Space Journalist/Blogger Rob Coppinger wrote an article tonight attempting to debunk “The Fantasy of Propellant Depots“, which he makes out to be some sort of religious mantra in the New Space community.Â I will admit that depots are finally starting to get a tiny bit of the attention they deserve, but that has only been within the past year or two, so calling it a religious tenet seems like a bit of a stretch.Â And while Rob tries to dismiss the claim that propellant depots have the potential to “open up this final frontier”, I stand by my claim that orbital propellant transfer is one of the key technologies required for a truly spacefaring civilization.
Before I get into my rebuttal of Rob’s post though, I had an interesting side note worth mentioning.Â In thinking of what to write, I decided to start by trying to figure out where I first became interested in propellant transfer and propellant depots.Â I’m pretty sure it was after the Return to the Moon conference in 2004.Â Because at that time I was still looking at “tinker-toys” style multiple propulsion module approaches to lunar transfer systems.Â So, I did a search on this blog for propellant transfer, and realized that all the way from my very first substantiative post on this blog in June of 2005, it’s a concept I’ve been trying to hammer home.
And quite frankly, I can’t think of a better place to start this discussion than by quoting one of the first things I ever said on this blog about propellant transfer:
After quite a bit of discussion, I realized that your perspective on what constitutes â€œspace developmentâ€ or â€œspace explorationâ€ will greatly impact your opinion on how to best go about achieving those goals. In fact, this is a fairly valid general point: your focus determines your path.
If you think that having a small McMurdo-on-the-Moon style lunar science base is space development, then your opinion on what is an ideal method to accomplish that will differ quite a bit from someone like me who doesnâ€™t see the moon as settled until there are dozens of settlements and hundreds of thousands of people living and working there.
If youâ€™re only planning on sending a few people a year to a small government camp, with no intention of ever opening the moon up to commercial exploitation, building a big Shuttle Derived Heavy Lift Launch Vehicle may make a perverse amount of sense. If on the other hand you actually want to make money on the moon through say, lunar tourism, you realize that the only way youâ€™ll ever get the costs low enough is if you have some sort of reusability built into your transportation system. Which means biting the bullet with on-orbit rendezvous, docking, propellant transfer, and vehicle assembly.
Ok, with that said, let’s jump into some of my thoughts on Rob’s article:
Missing the Elephant in the Room
In this article, Rob focuses on the economics of a propellant depot, and spends a lot of time trying to make the case that there’s no market for depots.Â “But where are the customers?” he asks, all the while missing the most obvious customer for propellant depots: NASA’s manned exploration program.Â I’m kind of confused really.Â I’m not sure if this is some sort of a strawman–trying to make it look like we think NASA should go away entirely and that with propellant depots, commercial space can do all of the exploration by itself?Â Because that’s not what any of us have been suggesting.Â We’ve been suggesting that if NASA transitioned to an open exploration architecture, enabled by propellant depots, that it would allow them to do better exploration, while at the same time helping catalyze demand for commercial spaceflight and provide an anchor tenant for the infrastructure that we’ll need if we ever want to get past Apollo reruns.
I guess he could have merely forgotten the comments that started this whole conversation. As I understand it, this whole discussion started with a commenter to his post on space policy recommendations linking to Rand’s article on the same topic, which suggested ditching the NASA-operated HLV approach for a depot-centric commercial launch approach to space exploration.Â Rand’s point if I may summarize was that if NASA opened up the lunar architecture to the concept of propellant transfer, not only could they launch the whole thing on existing EELVs (possibly in just two hardware launches), but they would also be opening up the largest launch market in history.Â The demand for propellant on orbit for even a modest lunar program would be amazing compared to the current launch markets.
No Such Thing as a Free Launch
Before I go on about other depot markets, there were some other points in Rob’s article that bugged me a bit.Â For example, one of them was when Rob goes on about all the costs of a depot.Â But instead of listing anything new, he goes and lists a bunch of stuff that are typical costs for any spacecraft.Â In fact, he lists several costs that are typically rolled into the cost of launch (like the cost of the launch’s ground support and the cost of shipping the rocket to its launch site).Â And then he acts like this is some sort of major news that none of us depot supporters have ever thought of before.Â Sure, propellant depots have costs associated with them.Â We know that.Â Nothing in life is free, but none of that is all that hard in the overall scheme of things.Â What matters is whether or not you can keep your costs sufficiently low compared to your revenues to turn a good enough profit.Â Unfortunately Rob doesn’t actually go into that at all.
LEO Satellite Demand for Depots
Getting back to markets for depots, I think Rob doesn’t really have a good idea of how a depot would likely be used.Â It’s really easy to setup strawman markets that don’t make any sense, and then knock them down.Â Sure, most satellites aren’t going to go “blast across to a new plane and dock for fuel.”Â There are some LEO satellites that could actually use the added flexibility/lifetime that could be had if there were an easy way of refueling themselves on a regular basis (or achieving that goal through alternative means).Â Rob dismisses these with the comment:
Telecoms and Earth observation satellite propellant resupply vehicles have beenÂ in development but to date, despite an obvious market and claims by developers of anchorÂ purchasers waiting in the wings, nothing has materialised
I think Rob is missing several things here. First off, the capabilities necessary to do such services has only recently been demonstrated by the US. Most of the technology required for doing something like what Orbital Express did wasn’t really that far past what we had in the 1960s, in fact the Russians demonstrated autonomous docking and fuel transfer at around that time. The problem is that while this market is real–most of the people I’ve spoken with in industry agree that there is real need for the added operational responsiveness that tugs and depots could give LEO sats–especially after China’s recent ASAT tests–there are some serious obstacles standing in the way of potential suppliers of these services. For one, no existing US satellite is designed to be refueled on-orbit. There are ways of getting around this (you don’t have to refuel the target satellite per se for the satellite to benefit from the capability of prox ops tugs and orbital refueling), but that provides at least one hurdle that has to be met. Another hurdle is that while there is demand, a lot of it is for government satellites. And while the government can purchase off-the-shelf services, and can set up procurement programs, they can’t typically sign a contingency contract for future delivery. Which means that without the military actually procuring the money to run the development as a typical government contract, potential suppliers would have to build the systems on speculation. And the reality is that most of the big publicly traded aerospace companies have a hard time taking up projects of this size on speculation–it’s just too much risk, and most of them don’t have huge amounts of free cash to invest in such concepts either. On the opposite side of the scale, smaller entrepreneurial firms might be able to handle the risk (and maybe even raise the money), but they face the hurdle that the AF or NRO is unlikely going to let some no-name company get anywhere near one of their “precious national assets” without first making them go through a gauntlet of paperwork. It isn’t impossible, and there may very well be a way of getting around those obstacles, but they are real barriers to entry. But remember: barriers to entry do not mean that there is no market!
Lunar Robotic Explorers
Rob then goes on to unfairly dismiss another market, deep space robotic probes:
The question I raise is this, are there governments around the world that can pay to put together a 20,000kg robotic spacecraft that would also have docking technology (maybe they can buy that off the Russians) and would need the services of an in-orbit propellant depot?
Why do you need to have a 20,000kg robotic spacecraft to take advantage of a depot? Why does the spacecraft itself need to do the docking (or berthing)? The explanation Rob gives in comments is lacking. Right now, there is only one vehicle that can even deliver a payload of 15klb to lunar orbit (that’s about what Delta-IVH can do). If you wanted to send even a 16klb payload to lunar orbit, a depot would be very useful. But more to the point, Delta-IVH’s are extremely expensive. A single Atlas V 401 (or even Falcon IX) could launch the satellite and the transfer state (a refuleing capable upper stage), and then buy propellant from the depot. If the depot is commercial, and in an inclination where it can buy from the cheapest suppliers, you might be able to do such a mission at a far lower cost than you could launching it on a Delta-IVH. With a propellant depot, even stages as small as the Falcon I upper stage can put sizeable payloads into lunar orbit. A single Zenit launched tanker could provide enough propellant for something like five Falcon 1 missions. There’s no reason why customers would have to build super behemoth spacecraft to take advantage of propellant depots.
In fact, some countries, whose launchers are still fairly small (like India for instance) could benefit a lot from propellant depots. The GSLV could launch the biggest GEO sat on the market (minus circularization fuel), and in conjunction with a propellant depot, the upper stage could be refueled could then deliver those satellites to GEO. Without having to develop a new booster to do it. In fact a refueled GSLV could easily deliver even a spacecraft the size of LRO/LCROSS to lunar orbit.
Anyhow, propellant depots can make a lot of sense for launching planetary/lunar probes, especially if:
- It allows you to launch your mission on smaller, cheaper launchers
- The depot can buy propellant internationally from the lowest bidder (ie is commercial in a proper orbit)
- It gives enough performance that your mission can be launched at all using your existing boosters (existing Atlas V Centaurs modified for refueling can deliver more than 3x what a Delta-IVH can put into lunar orbit without).
Anyhow, enough of that for now.Â My key point here is that there are potential users for propellant depots.Â Some of them (military LEO sats and possibly the Indian GSLV) that could benefit from the capability sooner rather than later.Â Even if NASA decides to continue ignoring propellant depots as Rob suggests, it is a technology that can probably make its way to market.Â But the path to commercializing those markets is going to be really tough without a solid anchor tenant.Â If NASA did adopt a more depot-centric architecture, it could go a long way towards providing the demand and breathing room for propellant depot operators that would allow them to grow those other markets until they’re ready.Â In other words, if NASA actually took seriously its legal mandate to “seek and encourage to the maximum extent possible the fullest commercial use of space”, it could make it a lot easier for propellant depots to come into existence sooner, and survive long enough for the markets to adjust to and adopt those new capabilities.
One last thought before I (belatedly) go to bed.Â In comments to the post, Rob states that:
A sustained presence on the Moon would help but I still think that building bigger rockets is ultimately more efficient than the entire depot Earth to orbit industry, infrastructure you would have to build to deliver this propellant, that could be on-vehicle using one big booster.
That last bit is the whole point. With depots, you no longer have to fit it all on one booster, which now has to be massive and bloated to deliver even a tiny payload to the Moon. With depots, you have a lot more size flexibility. The same depot that one day allows you to send a small lunar probe into orbit without requiring a huge launcher may on a different day allow you to send a 20-person crew to the Moon or Mars.
There are challenges that will need to be overcome on the way to implementing commercial propellant depots, but they are truly worthwhile, because without them, we’re never going to become a spacefaring society.
Latest posts by Jonathan Goff (see all)
- NASA’s Selection of the Blue Moon Lander for Artemis V - May 25, 2023
- Fill ‘er Up: New AIAA Aerospace America Article on Propellant Depots - September 2, 2022
- Independent Perspectives on Cislunar Depotization - August 26, 2022