Hating On Propellant Depots

Dan Adamo is a retired flight dynamics officer for the Shuttle program and does space mission trajectory, design and operations in his sleep. I recently saw him put together a design reference mission for a human mission to the moons of Mars, for fun, using only well understood technology. Back in February he did a Space Show Classroom where he discussed many topics and once again demonstrated his effortless grasp of orbital dynamics.

A listener asked about propellant depots and Adamo answered with what I think is the most articulate objection to the concept of low earth orbit propellant depots. He went on to talk more favorably about propellant depots at Lagrangian points and on the surface of destinations such as the Moon and Mars, but I think his objections are more interesting.

Starting from the ground up, here's where I have problems with propellant depots, in low Earth orbit.

Once you establish the depot, it can be empty for all I care at this point, but once you establish it's orbit you have an orbit who's plane is at the mercy of perturbation, namely the earth's oblateness, which is going to cause it to regress westward, like we talked about - a few degrees a day depending on the inclination. So there's only certain times that you can leave that orbit plane for the Moon, and there's only certain times where you can leave it for, let's say, Mars, or some other intermediate destination like a near-Earth object, an asteroid.

I can almost guarantee you, particularly with near-Earth asteroids, that you can't pick one orbit plane, and have it work at-all frequently for the asteroid. The inclination will probably be wrong and let alone the other angle that orients the plane which is called the right ascension of the ascending node.

So you're at the mercy of this initial launch that puts the propellant depot on-orbit. You cannot pick when you want to leave like you can from a launch site that is rotating, ya know, with the Earth - on the ground, there's basically a launch window every day to whatever interplanetary objective you have and you're only going to spend just part of an Earth parking orbit before you depart low Earth orbit entirely, and you can pick what the orbital plane is - and make it the optimal for your mission objective.

Dallas Bienhoff from Boeing was on the show in May to discuss propellant depots and addressed a few of the concerns but, in my opinion, not the primary one.

He explained that a propellant depot in LEO would need to reboost itself and station keep, and seeing as it has an abundance of propellant (and boiloff of around 0.1% of hydrogen per day) that should be manageable, so that answers "at the mercy of perturbation" objection.

Bienhoff also offered numbers for out-of-plane penalties of 10% of delta-v, which he considered an acceptable tradeoff for the capability that a propellant depot infrastructure provides, and suggested this could be mitigated by choosing an initial plane which has the most demand from customers, and/or having multiple depots in LEO. Adamo actually said something similar earlier in the Show Classroom episode in response to a question of whether or not launching a Moon mission from the ISS was practical - which is certainly not an optimal orbit for lunar missions - he said it was, if there's a good reason to accept that penalty.

Adamo's primary objection remains, and I'll paraphrase it as: there are less launch window opportunities from an arbitrary low Earth orbit than there are from an arbitrary launch site on the surface of the Earth. Is this, like the 10% penalty of out-of-plane maneuvers, just another trade and something we have to live with? What are the actual numbers?

If anyone knows the answers to these questions, please leave a comment or email me direct.

[update]

I asked Dallas Bienhoff at New Space 2010 and his answer was, and I paraphrase: live with it.

I've gotten a much more detailed answer from Henry Spencer and his answer was, and I paraphrase: live with it, or spend more fuel, you've got a lot of it.

I think I prefer the second part of Henry's answer the best, by doing a burn to put yourself into an appropriate LEO orbit you can leave whenever you want, it just costs fuel but considering you're at a fuel depot, which is there for more than just your one mission, that seems reasonable.

I've asked Dan Adamo for comment, we'll see what he says.