Perhaps I'm naive to think that the single large nozzle at the back of the X-37B is a nozzle for an engine, but regardless of the purpose, gas exiting from a nozzle will produce thrust. Is the X-37B's center of mass along this line? I can't think of any other reason why this would be offset.

Note that the most recent flight also tested ion propulsion, and that would not involve a large bell-shaped nozzle.

So, Why is the X-37B's large nozzle offset to right from the spacecraft's mid-plane? I suppose an answer should include some suggestion what this nozzle is for to begin with.

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above: Credit: Boeing. From Spaceflight 101

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above: Credit: Boeing. From Space dot com.

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above: "The military’s mini shuttle on the KSC runway this morning." Credit: Air Force. From Spaceflight Now

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above: From Fantastic Plastic.

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    $\begingroup$ Related: space.stackexchange.com/questions/19195/… $\endgroup$ Commented May 8, 2017 at 2:01
  • $\begingroup$ Followup question: What's inside this thing that the service personnel need to be wearing full hazard suits? $\endgroup$
    – aroth
    Commented May 8, 2017 at 5:44
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    $\begingroup$ @aroth: the remaining propellant for the engines. They might be hypergolic (en.wikipedia.org/wiki/Hypergolic_propellant), which are really nasty substances. $\endgroup$
    – Hobbes
    Commented May 8, 2017 at 7:18
  • $\begingroup$ @aroth it's probably safe to assume that that giant hose is not like one of these: en.wikipedia.org/wiki/File:Dumping-RV-Waste-Tanks.jpg or en.wikipedia.org/wiki/File:Aircraft_lavatory_service.jpg $\endgroup$
    – uhoh
    Commented May 8, 2017 at 7:27
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    $\begingroup$ @SF. I included the fourth photo (the model) for a reason - to better highlight that what looks like a flat black flange against the back of the shuttle is actually a tall, 3D cone. Look again. As you change viewing angle, it will appear to move left and right if you mistakenly assume it is flush against the spacecraft. You're experiencing an optical illusion, that's all. The nozzle is so black in the other photos that there aren't enough cues to see the true 3D shape. $\endgroup$
    – uhoh
    Commented May 8, 2017 at 8:30

2 Answers 2


First off, it's absolutely an engine nozzle. Some sources say X-37B has a storable biprop propulsion system with ~900N thrust. The Aerojet R-42 is such an engine and the dimensions look about right -- 24" long bell with 15" exit diameter. This engine uses MMH and NTO as propellants, which are toxic, requiring the use of hazmat suits by ground crew.

The X-37's propulsion system is used for orbital maneuvering and the de-orbit burn only; it rides a two-stage Atlas launcher to orbit, and glides to landing like the space shuttle.

For an engine only used in space, it's not necessary that it be mounted on the mass centerline of the spacecraft. As long as the engine can thrust directly through the center of mass, it can accelerate the craft without producing unwanted torque. Since the spacecraft will carry a wide variety of possibly-asymmetrical payloads, it likely needs a gimbaling engine anyway, so mounting it off-center with a small "toe-in" angle works as well as putting it on the centerline.

I don't know what drove the off-center placement of the engine in the development of the X-37, but the Boeing document X-37 Storable Propulsion System Design and Operations shows a symmetrically mounted pair of 450N engines instead of the single. That would point toward the earlier design using the Aerojet R-4, and:

  • A pair of R-4s replaced by a single R-42, giving about the same total thrust, with minimal changes to the mounting, or

  • A pair of R-4s replaced by a single R-4 with a larger bell -- the 300:1 expansion ratio nozzle for the R-4 is about the same size as the 160:1 nozzle for the R-42, and yields substantially better specific impulse.

I would assume the two-engine layout already had the mountings toed-in by a couple of degrees to minimize off-center thrust if one of the engines failed.

Alternately, since the spacecraft is intended for propulsion tests (among other things), offsetting the engine allows a test-subject propulsion unit to be placed closer to centerline than would be possible if the primary propulsion unit were centered.

Some illustrations show an asymmetrical solar array deployed on the left side of the spacecraft, but I don't think that's relevant because (a) it would move CoM to the left when deployed, and the engine is on the right, (b) the solar array would likely be retracted when firing for de-orbit, and (c) the illustrations that show the asymmetric solar array also show a centered engine.

  • $\begingroup$ OK this is so well explained that even I can understand it! Is the primary purpose of the engine the de-orbit burn (and thus on-demand reusability)? And this burn would be over before aerodynamic forces become significant? In space a few degrees tilt in attitude would be manageable, but I'd imagine you'd want only 0 or 180 degrees yaw once there was significant atmosphere. $\endgroup$
    – uhoh
    Commented May 8, 2017 at 4:56
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    $\begingroup$ I imagine it works much like shuttle: the deorbit burn just lowers the perigee into the atmosphere. space.stackexchange.com/questions/12011/… $\endgroup$ Commented May 8, 2017 at 12:58
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    $\begingroup$ Besides the de-orbit burn, the engine also provides for orbital maneuvers, and the project specified a remarkably high 3100m/s delta-v for the spacecraft at one point, but that would all be out of atmosphere. It gets to orbit on a two stage Atlas, under a fairing. $\endgroup$ Commented May 8, 2017 at 13:15

It is indeed the main propulsion system, powered by hydrazine and nitrogen tetroxide, hence the hazmat suits. The team wearing those suits checks the vehicle over and then hose it down, after which other people can approach without special gear on. They used to do that with the Space Shuttle too, and for the same reason.

Fun fact: the American crew of the Apollo Soyuz Test Project made the final return to Earth aboard an Apollo capsule. The mission had gone perfectly, but one thing went wrong during the return: a pressure equalization valve opened prematurely, while the reaction control system was still firing. They also used hydrazine and nitrogen tetroxide, and it was sucked into the cabin air. The crew survived, but spent weeks in a hospital in Hawaii to recover from lung damage.

The engine nozzle is offset simply because the original design, back when it was a NASA program, called for a choice of one or two engines, depending on thrust requirements for a mission. The USAF never exercised that option, but everything was already built to accommodate two engines. So it has just the one, off to one side. It would've cost time and money to modify it to put the engine in the middle, and it works well enough where it is.

  • 2
    $\begingroup$ No one "hosed down" the Space Shuttle after landing. $\endgroup$ Commented May 17, 2020 at 16:05

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