For those of you who have been following my Orbital Access Methodologies series, you’ll remember that I talked about the potential of using White Knight Two as the carrier plane for a small “assisted SSTO”, or a “TSTO with glideforward first stage landing” (to coin a phrase for the good idea John Hare mentioned in comments to my last post). The key question was what the lift capacity of the plane was. Now, I work across the street from Scaled here in Mojave, and I know some people on the propulsion team, but knowing Scaled’s usual desire for secrecy (and not wanting to get my friends in trouble), I had studiously been avoiding asking them for some time now.
So this morning I decided to do some googling, and I came across some numbers on Wikipedia that claimed the lift capacity was 30,500kg. Now, normally I wouldn’t trust numbers from Wikipedia that I couldn’t verify elsewhere, but the numbers jibed with some back-of-the-envelope Scaled “Kremlinology” I had been doing, so I decided to run some basic numbers on using it for an orbital launcher.
And the numbers looked good. Depending on assumptions, a 1000lb payload wasn’t completely out of the question for the assisted SSTO design (enough for probably 2 people, or a pilot and a microsat, or a bigger microsat, or some propellant). And the TSTO design was showing on the order of 2000-4000lb depending on assumptions (based on LOX/Kero for the first stage and LOX/LH2 with a stock RL-10 for the upper stage).
Rob Coppinger over on his Flight Global blog, Hyperbola, put up a blog post along the same lines (talking about using White Knight 2 for launching orbital payloads), but something in the article caught my eye:
So, now I was confused. Which was it, 30 metric tonnes like Wikipedia claimed, or 30,000lb like Rob was reporting. I brought this up, and Rob commented that the 30,000lb number came from a one-on-one interview they did today with Will Whitehorn of Virgin Galactic.
To me, that sounded pretty authoritative.
Out of curiosity, I decided to go back to Wikipedia and try to figure out where they had gotten their number from. I figured they must have misheard something–maybe they read the earlier Flight report and accidentally mistook 30,000lb for 30,000kg or something like that. Anyhow, the source for the data was given as a Pratt&Whittney press release and a video from a presentation at Oshkosh this year. The first release didn’t have too much information (just that the four PW308 engines each produce ~6900lb of takeoff thrust, for a total takeoff thrust of just under 28klbf). The second link was for an hour long presentation given by Alex Tai and Will Whitehorn. Being a nerd, I decided to listen through to try and figure out where that darned number came from. And sure enough, right around 32:45-33:00 into the video, Will Whitehorn says that they were building White Knight Two with extra range and lifting capacity, and that it could lift “30 tons” (or possibly tonnes) to about 60,000ft.
So, now I’m just confused.
As a comparison, WK1/SS1 combined had a takeoff weight of around 17000lb, with its two engines producing somewhere around 3850lbf each (if I’m reading the right engine), for a T/W ratio of about 0.45 at takeoff. If a similar T/W ratio is used for the WK2/SS2 combo, it would predict a takeoff weight of about 61,000lb. That would put the WK2/SS2 combo at a similar weight ratio (of WK2 to SS2 weight) as the WK1/SS1 combo. So at least that jibes.
So I guess what I’m wondering is, if anyone can shed any light on this? The most likely explanation is that when giving his presentation, Will meant to say 30,000lb and said 30 tons instead, and that in his interview with Rob, he stated the correct value. However, it’s also possible that he either mispoke in his interview with Rob, or both numbers could be right–30 tons may be the maximum payload, but 30klb is roughly the weight of SS2?
The big reason why this matters is that it effects my air-launch ideas substantially. If the later numbers Rob is quoting are accurate, then it’s highly unlikely that you could make a reusable orbital vehicle that could use WK2 for air launch. You might, if you make enough compromises get something, but it would be a rather marginal design. Which would mean that while WK2 could still provide an interesting carrier platform for TSTO ELV microsat launchers and for manned suborbital missions; real air-launch RLVs would need to either wait for WK3 or would have to find some other way of getting airborne–and that may very well break the case for them at least for the foreseeable future.
I hate it when an inconvenient fact kills a perfectly beautiful theory, especially when its my theory.