Random Thoughts: Tuggery and RCS

This is just a short one that someone pointed out to me over the weekend.  For spacecraft that have to do their own rendezvous and docking with say a space station or depot, you need an RCS capable of not just holding a stable attitude, but also providing translational control in all three axes.  Ie you need to be able to push yourself in x, y, or z directions, and rotate around each of those axes.  If you have a tug to do the prox-ops however, the actual spacecraft/launcher itself only needs RCS capable of holding a stable attitude (ie provide moments about the x,y, and z axes)…

Does this actually save you any complexity in the RCS system or avionics?  I think this may allow you to reduce the number of RCS thrusters, but I’m not sure (and don’t really have time to think it through today).

Comments?

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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.
Jonathan Goff

About Jonathan Goff

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.
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14 Responses to Random Thoughts: Tuggery and RCS

  1. The simplest setup for thruster-based attitude control would probably be to mount one thruster on the extremity of your longest axis (where it will produce the most torque) and a second on the next-longest axis, for controlling rotation about the primary axis. The primary one would need to be able to thrust in any direction perpendicular to the long axis, and the secondary one would need to be able to thrust perpendicular to both axes. So about six thrusters overall if they look like Apollo’s RCS.

    To translate without rotating you have to thrust through the center of mass. To do this with few thrusters you’d have to rotate the vehicle before each translation to line up the thruster and center along the translation direction. Less than ideal for docking operations where you want to maintain an attitude.

    If you only need attitude control and not translation capability there are other options like reaction wheels and gravity gradient stabilization. Those might be simpler/cheaper than a full RCS? I’m a game developer, not a rocket engineer, so I will shut up and let the knowledgeable have the floor.

  2. Jonathan Goff Jonathan Goff says:

    James, comments are good. For temporary attitude control prior to capture by a tug, reaction wheels or gravity gradient stabilization might work. But I would think that for stuff like reentry, etc you might want something faster. But yeah, at least my intuition tells me that if you don’t care about translations, you could probably knock it down to about 6 thrusters instead of the 12-16 you usually see on a stage.

    ~Jon

    PS game developers actually make fairly decent GN&C guys if they’re doing anything graphically intensive (see: John Carmack)

  3. Habitat Hermit says:

    With a tug ready and waiting in the target zone at launch (and with ample margins in capability) why would the vehicle/payload need anything at all?

  4. Martijn Meijering says:

    Is this different from your earlier idea of having an upper stage provide attitude control before handing its payload off to the tug?

  5. HH,
    I’d still want at least attitude control–it’s tons easier grappling something that is actively cooperating.

    Martijn,
    No, this is just more thinking along the same lines. For existing upper stages, it means they probably have all the control they need. For new upper stages or RLV upper stages it just means that they really have no reason to give them “the works” when it comes to RCS.

    ~Jon

  6. Randy Campbell says:

    Tet Ships:
    http://www.clowder.net/hop/tetships/tetships.html

    For the “heck” of it 🙂

    Randy

  7. Doug Jones says:

    +/- pitch, yaw, roll requires 6 thrusters minimum, and 8 (properly arranged) can provide partial redundancy. For full safety (and because we’re slightly paranoid) we plan to have two full sets of 6 in the A and B RCS for Lynx, a total of 12.

    A 16 thruster system can give six axis control and redundant 3-axis control, but if prox ops are _always_ handled by the tug, 12 should do just fine.

  8. Jonathan Goff Jonathan Goff says:

    Ok, so somewhere between 6-12 thrusters (with 12 being the paranoid but probably right number, and 6 being the bare minimum number)? Thanks Doug!

    So it’s not a huge improvement, but it does make a difference.

  9. Ian says:

    Jon,
    3 thrusters is the bare bare minimum, if you don’t mind unnecessary translations. 6 is the minimum that will let you rotate without translation. I haven’t done the math but I’ll believe Doug that 12 gives you redundancy. Besides, it’s not also the # of thrusters, but also the fuel. A LOT less if you’re only doing rotations, and you can use just momentum wheels, as it was mentioned before.
    Ian

  10. Aaron Williams says:

    I assume you’re talking fixed direction thrusters, is there something preventing thrust vectoring? If it was practical, it seems like 3 thrusters with a full hemisphere of motion would let you translate/rotate any way you want.

  11. Aaron,
    NASA did some work with Camfield-joint thrusters that had a full hemispherical aiming capability. The problem is that I don’t know how you would package those on an RLV or an upper stage. Maybe on a lander or a service module or something, but it would be tricky.

    ~Jon

  12. MG says:

    Ugh. At first glance, the title read “Thuggery and RCS”. My reaction: What has Jon gotten himself into THIS time?

  13. Dart says:

    This subject, like all really engineering ones, needs system level requirements to be answered correctly. By that I am certainly not criticizing your exploration of the subject but suggesting you develop a map of system functions and implications to be considered beyond simple redundancy. That, together with the great energy management requirements, are why rockets are so challenging.

    So consider: Do you have complete, redundant, control of the separation conditions or could you have rates induced when you are released? In addition to control redundancy, do you want any capability to maneuver for ascent underspeeds, collision avoidance, other cases? Do you care about cross-coupling with your control scheme? Do you have sufficient freedom in your configuration to assure the CG is convenient for the functioning of the thrusters? What kind of manifolding are you planning to manage leaks, thruster failed on, and other system level faults?

    Lots to think about…..

  14. Paul Breed says:

    Can’t you do full 6dof with 12 thrusters?

    Assume a sphere radius 1 with the center a 0,0,0
    Draw traditional x,y,z axis through the sphere.
    Place thrusters in pairs where the axis pierce the sphere.
    Assume x,y,z

    At 1,0,0 thrusters point toward y+ and y-
    At -1,0,0 thrusters point toward y+ and y-
    This gives rotation around Z and translation in y

    At 0,1,0 thrustors point toward +/- z
    At 0,-1,0 thrusters point toward +/-z
    This gives rotation around X and translation in Z

    At 0,0,1 thrusters point toward +/-x
    At 0,0,-1 thrusters point toward +/-x
    This gives rotation around y and translation in x

    6DOF with 12, 16 not required.

    Finding the proper locations for the 12 on something airplane shaped is “harder”

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