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I encountered this reddit post, which is a video of a test of some sort of device hovering precisely in place & maneuvering using some slightly underexpanded reaction control system. I'm also assuming there's some reaction wheels aboard, as it yaws without me seeing any corresponding plumes.

A commented added:

This is actually fairly old technology developed during the Cold War. It is designed to be used to intercept incoming ICBM’s in space.

I find this very unlikely for multiple reasons:

  • Why would such a system require such a precise maneuvering control system? To be able to hover like this is no small feat of control systems design.

  • Why would it have such limited reaction mass? In the video it burns out quite rapidly. To reposition to intercept an ICBM would not only require about a zillion of these things in orbit over the US at all times, but likely a massive amount of dV on each to be able to rapidly reposition to where a collision would occur.

  • The video quality seems too good to be from the 50s-80s.

  • It doesn't look like a vacuum optimized system. Sure, maybe it's just missing some nozzle inserts, but the thought was still in my mind.

  • I can't find anything on this through some cursory google searches. So I turn to you guys.

What the heck is this thing?

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    $\begingroup$ Does this answer your question? Does this kill vehicle contains momentum wheels? Watch how it moves! $\endgroup$
    – Russell Borogove
    May 19, 2020 at 21:48
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    $\begingroup$ No reaction wheels; some of the thrusters are offset from the center of mass, so depending on which combination it fires it can either rotate or translate. In use it'll mostly be translating -- it'll be put on a near intercept course by something else and just divert a bit in the terminal phase. $\endgroup$
    – Russell Borogove
    May 19, 2020 at 21:50
  • $\begingroup$ That's helpful information! However, my question is what is this thing, not how does it work. $\endgroup$
    – Anton Hengst
    May 19, 2020 at 22:02
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    $\begingroup$ If it's just a suborbital guided missile, why test it hovering in earth gravity at all? $\endgroup$
    – ikrase
    May 20, 2020 at 2:31
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    $\begingroup$ Thermos bottles are pretty dangerous at a closing speed on the order of 10km/s. $\endgroup$
    – Russell Borogove
    May 20, 2020 at 5:25

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That's the Multiple Kill Vehicle.

The filmed test took place in December 2008.

enter image description here

The MKV mission was to destroy medium-range through intercontinental-range ballistic missiles equipped with multiple warheads or countermeasures by using a single interceptor missile. During an actual hostile ballistic missile attack, the carrier vehicle with its cargo of small kill vehicles would have maneuvered into the path of an enemy missile. Using tracking data from the Ballistic Missile Defense System and its own seeker, the carrier vehicle would have dispensed and guided the kill vehicles to destroy any warheads or countermeasures.

Source: Wikipedia

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