dry on existing EELV equivalent vehicles. And if you then top them off in orbit, you can send a lot more in a given mission than could be done with a non-dry-launch architecture. You could probably send 6-8 person missions, or land entire Sundancer modules along with the 4-6 person crew. All without needing heavy lift launch vehicles.

Dependability: In a world of expendable launchers, where launcher reliability is still depressingly low, a propellant depot serves as a buffer or capacitor between a lunar or martian mission, and the launch vehicles that put the components up. A commercial propellant depot can buy from whoever can launch to it, and with the likely propellant demands for even modest lunar transportation architectures, it will be buying from lots of suppliers. If one launcher starts having problems, the show still goes on. Much like how many companies will put UPS systems between their computers and the main power grid, especially in areas where the power can be flakey or unreliable.

Incremental Developability: [Yes, I think I may have just created that word on the spot.] One of the main issues raised with propellant depots, is that they sound like big, very complex projects. When people hear propellant depot they often think of some ISS sized monstrosity and then extrapolate that only NASA could run something like that, and therefore it would cost as much, take as much time, and be as poorly run as ISS. The reality is that the first “propellant depot” probably isn’t going to be some sprawling 100% custom designed facility that has all of the features, bells and whistles. More likely you’ll see a gradual buildup of capability.

At first, you might see missions that don’t even use a depot–but transfer propellants directly from tanker to tankee without any special infrastructure. Some of that may be in the form of hitchiker satellites that tap the surplus propellants from their launcher so they don’t have to store propellants onboard during the flight (thus reducing the risk to the main, paying customer). Then, you might see someone moving into a first generation propellant depot. This will probably be nothing more than an upper stage possibly docked to a Sundancer module. It won’t have zero-boiloff capabilities, probably won’t have fancy sunshields or meteorite protection, it probably will only handle two propellants, and much of the propellant handling may involve manual connections and valves. Only once there starts to be serious money being made by depots will you start seeing them branching out, growing in size, adding bells and whistles, etc.

You’ll also likely see a lot of the technologies needed for these depots being developed not by big expensive NASA or DoD demonstration satellites like DART or Orbital Express, but by companies like Lockheed piggybacking experiments on the postflight portion of Atlas V launches, and other such, low-budget partially IR&D funded experiments.

Feasibility: One of the best things about propellant depots is that there really is a lot of prior art and experience that demonstrates that we should be able to make this a reality. Every time a Centaur upper stage performs an in-flight relight, it is settling propellants, transferring them through a series of valves and pumps, and then sending them into another system (in this case an engine). Starting with Gemini and Russian programs at the same time, we’ve demonstrated the ability to do orbital rendezvous and docking. The Russians have been doing autonomous rendezvous and docking for decades, and now that we finally got around to it with Orbital Express, we’re doing it too. The Russians for decades, the Shuttle, many other programs, and now Orbital Express have demonstrated the ability to make fluid couplings between spacecraft, both with and without manned intervention.

There are subtleties and tricky parts to tying everything together in the case of cryogenic propellants, but almost all of the toughest techniques and technologies needed for transferring propellants on orbit have been demonstrated already. Based on the plethora of past experience, one can have high confidence that this orbital capability can be refined and brought into practice in the near term. There is a lot of detail work to be done, and it’ll probably take a lot of hands-on experience and several iterations before we start converging on the best ways of doing things, but the initial capability is relatively low-risk, and near-term.

Anyhow, that’s a basic introduction to some of the benefits I see from orbital propellant transfer and storage. I’ve got lots of other articles on this blog detailing some of the technical challenges and some ideas for how to handle them. I’d strongly suggest doing some searches if you have the time and are interested.

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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13 Responses to Benefits of Orbital Propellant Transfer: Adaptability, Capability, Etc.

  1. john hare says:

    “Some of that may be in the form of hitchiker satellites that tap the surplus propellants from their launcher so they don’t have to store propellants onboard during the flight (thus reducing the risk to the main, paying customer).”

    This sentence caught my eye. It seems to be a quick way of enhancing performance on almost any payload that needs propulsion capability.

  2. Anonymous says:

    How easy would it be to store ammonia and using an ammonia-arcjet for orbital transfer?

  3. Jon Goff says:

    Anonymous,
    Storing ammonia should be relatively easy. To get decent density you’ll be storing it cryogenically, but as I understand it, it’s a pretty mild cryogen. That said, arcjets and other electric engines tend to be very low-thrust, which makes for very long trip times. This is a challenge for transporting humans, or cryogenic propellants, but might be workable for transporting non-perishable, time-insensitive bulk goods…but if you’re going through that much trouble already, an ion tug might be a better choice. I’m not sure, most of my background is in chemical propulsion.

    ~Jon

  4. Kellyst says:

    I strongly agree that on orbit fuel depots make a lot of sense. I’d add a couple reasons:

    Increasing flight rates for smaller fuel tankers, would drive down cost per pound by amortorizing overhead over a greater number of flights.

    Increased effective performance of RLVs. Refuel a DC-X like RLV in orbit and it can then fly to any Earth orbit, lunar surface and back, or to Mars. More mundane, a RLV that can only just get to a low useless orbit, could refuel at a depot parked there (or a tanker sister craft) and boost to any other desired orbit. Making designing said RLV a lot easier.

    And just to play devils advocate.
    Most stacked launch vehicles can’t take launch stresses on empty tanks, and the bulk of such tanks (even if empty or loaded with presurent) would drive you to a larger launch vehicle – even with lighter cargo. You could wind up with similar launch costs since you have to use the big expensive bird either way.

    > Adaptability: Propellant transfer and storage technologies (especially
    > in the form of propellant depots) allow a space transportation system
    > to take advantage of improvements in launch vehicles over time.
    > By separating the launcher from the interorbital transfer stages,
    > landers, and other in-space hardware, it makes it a lot easier to
    > take advantage of upgrades over time.

    I’m not sure this really relates to on orbit refueling as much as multiple launches per cargo. Say as a argument for on orbit (hopefully reusable) inter-orbital tugs. Obviously you need to launch your tran stages from Earth eiather way. So taking advantage of new upper stages by integrating them onto a existing multi stage LV, works as well with or without on-orbit refueling. The same way you reintegrate new parts to your PC, rather then separate its function into separate processor blocks you transfer the data between by hand.

    πŸ˜‰

    > Dependability: In a world of expendable launchers, where launcher
    > reliability is still depressingly low, a propellant depot serves as a buffer
    > or capacitor between a lunar or martian mission, and the launch
    > vehicles that put the components up. A commercial propellant depot
    > can buy from whoever can launch to it, and with the likely propellant
    > demands for even modest lunar transportation architectures, it will be
    > buying from lots of suppliers. If one launcher starts having problems,
    > the show still goes on. ==

    A counter argument is since you still need to launch the cargo either way – you have a equal exposure to being grounded. If you launch everything at ones. If it makes it your clear. If not – your not. If you launch it in parts (fuel on some flights, systems on others, upper-stages somewhere else) any single systems launch failure grounds you. You in theory can absorb fuel launch failures by having the fuel launched by other launch services – but how long before their will be enough demand for such services to keep multiples in business?

    So statistically, how much more likely would you be to get a successful mission?

  5. tankmodeler says:

    Jon,

    You mention cryogenic propellants and the nontrivial problems attendent with that. Why not build an arctitecture around hypergolics? Yes, the Isp is lower, but the flexibility is greater, transfer is simpler and if you are not as concerned about mass in orbit (reusing your LSAMs) does it really matter if you are using three flights of lower Isp fuel versus two flights of cryogenics?

    Also, we are talking here primarily about cislunar operations, but the benefits to Mars exploration are much larger and, if we are looking to develop Mars technologies, then surely this is a critical Mars technology?

    (Yes it is and don’t call me Shirley!)

    Paul

  6. James says:

    I had never thought that the propellant for an electric engine would be easier to transfer than that for a chemical engine. Although, if it is, that would be a great way to do it for long trips.

    The best way to get to mars isn’t a fire every now and then chemical rocket, its an electric engine which is firing the entire trip. That way, if you design the ship correctly, you’ve got at least a bit of “gravity” for most of the trip (the part where you don’t have it is where the ship does a 180 degree turn half way there).

    I guess the result of all that is if cryogenic propellant transfer is really going to be that hard, electric propulsion with non-cryogenic fuel might mean mars would be easier to get to than the moon πŸ™‚

    Anyhow, thats just an idea from my uneducated corner of the world. I’m sure I got it wrong somehow.

  7. Gaetano Marano - Italy says:

    .

    a little off-topic, but interesting…

    believe it or not… (both) EELVs and Ares-I could be completely unnecessary to launch the Orion for moon missions… πŸ™‚

    http://www.gaetanomarano.it/articles/026shuttleorion.html

  8. Brad says:

    Ammonia for LEO Propellant Depot

    http://www.astronautix.com/props/ammonia.htm

    Ammonia fuel was used with great sucess in the X-15 spaceplane.

    But the greatest potential of Ammonia is as a propellant for a Nuclear Thermal Rocket (NTR) engine. With an ammonia NTR it’s possible to get the best of both worlds of chemical propulsion, the storage and transport ease of hypergolics plus the high ISP of cryogenics. Dry launch and orbital refuelling fits ammonia NTR like a hand in a glove.

    Ammonia is easier to store and transport than typical hypergolic propellants, yet an ammonia NTR could have an ISP greater than 425 seconds, as good as cryogenic propellant engines. A simplified design low-pressure NTR engine using pressure fed ammonia might even have an ISP in the range of 650 seconds.

    There is already extensive experience with storage and orbital refuelling of hypergolic propellants. Ammonia could easily piggyback directly on that experience without having to figure out new methods for transfer or long term storage of cryogenics. Liquid ammonia is dense like liquid oxygen and would only requre small lightweight tanks.

  9. M. Simon says:

    If you are using some kind of nuclear drive then water is excellent reaction mass.

    For higher ISPs electrolyse the water, throw away the O2 and use the H.

    Transfering water should be easy. Couple the two vehicles and micro G accelerate. Man the pumps.

  10. M. Simon says:

    Did I mention that water is abundant in space?

    Which means you don’t have to carry all your reaction mass with you. Or even send it ahead (except maybe as a back up for the first few missions).

  11. Brad says:

    Water has many problems for use as NTR propellant.

    1)High temperature steam is a powerful oxidizing agent and would destroy an NTR engine of the NERVA type. An NTR engine that could stand up to hot steam would be a brand new develpment project

    2)Water is a mediocre propellant and would provide inferior ISP to ammonia. Hydrogen has a molecular weight of about 2, Water a weight of 18 and ammonia a weight of 17. Water would only have an ISP of about 300 s, 1/3 the ISP of hydrogen.

    But ammonia at relatively modest temperatures breaks down into nitrogen and hydrogen so the average molecular weight becomes much lower, about 8.5 which is less than half of water. So ammonia has a storage density which begins to approach water while still providing a propellant of relatively low molecular weight. Ammonia should have an ISP of 400 s at worst. Ammonia is great stuff for an NTR engine.

    3)Trying to improve water’s performance by electrolyzing a tank of water and only expelling the hydrogen as NTR propellant gains nothing in rocket performance. The oxygen dumped over the side is still mass carried and pushed by the rocket.

    4)If water can be found at the destination of a mission, even then running water through an NTR engine is a poor second choice. Better choices for using that found water would be…

    a)Electrolyzing the water into hydrogen and oxygen for a liquid bi-propellant chemical rocket engine, such as the RL-10. A much smaller reactor would suffice than for an NTR engine and the chemical rocket performance would be much higher than for a solid core NTR engine using water.

    b)If refuelling an NTR spacecraft, process the found water into a lower weight propellant such as hydrogen, methane or ammonia. This could mean exploiting atmospheric resources besides the water to accomplish, such as making methane on Mars.

    If an NTR engine was used to drive the spacecraft to the destination where you find water for refuelling, it’s best to always use the same kind of propellant the NTR engine was designed for. An NTR engine designed for hydrogen would be destroyed by hot steam and an NTR engine designed for hot steam would be destroyed by hot hydrogen. It’s impractical to design an NTR engine that can stand up to both oxydizing and reducing agents!

  12. Tom says:

    Brad-

    You appear to have given NTR using ISR a lot of thought. I’ve been doing the same. Please contact me through my web page.

  13. Anonymous says:

    You need to be told that our project team at Worcester Polytechnic Institute is developing an orbiting atmospheric gatherer.

    –The gatherer uses a molecular pump designed to capture ions coming from one particular direction at orbital velocity. A gatherer with a collecting area of
    1 square meter traveling at 29,.000 kilometers per hour will gather a volume of 1 square meter times 29,000 kilometers every hour. Even in nearly empty space, the kilograms of gases in such vast volumes add up quickly.

    –Our target altitude is a circular orbit in the vicinity of 400 km. A gatherer with a collecting area of 1 square meter will gather on the order of 10 tons of atmosphere per year at this altitude. The output pipes will deliver 7.8 metric tons of oxygen, 1 ton of water, 0.7 tons of nitrogen and 0.5 tons of helium per year. We assume that the gatherer spacecraft will have a ten year lifespan. Liquid oxygen, water and nitrous oxide are propellants, and we can also refill ion propulsion engines with nitrogen and oxygen.

    –1 square meter of collector will induce a perpetual drag of 2 newtons on the spacecraft. We will use an electrodynamic tether to produce 2 newtons of steady
    thrust.

    I’ve been working on this project with Dr. John Wilkes of Worcester Polytechnic Institute and with three WPI engineering students. Next year we will have seven new students on the project. We hope to be pumping propellants into space tugs long before NASA returns to the moon. In fact we may fuel NASA’s lunar landers. We hope to be profitable.

    –Paul Klinkman
    psychware att yahhooo dott comm

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