I was going to write a short post today about a variant on the Dual-Fluid, Single-Launch Propellant Depot idea, when I realized I had never actually gotten around to explaining the idea here on the blog. While this idea has been now discussed in a few of our AIAA papers ULA and I have published on the topic, I figured it was still worthwhile to do a brief blog post for those who haven’t had the time to read any of these papers.
Back in early 2009, Frank Zegler (of ULA) and I both independently came up with the concept for a LOX/LH2 propellant depot of decent propellant capacity that could be launched on a single Atlas V. The basic idea, illustrated in the picture below (lifted from Bernard’s presentation from last month that is the third link above) is pretty simple:
- You have three parts, the Centaur upper stage that is used to launch the depot, a central section that holds the depot controls, docking interfaces, propellant transfer interfaces, etc, and a depot LH2 tank that is manufactured using the same tooling and processes as the Centaur tanks.
- When you get to orbit, you transfer all the excess LH2 from the Centaur to the depot LH2 tank, you then vent the Centaur LH2 tank down to vacuum, and permanently seal off the connection between the Centaur LH2 tank and the depot LH2 tank.
- You then close off the Centaur LH2 tank, and transfer the remaining LOX from the Centaur into it. It now becomes the LOX tank for the depot.
- In order to reduce propellant boiloff to reasonable levels, ULA has typically suggested either the use of MLI and/or deployable sunshields to cut down on the heat flux into the tanks.
- Boiled-off hydrogen is used as a “heat sponge”, to intercept heat flowing into first the LH2 tank than the LOX tank. Eventually the now much warmer GH2 is run through a rocket nozzle to provide settling force and station reboost. It turns out that the boiloff rate achievable with good passive system design is lower than the amount of propellant you need to use anyway for stationkeeping/reboost, so for depots in LEO or L1/L2, you get the benefits of a ZBO system with separate reboost capabilities without the difficulty of building a ZBO system.
The end result is that with a Centaur diameter depot LH2 tank, you can store around 30 tonnes of propellant. It’s a bit on the small side, but enough to fully refuel a Centaur or Delta-IV upper stage in orbit. It has both propellant. It requires no orbital assembly, no EVAs, no new tank tooling, etc. The depot tank and depot center section together weigh less than 2 tonnes, so you can actually use the Centaur to deliver a depot like this to anywhere in cislunar space, and with good passive shielding (sun shields etc) to Mars orbit or to a NEO you want to visit.
There are ways of going bigger than 30 tonnes that have been discussed, the two primary ways suggested have been having the depot LH2 tank built into the 5m diameter fairing (instead of keeping it Centaur diameter). That gets you up to ~60-70 tonnes of propellant. Going with ACES style tanks can get you up into the 110-120 tonne capacity, but require the development of the ACES stage. Even 30 tonnes of propellant, if you have depots in both LEO and at L1 or L2 is getting into the range that you can do very interesting things.
Anyhow, just wanted to introduce people to the idea if they hadn’t heard of it before. Read the papers for more details.