Additional Thoughts on the SA'08 Propellant Depot Panel

It's been long enough since I put up the presentations from the panel, that I figured it would be worth starting a new post to mention some of my thoughts on what we discussed on the panel. Some of these ideas are thoughts that were briefly discussed during the panel, and others are ideas I just didn't have time to bring up.

While the panel went much better than expected, I also noticed several things I could do next time to make it even better.

One thing is that I probably should have mixed audience participation, discussion, and presentation a little more thoroughly. Expecting people to stay awake through an hour of presentations right after dinner was a bit much--surprisingly enough most of the attendees did, but I think mixing it up would've been better.

Another thing that in hind-sight would've been useful is to set aside more time in the panel for actual discussion, particularly on points where one or more of us had a different preferred approach than the others. For the most part we all agreed on the importance of propellant depots, the benefits to commercial and governmental programs, and the fact that a lot of the technology either exists, or is on the cusp of existing. But we disagreed on some of the details, and much like statistics, it's the disagreement or outliers (and the reasoning behind them) that hold the most information.

Tugs or Reusable Tankers or Both

One point of disagreement was between myself and Dallas Bienhoff regarding the best way for handling prox-ops for propellant deliveries. I am a big proponent of using space tugs to offload as much of the weight and complexity of a prox-ops system to an asset that stays on-orbit and gets used a bunch of times. Dallas on the other hand disagreed with me on that point and suggested that a reusable tanker with autonomous rendezvous and docking capabilities was a better way to handle complexity and reduce cost. Both of us were advocating for reusing as much of the complicated prox-ops hardware as possible, but going about it in different ways.

To me, there are several benefit of having the tanks dumb as possible, with all of the complicated subsystems on the tug:

• Performance: the tug stays in orbit, and is very weight insensitive. By not having to relaunch the tug every time, you get a lot more propellant per pound of delivered mass on orbit. When you look at unmanned delivery craft right now, they typically have a really poor payload to drymass ratio. For a tug you want as high of a payload to drymass ratio as possible.
• Easier Open Interface Standards: It is probably a lot easier to get ITAR approval for openly publishing a very simple common interface specification if that interface spec is just a few handholds and a commercial, off-the-shelf quick-disconnect receptacle. That way, anyone who can launch stuff into your station's orbit can deliver propellants to your station, even if they're durned furiners. An Autonomous Rendezvous & Docking (AR&D) system with automatic fluid couplings would likely be a lot harder to get export approval for.
• Flexibility: The simpler the propellant module, and the less smarts it has built-in, the easier it is for people to just stretch it to whatever size best suits their launch vehicle. Unlike a reusable tanker which might require significant redesign to be used on a different launch vehicle.
  
• RLV Simplicity: Making a high-flight rate reusable launch vehicle is going to be hard enough already without trying to also make it into a satellite as well. For RLVs, you want a relatively dumb tank that never leaves the payload bay, but you don't want to have to have all the prox-ops stuff cut into your already very limited payload budget.
• Other Tug Uses--On-Orbit Assembly: Tugs are useful for lots of other things too, especially if they have arms. They can help in assembly of stations and large in-space vehicles. No need to make each and every space station or space vehicle component be its own independently operable mini-station complete with its own GN&C, power, etc.
  
• Other Tug Uses--Satellite Recovery: They can also perform satellite recovery missions. Imagine if there had been a tug already developed, and set aside on standby in case of a botched launch like the recent Proton upper stage failure. If done properly, the tug could've been launched on short notice on an otherwise empty upper stage. The tug could've then transfered the satellite from the malfunctioning upper stage to the still functional, and mostly full upper stage that delivered the tug. There are some very tricky technical details I'm glossing over, but its a capability that could become rather standard once you have tugs available. In case anyone from the DoD is reading this, yes, I'm saying that tugs are an important part of ORS.
• Other Tug Uses--Rescue Missions: They can perform rescue missions. Right now, one of the most hazardous parts of a lunar mission is the ascent, rendezvous, and earth return legs. Imagine if there was a staging point in L1, L2, or LUNO, instead of basing all lunar missions from earth's surface. You could store one or two of these tugs at the small staging/refueling base. If something went wrong with the LSAM US or CEV, you could send a tug in to help out. If you were using lunar ejector seats, and had to abort to orbit, this would give you a quick way of getting a rescuer to a stranded astronaut. This would greatly reduce your odds of losing a crew due to a LSAM/CEV rendezvous failure, or CEV propulsion failure prior to (or during) TEI.

And there are probably other ideas I'm overlooking.

On the other hand, Dallas a good point in favor of reusable propellant tankers, and I can think of some others as well:

• The more expensive propellant handling hardware your tanker needs, the better it would be to reuse it. For instance, say you don't think you can get first-orbit or even first-day rendezvous with your propellant depot. You might want to invest more heavily in insulation, zero boil-off systems, and other cryo handling hardware. You don't want to be tossing that away after every flight.
• You're eventually going to want to have smaller depots located on the other ends of your transportation system (ie in the lunar vacinity, around Mars, around Venus, etc). Some of these locations, especially at first, will need to be fueled from Earth. That means tanker modules are necessary. Once again, once the flight duration gets longer than a couple of hours, you're going to start wanting to add other bells and whistles. And those bells and whistles are expensive enough that not throwing them away after every flight is a good idea.

Dallas may have had some other points that I'm not remembering right now, but I think that both sides have valid points, and that the best option may be to do a little bit of both.

Full-Service vs. LOX-Only?

I had been somewhat surprised when Dallas (who works for the company that was the lead on developing and flying Orbital Express) suggested against using a tug. I was even more surprised when Frank Zegler suggested a LOX-only depot. Before I had met Frank over the internet, I considered LH2 to be an unmitigated evil, almost on the level of Nitrogen Tetroxide and UDMH. But he was one of the main people to talk me into thinking that Hydrogen isn't always evil, and sometimes can be tamed, and can make a lot of sense. So, when he sided with the sentiment that meiza and several other regulars here at Selenianboondocks have expressed--namely that your first depot should probably be LOX only--I was very surprised.

I didn't have time to bring up this point of disagreement at all during the panel, but here were some of Frank's points in favor of LOX-only depots:

• LOX is much easier to store and handle cryogenically due to its much higher boiling point.
• LOX is much denser, and thus you can store a lot more of it in a given size tank.
• LOX makes up the majority of the propellant mass than for any fuel combo you would likely use.
• Storing only one liquid is much easier than two, because you can eliminate the heat transfer from the warmer propellant (LOX) into the colder one (LH2). Even with a sunshield or a ton of MLI, you still have a significant heat source in the fact that your LOX is way hotter than the boiling temperature of your LH2. You probably never thought of LOX as a heat source, did you?

If these arguments sound familiar, its because they're the same ones that many of you have made over the years. I can see Frank's point, especially if you think that your main (or only) market is going to be "topping up" EDSes and LSAMs for NASA. I've never disputed these facts. But I still think that going all the way and providing at least one fuel to go with that LOX is a good step. These arguments probably aren't new, and probably aren't going to change anyone's minds, but in case you haven't heard my spiel before here's my case:

• Without modifying existing or future stages, they only have so much hydrogen capacity. Unless you launch a complete stage as your payload, topped-off to the brim, you're going to use some of that capacity getting to orbit in the first place. Which quickly cuts into your maximum payload you can deliver to your final destination (and also how much LOX you can actually use).
• For many payloads, prior to the time when reusable LEO-GEO or LEO-Luna ferries are available, the best way to use a propellant depot is to launch the payload on a refuelable upper stage, top that upper stage up in LEO, and then immediately go to your destination. If you reuse your upper stage as your transfer stage, the inability to top of the hydrogen as well effectively halves your payload you can deliver to other destinations. If you fly a separate refuelable stage that has a full load of LH2, you're greatly cutting into how big of a payload you can put into LEO in the first place. For instance, a Centaur stage with a full load of LH2 weighs about half of the payload capacity of an Atlas V 401 to LEO.
• If you can't provide both oxidizer and fuel, you can't reuse interorbital transfer stages/ferries.
  
• If you can't provide both oxidizer and fuel, you can't reuse lunar landers.
  
• If you can't provide fuel on-orbit, you can't make up for boil-off caused by unexpected delays, variance in the thermal properties and boil-off rate of your stages, equipment malfunctions etc.
• On a psychological level, going LOX-only allows people to continue to disbelieve in the feasibility and utility of propellant depots. Look at the mindset two years ago. It said that propellant transfer of any sort on orbit was deep, black magic, and that it should be avoided at all cost. Now that Orbital Express has shown that it is doable and not that hard for storable propellants, critics say "well, that's all good and fine, but cryogenic propellants are a whole different beast entirely." If we went to LOX-only depots, those same critics would likely say "well we knew LOX was doable all along, it's the hydrogen that's the unrealistic part--there's no way you could store that long enough to be practical." At some point I want to stop giving skeptics ammunition.
• More importantly, both Dallas and Frank agree that LH2 storage on-orbit is completely feasible. Dallas going so far as to say that for 1-2kW and 50-60kg, you could install a ZBO system that could completely eliminate boil-off.

I guess I'm still convinced that in spite of the added extra difficulty, that the real markets that I think there will be for propellants on-orbit will be much better served if you can provide fuel as well as oxidizer. But you can draw your own conclusions.

To ZBO or Not to ZBO?

Another disagreement (this time between Dallas and Frank) was on whether or not to go with a Zero Boil-Off system for long-duration cryo storage. Dallas seemed to think that not only would it not be that hard to implement, but that it would be very desirable, while Frank seemed to prefer passive storage techniques, and in fact considered ZBO to actually be a detriment! I think both sides have points, but that in a way they're somewhat talking past each other. And in the end, I think I side more with Dallas on this one.

Frank is right that ZBO systems, done the typical way, (without doing a proper passive-storage design and without settling propellants) is likely going to be an expensive development project, and a complicated system in operations. Frank also made the point that at least some of the boiled-off hydrogen is actually useful. That warm GH2 can be propulsively vented to cause the other propellants to stay in a settled orientation. It is still pretty cold, so it can be used to pull heat from the avionics away from the propellants. It can be used for prechilling lines and valves. It can be used to provide propellants for GOX/GH2 RCS engines. It can be combined with oxygen in a fuel cell to provide water. The single most important benefit for Frank and ULA is that first one--settling the propellants makes everything easier, and propulsive settling is by far the highest maturity and easiest way of settling propellants.

As Dallas pointed out, a properly designed ZBO system when added on-top of a good passive insulation system doesn't need to be that big and complex. 1-2kW isn't that much power. And especially if the propellant is settled in some fashion, running a cryocooler becomes even easier, because you can avoid two-phase flow. If your cryocooler works better taking gas in and spitting out liquid (or chilled gas), settling allows you to guarantee you're only pulling in gas. If pulling in liquid, subcooling it, and spraying it through the gas is more effective, settling allows you to pull liquid from a part of the tank where you know liquid will be, and to inject it into a part of the tank where you know it will be gas.

Lastly, there are other ways to settle propellants, and if you use them, you no longer need boil-off gases to provide the settling. You might still intentionally allow some LH2 to boil-off, for use in RCS engines for stationkeeping or to run fuel cells. But with a ZBO system, you have a choice.

A more convincing argument against the complexity of a ZBO system, that can be derived from Frank's presentation, is the fact that a good passive system can get boiloff rates low enough that you just don't care about them anymore. In those cases, a ZBO system might not buy you that much extra performance. Now, there's an argument against ZBO that I'm more willing to buy.

So, I guess the real answer may be--it depends. In situations where your propellant is expensive enough to deliver to, where deliveries are somewhat infrequent, and storage times are long, a ZBO system might make a lot of sense. But in situations where the propellant can be readily topped off from tankers on a regular basis, even though ZBO is doable and not that hard, it still might not be worth it.

Once again, draw your own conclusions.

NASA vs Other Markets

This last topic isn't so much a disagreement, as an area that I thought deserved a little more commentary. I think that all of the panelists would agree that NASA is unlikely to have a change of heart tomorrow, and completely overhaul Constellation to take more advantage of propellant depots. However, in spite of this recognition that NASA isn't likely to become a good customer anytime soon, most of the panel was still very NASA- and Constellation-centric. While there were mentions made by all the panelists about performance benefits that normal ELVs could get for delivering payloads to GEO and beyond, most of the discussion of benefits was focused on augmenting NASA's return to the Moon.

Admittedly, if NASA ever gets its act together and actually makes it back to the Moon, it will be annually consuming more propellant mass in orbit than the combined launch mass of all other launches combined, and if they were actually buying that from propellant depots, it would be a truly transformational event. But lets face it guys--it makes too much sense for NASA to willingly go along with it. Much like Zero-G demonstrated, while NASA might eventually be willing to abide by the law and purchase commercial services that they used to provide for themselves in-house, it will take many years to get them to change. As it is, it'll take NASA a decent amount of time before they can even take advantage of depots, even if they recognized the potential right away.

As I said in my presentation, it doesn't matter how critical propellant depots are for creating a spacefaring society, or how much better lunar exploration would be with propellant depots involved. If you can't find a way to get enough real customers to wrap a business case around, it will never happen.

That said, I also wanted to note that NASA and the DoD are actually doing some good things regarding propellant depots. First, they've regularly put out SBIR solicitations for technologies that could be relevant to propellant depots. Second, on the larger scale, they've funded actual technology demonstration missions like Orbital Express, DART, and XSS-11 to demonstrate useful related technologies. Third, even though Michael Griffin's NASA hasn't been doing much action-wise to enable propellant depots, Griffin has at least been a vocal proponent of the capability in many public forums. Fourth, NASA was at least interested in offering a propellant depot related Centennial Challenge--if they had actually been given any new Centennial Challenges funding in the past three years.

Even though I don't think either the DoD or NASA is likely to outright fund a propellant depot anytime soon (and personally I wouldn't want them to!), there are lots of things that can be done within the system to help move things closer to reality. Better, clearer ties can be made between the technologies needed for propellant depots, and the needs and desires of NASA and the DoD. Tugs and depots, for instance, are an important part of a truly Operationally Responsive space transportation infrastructure. By making that connection more and more in public, additional funding for research, development, and demonstration might become available. NASA also desperately needs good long-duration cryo storage and handling technologies in order to make ESAS work, and at least some of those technologies will also be useful for propellant depots. Propellant depots (especially commercial ones) might allow NASA to launch larger interplanetary missions than would otherwise be possible, etc. So while I think the key to propellant depots lies in markets outside of NASA, I think there's a lot of good NASA and the DoD can still due, even in spite of the political environment and constraints that both of them operate in.


Anyhow, those were some of my thoughts I wanted to discuss from the panel. Once I get the video from Henry, I might find a couple changes or additional comments to bring up, but for now those are my thoughts.

Comments?

[Update: 4/4 8:30AM PDT]
Here is the link to a thread on NASASpaceFlight.com where I have been discussing propellant depots with some of the other regulars.

Space Access 2008 Propellant Depot Panel

I'll update this after work with some commentary (and sometime in the next week or two, I'll see if I can upload a video of the session as well). But for now here are the four presentations that were given.

I started out with an introduction to the concept of propellant depots, a short discussion on some of the general technical and business challenges, and then an introduction to the panelists.

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Rand Simberg (of Transterrestrial Musings and also of Wyoming Space and Information Systems) gave a presentation talking about some more of the business and policy issues related to propellant depots.

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Dallas Bienhoff of Boeing discussed the work he's done over the past several years on propellant depots, and he also discussed in more concrete terms some of the advantages (both to ESAS and to commercial operators) of propellant depots.

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The final presentation was given by Frank Zegler, of ULA. He talked about their thoughts on propellant depots, and some related work they've been doing (and will be doing in the near future. Frank had some cool eyecandy for some of the hardware they've been building and testing lately for the new Advanced Cryogenic Evolved Stage that ULA is working on.

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Propellant Depot Panel Questions

I'd like to solicit questions regarding the technical or business aspects of orbital propellant depots for the panel I'll be chairing this coming Friday. I've got a great lineup of panelists (Rand Simberg of TransTerrestrial Musings, Dallas Bienhoff of Boeing, and Frank Zegler of ULA), so if you have any good questions about propellant depots, just post a comment here, and I'll try and pick the best two or three to start off the Q&A part of the panel discussion.