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This comment below this answer about 'spacecraft' or at least 'craft' that can maneuver near the surface of low gravity bodies links to this video about Lockeed Martin's Multiple Kill Vehicle.

If I understand correctly it is designed to operate for a short time (destructively) in space, but this test happens on Earth, presumably in normal atmosphere. I don't know the size but Gizmodo says it was at an altitude of 23 feet.

I see a "main" thruster to maintain altitude (in the test, against Earth gravity), and a "left" and "right" thruster for translation and stopping, and four diagonal thrusters for "fine positioning", perhaps? Although the axes of the top pair meet above the spacecraft axis, and that of the bottom pair meet below, so perhaps they add translation and torque simultaneously?

No matter though, it looks like all thrusters lie within a single plane of symmetry.

The attitude seems quite stable throughout, even after falling and landing. By the way it moves, does it look like it has rotating intertia wheels of some kind, to maintain attitude?

The YouTube video shows the same 30 second test "flight" from three different angles:

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above: GIF made from frame numbers 300 to 326 extracted from this YouTube video.

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above: GIF made from frame numbers 459 and 460 extracted from this YouTube video.

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above: Lockeed's Martian Multiple Kill Vehicle - cropped from here.

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3 Answers 3

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The thruster configuration can be seen better in this image:MKV-L image(cropped from this document ).

The attitude control thrusters are not in plane with the direction control thrusters. This Raytheon patent on MKVs states that "attitude control system includes multiple thrusters offset from the center of gravity that provide yaw, pitch and roll control." All four of them are offset towards the aft, and the top and bottom pair are offset from the roll-axis. Firing one thruster provides torque on all three body axes, firing a second one compensates two of these axes and enforces the third.

This does not rule out that reaction control wheels are used in addition. This older video mentions that the platform is "gyroscopicly balanced", which probably means that an IMU is used as guidance input.

However the design objective of this device is to have many vehicles with independent steering at low cost and effort. This makes it feasible to choose such a simple thruster arrangement and put the burden on the guidance system (hard- vs. software). It is unlikely that reaction control wheels are employed from this perspective.

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  • $\begingroup$ Excellent image and information! They really do look like all eight lie in a single median plane, or extremely close to it. The language you mention in the patent seems to naturally fit the older video mentioned in the answer by @SF. but I think the thruster configuration in the video in my original question are all in a single mid-plane, or extremely close to it. $\endgroup$
    – uhoh
    Commented Oct 26, 2016 at 11:37
  • $\begingroup$ Here's a question about the term "gyroscopically balaned" in the older video $\endgroup$
    – uhoh
    Commented Oct 26, 2016 at 11:38
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    $\begingroup$ @uhoh I also noticed they likely have reduced the offset, but I can still clearly see it. BTW: I was not able to find a term for such a thruster configuration, that would be useful to find further documentation. $\endgroup$
    – Andreas
    Commented Oct 26, 2016 at 11:52
  • $\begingroup$ Excellent answer. Any idea why the ACS thrusters wouldn't be placed further back from center of gravity to deliver more torque? $\endgroup$
    – Russell Borogove
    Commented Oct 26, 2016 at 12:47
  • $\begingroup$ @RussellBorogove No idea. Less torque may be beneficial if you look at that roll-test video, it already rolls at quite high rate. $\endgroup$
    – Andreas
    Commented Oct 26, 2016 at 12:52
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As you can see in this older video, the main thrusters at center of mass (probably 4, but the upper one never fires; no need for it due to gravity pull) - are used for translation (dorsal/ventral; port/star) . Four smaller diagonal thrusters are used to provide pitch and yaw stabilization. There doesn't seem to be any fore/aft control because this being an interceptor/impactor it will be traveling in the 'fore' direction with as high a speed as the delivery vehicle can provide, no need to regulate it. The only remaining axis is roll, and it seems like indeed it's either maintained by a (single) reaction wheel - or not maintained at all - the device could roll and simply another thruster would take over given direction, but the test takes too short for that to matter. Alternatively the thrusters for it are minimal - comparing the thrust necessary to maintain roll comparing to other axis, they may be too minimal to notice their activity.

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  • $\begingroup$ No that's a differently configured spacecraft. It doesn't have the same symmetry about the mid-plane, and the diagonal thrusters are out on one end. $\endgroup$
    – uhoh
    Commented Oct 26, 2016 at 9:44
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    $\begingroup$ @uhoh: Nevertheless, the diagonal thrusters are NOT in the same plane as the "straight" ones. Watch the three bottom thrusters in the OP's video around 1:10. The flame of the nearer diagonal one practically overlays the flame of the main thruster, while the flame of the farther one is clearly visible. If they were in the same plane as the main thruster, they would frame it symmetrically from both sides. $\endgroup$
    – SF.
    Commented Oct 26, 2016 at 9:48
  • $\begingroup$ OK I'm going to extract the frames because now I can't think about anything else until I figure this crazy thing out, but I can almost see what you mean now. So do I understand correctly that roll could be stabilized with a wheel rotating about the roll axis, or it could be stabilized with the existing diagonal thrusters as well without need of any wheels, (and maybe it's not stabilized at all in this test)? $\endgroup$
    – uhoh
    Commented Oct 26, 2016 at 9:54
  • $\begingroup$ @uhoh: Not "with existing diagonal thrusters" - either extra thrusters (but really weak and possibly never firing in the video) or RW or not stabilized at all - the vehicle simply accepting random roll and changing role of thrusters to adapt to it. $\endgroup$
    – SF.
    Commented Oct 26, 2016 at 10:25
  • $\begingroup$ ...or other means - there are many. Like, absolutely minimal thrust vectoring - just tiny actuators entering side of the stream from the main thrusters, offsetting the thrust a little. $\endgroup$
    – SF.
    Commented Oct 26, 2016 at 10:28
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No, I don't see the need for Inertia wheels

I can only see 2 sets of engines used: Strong main engines (one on each surface) for up and side movement and 4 RCS ports for stability.

As I viewed the video, I saw it was firing 2 RCS ports in almost each spot to maintain the position. This does indicate to me, that the whole stability is achieved by those RCS ports, as the sum of 2 orthogonal ports firing at the same time gives us a force along the mid of that angle between.

To fight or induce a roll, just one of the ports would need to open or stays open a split moment longer.

As this device seems to aim to be used in space, I would deem inertia wheels harmful even: the use of many moving parts in a device operating in a 3K environment (space) needs a very reliable method to reduce friction on permanently used parts like the axis of the inertia wheels. Such a method that also works equally good in a 0G environment I don't know of, indeed, a dry grease resulted in a catastrophic failure of a rover prototype.

The different engines this device will need for mobility already demand the use at least the same ammount of motors or electric valves for the main engines and RCS ports to work and several more.

However, I see some problems as that machine on the video does not yet seem to have a front/back engine set and lack RCS to control Pitch/Yaw too.

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  • $\begingroup$ I agree - I think the thrust axis from all eight thrusters lie in the mid-plane. Because the axes of the smaller "diagonal" thrusters don't pass through the axis, they can generate torque for roll, but I think in this particular configuration the thrusters can not generate pitch or yaw. $\endgroup$
    – uhoh
    Commented Oct 26, 2016 at 11:46
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    $\begingroup$ Lots of satellites uses reaction wheels in space. They are warmed by the sun or use internal heaters. If it was 3K inside a satellite nothing else would work either, computers, transmitters, not even reaction thrusters - propellants would be solid. $\endgroup$
    – uhoh
    Commented Oct 26, 2016 at 11:48
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    $\begingroup$ 3K is the temperature in deep space, from the cosmic background radiation. Near stars (such as the sun), the temperature is obviously much higher. $\endgroup$
    – MSalters
    Commented Oct 26, 2016 at 13:21

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