Random Thoughts/Rocket Legos: Masten Xephr as a Vulcan SRB Replacement?

I’m sick at home today, so had extra time to think about fun technical concepts, so here’s an interesting one to think about. Right now, ULA’s plans for Vulcan include using expendable Orbital ATK GEM 63XL sold strapon boosters to enable higher performance missions, as shown below.

Vulcan Centaur Exploded View (credit ULA)

Here’s my crazy thought–what about a future upgrade replacing the expendable solid-fueled strapon boosters with fully-reusable VTVL liquid-fueled boosters?

Here’s some thoughts/considerations:

  • The boosters usually stage pretty early in the flight, just like the side boosters on Falcon Heavy, meaning that the required dV for boostback and landing are fairly modest, probably enabling all landings to be RTLS land landings.
  • Historically people have estimated that the cost per booster is in the $5-10M range. With a fully reusable booster stage, a much lower number should be possible, meaning that heavier Vulcan flights would be a lot cheaper than otherwise.
  • The GEM63XL has as best I can tell ~450klbf of liftoff thrust. That’s right in the same size range as the Masten Xephr XS-1 concept1, which has ~7x 65klbf engines last I saw.
  • A LOX/Methane stage like Xephr would have ~20% higher Isp than a solid stage, and being pump fed, even with reusability is likely similar or better pmf, meaning there’s a good chance you could still provide the same impulse to the Vulcan stage while maintaining enough prop for RTLS maneuvers.
  • I’m a bigger fan of VTVL powered landing than SMART reuse, but for missions like this, the core booster would be going so fast at staging that recovering just the engines might not be a totally awful approach.
  • It might be possible to design the Xephr boosters to serve both ULA Vulcan strapon booster markets, as well as flying their own expendable or fully-reusable upper stage (for much smaller LEO payloads–maybe in the 1mT class). The strapon booster market would mean you’d want to have a small fleet of 8-10 of these boosters, and ULA would only be using them a few times a year, meaning that the rest of the time you could have them flying smaller payloads such as propellant (enabling lower cost ULA distribute launch), megaconstellation satellites, or cargo deliveries to space facilities.
  • The current Xephr concept already used parallel staging of the upper stage, which would be attached via struts much like strapons are attached onto a core stage. Would it be possible to design the struts to work for both applications?

    Masten’s XS-1 Concept Xephr (credit Masten)

There are a few challenges that would need to be addressed to make this work though:

  • Solids don’t have a lot of propellant on them compared to a liquid fueled first stage like Xephr. GEM63XL has a GLOW of ~117klb, in spite of having 450klbf of liftoff thrust. It might be possible to partially load the stage for Vulcan boost applications to provide more thrust to the main vehicle. Though this will require some optimization and analysis to see how best to balance things.
  • Another challenge is that Xephr is probably more squat than the planned GEM63XLs. Right now some of the pictures show the boosters evenly distributed around the stage, but others have them clustered tightly together on both sides. It would be relatively easy at this point to change the spacing on the boosters, but will become significantly more expensive to change down the road.
  • Another aspect of the fine aspect ratio of the solids relative to the Xephr stage is that it might force you to a higher aspect ratio booster than Masten would ideally like, in order to keep the attachment points the same. Though the LOX/Methane stage is lower density than a solid so it might still be tall enough, just wider. Once again, more analysis and optimization would need to be run.

All told, it’s an interesting idea that would be worth running some numbers on. None of the challenges are obvious show-stoppers, but the devil is in the details. This concept would make Vulcan more competitive down the road, and could potentially have a lot of additional synergies between the two companies. This also would provide a company like Masten with an existing anchor customer for a Xephr-class reusable first stage. Rockets aren’t Legos, but during the early design phase, there’s a lot more flexibility to easily accommodate crazy ideas like this.

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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.
  1. At least as best I can tell from public sources.
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6 Responses to Random Thoughts/Rocket Legos: Masten Xephr as a Vulcan SRB Replacement?

  1. Paul451 says:

    Could you fit 6 Xephr’s around the Vulcan first stage, even with even spacing?

  2. Jonathan Goff Jonathan Goff says:


    I think so, but obviously you’d need to check. You might have to go with a little higher of an aspect ratio for the boosters than the current Masten concept. As I said, there’s a decent amount of analysis that would need to be done to see if this closes. If I were independently wealthy, I’d be paying SpaceWorks to run the numbers–this sort of analysis/system optimization is right in their wheelhouse.


  3. Dave Salt says:

    I wonder if the New Shepard booster would make a credible alternate, though I suppose it would involve far more ‘units’ than the Xephr option?

  4. Jonathan Goff Jonathan Goff says:

    I’m doubtful–my guess is it’d be too low of thrust, and too low of density (ie too high of diameter) to fit around the stage. This is one of those areas where LOX/LH2 is probably at a clear disadvantage.


  5. johnhare john hare says:

    I find it annoying that nothing comes to my mind in terms of improving or criticizing the idea. The best I could come up with was a universal booster that could be used to assist many companies. Pretty sure that wouldn’t work in the foreseeable future for reasons of business, compatibility, and personalities.

  6. ken anthony says:

    I had the same idea John, but starting in LEO or Lagrange. it would have a frame that adapted to any vehicle below its maximum size. That would allow a fully fueled vehicle for a substantial part of the mission getting more acceleration during gravity slingshots (a real rocket scientist would have to run the numbers.)

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