# More feasible geometries for counter rotating variable pitch mechanical launch system?

Included as a source in recent answers was this paper which while focusing on a tethered ring design best described as ambitious included maths for a mostly mechanical space gun system starting on page 11 of 20. The concept featured a pair of counter rotating threaded drive shafts with thread pitch changing to achieve constant acceleration down the launch tube with the coupling of vehicle to guide rails and drives being magnetic.

(image from page 12 of paper)

This seems initially plausible, and attractive in that the energy storage issue of most space gun systems is bypassed by directly 'storing' the launch energy as mechanical inertia that is then directly used 'at source' to propel the vehicle down the launcher.

The magnetic field strength achieve low friction/high force coupling is a problem, as discussed on page 13 with needed magnet being questionable on current technology. A more substantial issue is at page 14 examining the forces during launch where along with the useful force propelling the vehicle down the launcher there is also a component at right angles across the launch tube of several hundred tonnes. This is a problem for the drive shafts, since while it might be possible with regularly spaced bearings to engineer the shafts to handle hundred tonne static loads these shafts will be spinning at high speed where deflection will tend to amplify and result in self destruction. The proposed solution is flywheels within the shafts that are used to generate a counter force during the launch.

This takes the design from one where the 'long' part of the launcher is a pair of cheap(ish) passive shafts with some bearings and drive motors to one with very not passive gyroscopes in massive numbers that need to operate with perfect reliability in a very not cheap manner.

Is there any alternative to the geometry in picture above (payload above two drive screws) where adding either more screws or more screw contact points on the payload sled would allow the forces to be balanced within the payload sled system and leaving just the 'push down the tube' forces on the drive screws?

It is to be noted that there are several other obstacles to viability of this system, those shared with all gun launch systems and some unique to it (a magnetic coupling to changing surface geometry). This question is just if the only solution to the lateral forces is flywheels every couple of meters of a 100+ kilometer launcher.

• If the screw shafts are only supported on the ends, they will flex and become unbalanced. At high speed, the device will self destruct. Commented Dec 3, 2023 at 16:34
• @Woody rather a lot of ways this thing can self destruct! Picture that came to mind was a segment between two bearings starting to wobble, impacting the segments either side both from torsion, and because the length change pulls them inwards. High potential for a minor issue to become 'entire 100km death noodle is on the loose' very very quickly. Commented Dec 4, 2023 at 8:44
• Mechanically, I can't picture how there could be bearings anywhere but at the ends. This is a pair of lead screws. Google can't find me an single example of a lead screw with a mid-shaft bearing. Death Noodle indeed ! Commented Dec 4, 2023 at 9:19
• @Woody if the bearings can run on the outside diameter of the screw threads, or be noncontact, or if the carriage can skip an interruption in the screw threads, you can have bearings. Commented Dec 4, 2023 at 9:34
• @woody, as long as the coupling on the carriage is wider than the interruption in the thread for the bearing it is not automatically fatal to the design as shown, since it rides on top and quite long. A conventional lead screw nut wrapped around the shaft solves most of the side force problem but certainly would be impossible to mount bearings for. I suspect the absence of real devices using mid span bearings to allow shorter/thinner (eg cheaper) lead screw segments is possibly informative here. Commented Dec 4, 2023 at 10:00

I'm the author of the paper you referenced. You are asking some great questions here!

Is there any alternative to the geometry in picture above?

Yes, as Woody pointed out, if the screw is spinning fast and the bracket spacing is too large, the force of the passing launch sled would apply inward lateral forces that could destabilize the spinning screw, leading to the aptly named "death noodle" scenario.

Depiction of single flight per screw coupling geometry (rail hidden to make brackets more visible).

To prevent this, at each position along the length of the sled, the sled could couple to at least two flights per screw instead of one, with the at least two flights being on opposite sides of the screw. In this way the lateral destabilizing forces on the screw would cancel out. However, the grappler devices that engage with the screw flights (green and orange in the figure above) would become heavier and more complex. In function, they would be closer to a traditional full-wrap-around lead-screw nut but they would still need to be a partial-wraparound nut. If they weren't, it would not be possible to support the screw with lots of brackets.

The grapplers would have to either reach over and around the screw from above, or the brackets would need to be larger and C-shaped so that the grapplers could access the bottom screw flight from the inside without reaching around.

There is a design trade here between: a) increasing grappler complexity or increasing bracket size and geometry to reduce lateral forces, and b) specifying stiffer screws and more closely-spaced brackets sufficient to handle the transient lateral loads.

One final note, the interior flywheels are not meant to be a solution to lateral forces problem but rather to the problem of how to rapidly transfer a lot of power to increase the kinetic energy of the passing vehicle without requiring lots of pulsed-power electronics and oversizing the electric motors. There's some math (its derivation is unfortunately out of scope for this answer) that shows that the cost of power electronics scales with the cube of the muzzle velocity for the coil gun type of mass driver. The flywheels help the cost of the twin-screw launcher to scale more gradually - much closer to the square of the muzzle velocity. (Note that rocket mass scales exponentially with delta-v.)

• Thanks for the answer that yes going opposite sides solves part of the side forces problem, but does have trade offs. If you are doing future renders I'd suggest considering making the payload center of mass more clearly between the drive screws since as drawn with it up high there would appear to be a substantial pitch force during acceleration that somewhat defeats having twin screws. Commented Dec 10, 2023 at 4:11
• Good suggestion, thanks! Commented Dec 10, 2023 at 4:50
• if the orange things go all around the screws and hold them at the required distance, payload could ride some "converging diverging noodling wave" like a warp drive
– user19132
Commented Dec 10, 2023 at 15:02