Mass Confusion and White Knight 2 Kremlinology

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:

Flight learnt that WK2 has a 13,600kg (30,000lb) payload capacity sometime back.

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.

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
Jonathan Goff

About Jonathan Goff

Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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19 Responses to Mass Confusion and White Knight 2 Kremlinology

  1. Habitat Hermit says:

    I have no idea whether it’s 30000 lb or kg but if is kg wouldn’t it have made sense to go a bit further? 32,650 kg is the total mass of Quick Reach 1 according to and it is just a tad longer and beefier than SS2, I’m sure something could be figured out for the release mechanism.

  2. Rüdiger Klaehn says:

    I think 30000lb is more consistent with what to expect given the mass of SS1 (7920lb).

    But remember that this is to a release altitude of 15km. I would not be surprised if the WK2 could lift significantly more to a lower but still useful staging altitude of, say, 8km.

  3. James Robertson says:

    I would consider a Lockheed L1011 if I desired more lift capacity than 30,000 lbs. The L1011 can lift 52,000 lbs of Pegasus XL, but only to 40,000 feet which is somewhat lower than the reported staging altitude for WK2. I wonder how expensive an L1011 is compared to WK2. 🙂

  4. Ed says:

    Why not just go across the street and ask Burt?

  5. Paul Breed says:

    More speculation…
    On the cutaway pictures of SS2 the oxidizer was not named. It still talked about a hybrid propulsion, but it did not name the oxidizer, do you think that the Nitrous accident is causing them to consider something else?

  6. Jon Goff says:

    AFAIK, while they have investigated other options over the past several months, I think that Nitrous/Rubber is still their primary plan. They’ve looked at other options, but since they need ~60klbf worth of thrust, there aren’t too many options that are also low-cost. I wouldn’t read too much into it yet.


  7. Jon Goff says:

    Ha! Don’t I wish. I’m not even sure Burt’s aware MSS exists. I know several people on his propulsion team, but I’d bet that someone like Gary Hudson or Antonio Elias would have a much higher chance of getting such info from them. Burt’s usually pretty secretive about these sorts of things (and its understandable why), and I don’t think that having a shop on the other side of Sabovich street really makes that big of a difference for him.


  8. Jon Goff says:

    While the L1011 does have higher lift capacity, it’s also very volume constrained. Pegasus barely fits AIUI, and it’s a high density solid propellant vehicle. Maybe if your spaceplane was loaded with like Peroxide/Peanut Oil or something super high density like that. But then actually getting to space would be nontrivial due to the much lower performance….

    It’s not a totally lost cause, but WK2 was the best shot for an off-the-shelf carrier plane. All of the other planes would require enough modifications and operational headaches that it makes you wonder if it’s really the best approach. I’ll get into some of the other ways to skin that cat soon enough.


  9. Jon Goff says:


    I agree that the 30klb number seems to fit better with other numbers, but there’s just enough evidence (direct and circumstantial) to make me wonder.


  10. Jon Goff says:

    Habitat Hermit,
    Well, Will did say “about 30 tons”, which could mean plus or minus if that’s what he really meant to say. And with Gary Hudson and Burt being friends, it would increase the odds of them wanting to make WK2 “Quickreach compatible”. But I still think that the most likely explanation is that it really is only 30klb, and Will just misspoke in that presentation. It happens.


  11. Anonymous says:

    I think that 30,000 lbs is the WK2 payload mass to 50,000 feet and 30,000 kg is the payload mass to 20,000 feet. The payload mass is cut in half at high altitude as effective engine thrust and lift is cut in half at 50,000 feet. Also SS1 was more like 6,600 lbs so SS2 is probably 33,000 lbs. I am sure that Burt Rutan is contemplating launching the 30,000 kg AirLaunch LLC rocket from WK2 with a DARPA approved payload of 1,000 lbs into low earth orbit if Air Launch LLC makes progress. A 1-man, 1,000 lb capsule that “self” re-enters after 2 or 3 orbits is your best bet.


  12. gravity loss says:

    Regarding the T/W numbers, if total takeoff mass of WK2 is 30 t (t=1000 kg) then it’d be reasonable if the payload was 15 t ie 30000 lb. Rest is engines, structure and fuel.
    With a T/W of 10 the jet engines alone would mass 1.5 t. Add 5 t of fuselage and 8 t fuel.

    Actually that would make it a 50/50 staging system with wk2 and ss2 having same fueled mass. 🙂

  13. Ivan Vuletich says:

    This could well a units mixup. 30 tons = 60klb or 67.2 klb depending on whether you are using short or long tons.

    If you are using long tons then 30 tons = 30,481 kg, i.e ~ 30,500 kg

  14. James Robertson says:

    Re: L1011. I agree that the vehicle diameter is more constrained under the L1011, but I don’t think the volume constraint is as bad as you suggest. The density of Pegasus XL is 987 kg/m^3 (taking the overall weight divided by the volume of the body), which is much lower than the density of solid propellant: around 1350 kg/m^3 according to I would hazard a guess that that is due to “empty” interstage space needed for nozzles and such. A single stage LOX/kero vehicle might not be far off the Pegasus density due to less wasted interstage space.

    In addition, it looks to me like there is room under the L1011 for a slightly longer vehicle with a larger wing. A wet wing would also provide more propellant storage.

    I do agree that it is harder to package under the L1011, but the possible lift capacity advantage may make it worthwhile.

  15. ザイツェヴ says:

    One thing everyone seems to forget is how keeping an aricraft and pilots on your books is a big liability. If you fly every week, it may be acceptable, but otherwise all this talk about L-1011 is utter nonsense unless they want to persuade Orbital to lend the specific plane, the Stargazer.

    QuickReach’s ability to launch from an unmodified C-17 is a big advantage.

  16. Anonymous says:

    Work backward from the recent announcement:

    “Rutan has calculated that it is theoretically possible to use WK2 to place a single human passenger into orbit.”

    So if your calculations show it can launch more than one person, your probably working off the wrong starting number.

  17. Jon Goff says:

    Unfortunately, that depends a lot on your assumptions going in. There’s a large variability in stage performances for instance, even among the big boys. Just because Scaled thinks they could do one thing doesn’t mean someone else couldn’t do better (or worse).

    That said, I think the other scaling factors do point more towards 30,000lbs not 30,000kg.


  18. Rob says:

    Jon. There is no mistake in my report on what Will Whitehorn or others have said to me on this issue. Like any journalist I have a note book and I can write down what people say as fast as they say it. And I question them to confirm that I understood what they said.
    I actually got the original WK2 payload figure from an individual from the Swedish Space Corporation (SSC) when I was at the Kiruna spaceport event last year. SSC wanted to launch sounding rockets from WK2 and so they had been told the payload capability. At first Galactic did not want to confirm this but have since.
    At the UK’s Rutherford Appleton Laboratory space event Whitehorn said that it could carry 30,000kg but then did not reply to my email asking for clarification.
    But on Wednesday this week his exact words in my one-on-one interview were that WK2 could “carry more than 30,000lb to 50,000ft”.
    But I prefer to stick with the SSC officials statement. Why?
    When I asked Will what the landing speed of SS2 was he said “at least 115kt”. Did that mean 300kt I asked. No, he said, laughing, that would be too much.
    So now you understand the game that is being played.
    For commercial reason they don’t want to give exact figures but when potential customers blab them there is a standard public relations reaction of fudging the issue.
    With the evidence I have I think that WK2 can carry 30,000lb safely but when needed perhaps, as a guess, 2,000lb more but it will be pushing the envelope.
    I will explain in more detail about this at my hyperbola blog in a few minutes (1630h GMT) when I will also be stating the mass for SS2 and why I know it.
    I should add that the reason for four engines is that they wanted an engine out capability. Alex Tai told me they wanted an ‘on take-off’ engine out capability last year.

  19. Chuck says:

    Could the White Knight 2 gain performance from in flight refueling? Specifically, is the lift and ceiling limited by wings and power, or lift capability or fuel (time to climb?)

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