It’s been a rather interesting month so far, and I’ve been under a bit too much stress lately to blog much, but I wanted to put up some of the presentations from the Propellant Depot panel I was on at Space Access this year. If I had found the time sooner I would also say something about the advanced technology panel I was on, but it’s now been long enough I can’t recall what I was going to say.
Here is the humor slide I started out with:
My actual presentation:
Bernard Kutter’s presentation for ULA:
and Dallas Bienhoff’s presentation for Boeing:
I haven’t been given a copy of Rand’s presentation yet.
[Edit, here it is, Rand says he’ll probably get some annotations up later]
Anyhow, a few quick random thoughts that I don’t think anyone else has really hit upon on the intarwebs:
- One of the concepts out of Dallas’s presentation I liked was the idea of having a space transfer tug that takes landers from EML1 (L2 would also work) to some perilune trajectory, and then returns to EML1. I’ve been toying with variants of this idea for some time. With a Centaur-sized trasnsfer tug, fully-tanked-up in EML-1/2, you can actually bring pretty darned big landers most of the way to the lunar surface (ie leaving 1000m/s or less of delta-V for the descent), while still having enough propellant to return to lunar orbit and from there to the L-point station. That segment is probably one of the easiest in-space segments to start doing reusable stages, since you don’t need an aerobrake, and don’t have to deal with lunar dust, just propellant transfer, and lots of engine relights.
- In a conversation with Jeff Greason late one night at the conference, we got off onto the topic of RLVs and propellant depots. One of Jeff’s opinions is that in order to really have an industry for some service, you need enough demand to allow for 2-3 healthy competitors. With only one provider, you get monopolies, three is ideal. But for RLVs you probably want a small fleet (~3 vehicles) of RLVs so that you can provide dependable service even if you either have a mishap or have to pull one of the vehicles for maintenance or repair. Having a single vehicle may work during the development phase where you’re transitioning into operations, but once you’re in full operations, you want enough demand for 2-3 companies with probably 2-3 vehicles per year. And for each of those vehicles, in order to get the per flight price in a really good range, you need to fly often–Jeff says 100 times per year, but I’ve heard numbers as low as 30-50 (but any way you slice it, it’s a lot of flights). That comes out to somewhere in the 120-900 flights per year range. The interesting thing that Jeff mentioned was that if you postulated very small RLVs to start with (say 300-500lb to LEO net payload capacity), just one lunar mission per year would be enough to provide enough demand for an entire healthy industry by itself. Towards the lower ends of that scale, you’d only need one “soyuz around the moon” flight, or 1-3 GEO flights that used a propellant tank-up in LEO (say using a Falcon 1 with a mini-Raptor type LOX/LH2 upper stage?) to provide enough demand for at least the starting of an industry.
- While 300-500lb to orbit sounds tiny, that’s actually a pretty reasonable size for a first-generation RLV. The first stage doesn’t end up being that much bigger than existing or planned suborbital vehicles, doesn’t have to have much more capability either. The upper stage ends up down in the middle of the size range for proposed suborbital vehicles. While it has a much higher performance requirement, and much nastier reentry environment, it’s on a size that you can realistically work with a lot easier. Also, a lot of the TPS work can be refined by flying “expendable” upper stages on these first generation commercial suborbital launchers.
- This would definitely require the sort of RLV-friendly depot setup I described in my presentations–you’d have to have tugs that carry all the rendezvous/docking smarts, and keep the RLV-side of the propellant system as dumb as possible
- Propellants are a much less demanding payload than people. Not only does this keep up-front development costs down, but it also reduces the business risk if you happen to lose a vehicle occasionally. While high flight-rate RLVs should be capable of high reliability, we’re also talking about 1st or 2nd generation systems here, where we’re still learning a lot–and learning can be painful.
- I also liked Bernard Kutter’s graphic of the simple, single-launch, dual-fluid depot concept. This is a simpler version of the ideas Frank Z and I came up with last year (it uses a stock Centaur-sized tank for the LH2 side of the depot), but is still quite capable–on the order of 30mT capacity is nothing to sneeze at. With one of those in LEO and one in L2, that’s actually enough to do an ESAS-capacity lunar transportation system without Heavy Lift.
- One of the really interesting possibilities is that if something like this demonstrator depot were chosen as a part of the money Obama has proposed for orbital refueling technology demonstration (it wouldn’t need anywhere near the full $400M-1B that Obama mentioned per technology area), if the demo system worked, it would actually be operationally useful. Sure, you’d want to replace it and/or upgrade it down the road with lessons learned, but I’m a fan of pressing technology demos into operational service, as that’s a good way to get a lot more data out of the deal.
- I also liked how Bernard explained a lot of the cryo storage issues. A lot of this stuff still needs to be proven in space, but they (ULA, LM, and Boeing) have a lot more experience doing related tasks than most people realize.
I probably have some more thoughts on the matter, but I’m home at sick with a cold today, so I’ll leave it at that.
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- AAS Paper Review: RAAN Agnostic 3-Burn Departure Methodology for Deep Space Missions from LEO Depots (Part 1 of 2) - September 15, 2018