SNC Abandons Own Hybrid Motors on Dream Chaser

Dream Chaser Main Propulsion System Test. (Credit: Sierra Nevada Corporation)

Dream Chaser Main Propulsion System Test. (Credit: Sierra Nevada Corporation)

Sierra Nevada Corporation won’t be using its own hybrid rockets for its Dream Chaser space shuttle, making it the second company in recent months after Virgin Galactic to dump the nitrous oxide-rubber motors.

Kathy Lueders, program manager for NASA’s Commercial Crew Program (CCP), revealed the change in an update during the third quarterly meeting of the Aerospace Safety Advisory Panel (ASAP) on July 24.

“SNC has also baselined a new propulsion system design (a pure liquid system design rather than a hybrid) in conjunction with their purchase of ORBITEC,” according to the meeting minutes.

Dream Chaser would have used two small hybrid motors per flight. Virgin Galactic’s SpaceShipTwo would have used one larger nitrous oxide-rubber hybrid motor.

I asked Charles Lurio, who first broke the story a few weeks back in The Lurio Report, what lay behind Sierra Nevada’s decision to abandon their own motors. He said sources told him that ORBITEC, which Sierra Nevada recently acquired, had a better engine solution.

I’m skeptical of whether that is the full story. The Virgin Galactic contract would have been quite lucrative for Sierra Nevada assuming a high launch rate for the SpaceShipTwo suborbital tourism vehicle. It also would have offset some of the fixed costs for the Dream Chaser engines, which would likely produced in much smaller numbers.

A liquid engine for Dream Chaser would be less expensive than a hybrid one assuming it could be reused multiple times. The hybrid engine would have to be replaced after each flight.

  • Solartear

    So NASA’s technology readiness estimate was correct to doubt some of their choices. Good to see they are quickly switching to a much better system, no doubt thanks to the benefit of seeing SpaceShipTwo’s lengthy delays.

  • Aerospike

    Some positive news about Dream Chaser for a change.
    I was highly skeptical about using two hybrids (with potentially “unstable” combustion) in a side by side “tandem” configuration.

    Switching to liquid propulsion should provide a much smoother ride!

  • HyperJ

    Yes, agreed. One hybrid, sure. But two, spread that far, seemed like a recipe for disaster.

    This is good news for the safety and ultimate cost of Dreamchaser, IMO. The biggest drawback for them is how this will impact their schedule.

  • JustAGuy

    Wow, you should really know the whole story before jumping to conclusions like that. The hybrid motor has been developed to the point that it would provide a smooth combustion for the duration of the burn. Also, liquid engines are as susceptible to pressure oscillations as hybrid motors. Just look at the development of the Saturn V F1 engine. Over 2700 tests and refinements before the engine was deemed stable enough to put into service. We’re not afforded that luxury today because of cost issues and the intolerance for failures during development. As to a liquid propulsion system being less expensive, that is simply not true. First, look at how that worked for the space shuttle. Second, the components and fuel for a hybrid rocket are much cheaper than for a liquid engine.

  • Terry Stetler

    The next question is what engine cycle, if pressure fed, or if it’ll be ORBITEC s Liquid Vortex. Other sources say ethanol based. The speculation light is on.

  • Jeff Smith

    The problem with nitrous oxide based hybrids is not strictly combustion instability, it’s a flame holding instability. The literature refers to it as Inherent Low Frequency Instability. It’s common to propellants that aren’t classically “propellants” at all: nitrous and rubber are NOT propellants. Nitrous disassociates at rather high temperature and trying to burn tires in a pile doesn’t happen unless you have lots of gasoline. The propellants is always going through an light/extinguish cycle that you see clearly on the SS2 motor. One solution (that Virgin picked) is to go with at least one material that disassociates more easily: nylon. If you look at AMROC, they used LOX in their hybrid (which is the IP that SNC is using). Both of these make the problem much easier to solve. Just remember, hybrids have the advantages of both liquids and solids, AND their disadvantages. As long as you go in with your eyes open, you won’t be disappointed.

  • Charles Lurio

    No, it’s off. The fuel is propane, the system is Orbitec’s Vortex.

  • Jeff Smith

    It’s be pressure fed in some manner. How else could it stay on station for 6 months without use and then be ready when you want to come home?

  • yg1968

    Is it LOX-propane?

  • therealdmt

    On the plus side, they were smart and flexible enough to switch from a losing technology to a winning one.

    On the negative side, they stuck with hybrids for years after it appeared there were troubles with the tech and are only switching now, late in the game (the very month final contracts are to be awarded). When combined with their failure to get something as basic as landing gear deployment right (or to have a fail safe “all gear up belly landing mode” in case of failure to deploy, so as to avoid a catastrophic or life threatening [if passengers had been aboard] touchdown) on the only flight test they’ve managed to conduct – and – the uncertainties surrounding lifting body aerodynamics in hypersonic flight (shifting CL and how to deal with it, iirc), I have to admit that backing Boeing’s CST-100 would be the clearly less risky investment for NASA as backup/competitor to SpaceX.

    I’d still like to see DreamChaser fly (and so Sierra Nevada win one of the contract awards), and the vortex engines are exciting in their own right, but SNC does leave me wondering if they can pull this off In a timely manner. Since it’s not a cost plus contract though, we don’t have to worry about them blowing through tons of taxpayer dollars to get things right, so that is a good thing for sure.

  • Charles Lurio

    No, nitrous-propane is what I was told and printed in my July Lurio Report.

    You can see the advantage…simpler changes than with LOX tanks.

  • Jeff Smith

    That implies that would be using a self-pressurizing system, or a hybrid self-pressurizing/pressurant gas system. Nitrous/ethane is a better pairing based on vapor pressure (what Tim Pickens was working on), but nitrous/propane can work too. Considering it’s nitrous/propane, that also means they probably have a electric ignition system.

  • Eric Lucero

    JustAGuy is so right-the issue is propellant burn stability and the thrust delivered vs payload weight to be inserted into a given orbit/arc, regardless the vehicle system.

  • HyperJ


    Yes, there are combustion instabilities with liquid engines, but they mostly are an issue for larger scale engines – Like the F1 or SSME’s, as you noted. But the liquid engines needed for DC aren’t even close to that size, and over the last decade there have been many smaller liquid engine development across many small engine builders, all fairly successful.

    The problem with hybrids is that for the same thrust, the combustion chamber is MUCH BIGGER than it would be for a similar thrust level liquid engine. And so you would expect to run into combustion instabilities much sooner. And not only that, it is an evolving combustion chamber as the solid propellant burns off, whereas a liquid combustion chamber stays the same size.

    “Second, the components and fuel for a hybrid rocket are much cheaper than for a liquid engine.”

    Then why is DC switching? Safety? Performance? Restart issues? Something else?

  • J Michael Antoniewicz II

    Considering what ORBITEC is bringing to the table, in brains, ideas, technology, etc, power .. no duh they are dumping the bunny-hugging ‘green’ hybrid engine for a robust, good to great performance margins, re-usable engine that, in time, will shake out to have tens of minutes between major maintenance cycles. And if not of the same thrust class as SpaceX’s Merlin Family, expect some tech application envy from Musk et al as Sierra Nevada-ORBITEC sidesteps several issues…. 😉

  • J Michael Antoniewicz II

    Flowmetrics comes to mind. 😉

  • J Michael Antoniewicz II

    Wait till they start doing chamber wear tests/examinations on the test/prototypes .. there are going to be people not believing the numbers and thinking they are faked tests or made up…. But what those numbers are going to due to the robustness, reusability, and reliability engine run time between maintenance calculations are going to be nuts. 😉

  • Jeff Smith

    I haven’t heard a peep out of them in years. No one wanted the technology and I thought they just gave up marketing it. The last time they were at Space Access was some time ago.

  • Fortunately SNC is getting funding from both ESA and JAXA in regards to achieving manned spaceflight systems. So I think it is likely the Dream Chaser will be completed even if it is not one of the final winners of the commercial crew program.

    So NASA’s commercial crew program, which allowed the Dream Chaser to get started, may in effect wind up allowing both ESA and JAXA to also achieve manned spaceflight.

    Bob Clark

  • Remember for the hybrids the combustion chamber, which also holds the solid propellant, and the nozzle, have to be replaced with each flight.
    That makes it more expensive for a reusable system than a liquid fueled one.

    Bob Clark