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Comment by floxy | Hacker News Reader

Since momentum is conserved, why not just have a 2 of the 1 g probes strapped to each other with a spring in between. When you need a course correction at 100 AU out (or whatever). The probes calculate how much of a correction is needed, adjusts a screw that tightens or loosens tension on the spring, reorients itself appropriately with a reaction wheel, then the two probes are released from each other, begin pushed apart with the spring. One probe gets the trajectory correction it needs, and the other gets further off course. Maybe with some gravity assists with nearby objects.

https://en.wikipedia.org/wiki/Gravity_assist

also:

Roundtrip Interstellar Travel Using Laser-Pushed Lightsails

https://ia800108.us.archive.org/view_archive.php?archive=/24...

View on HN

Isn’t that basically how a rocket works? Throw stuff out one side to get the thing on the other side moving. Not sure how this would compare to a rocket engine with hyperbolic fuel.

Yes.

It compares in that it doesn't require said hyperbolic fuel. That fuel is heavy and finite.

Uh, it does - the ‘fuel’ is the other probe.

Notably, this also has a particularly bad ISP?

Also, probes are presumably also heavier and rarer?

You can think of the hypergolic fuel as a type of spring. The chemical energy stored in the bonds of the fuel is what pushes the fuel products apart when it reacts. This is what pressure is, it's nothing more than KE of molecules.

The 'spring action' of fuel is very good because there's a lot more energy (per unit mass) stored in the bonds. Orders of magnitude more than a mechanical spring.

But a mechanical spring will reset itself to its natural position once tension is released

Only in the sense that throwing a knife at someone is the same as shooting a howitzer at them.

Specific impulse.

As far as impulse goes, the spring will probably be pretty inefficient relative to mass.

The point we are trying to make is that there hardly anything that would be less efficient relative to mass.

No. Very much no. The spring system would literally throw away half the mass of the craft for, maybe, a 10m/s delta. Fireworks would be more efficient. A pitching machine attached to a huge pile of baseballs would be more efficient (ie the baseballs could be thrown faster).

They all weigh a lot more than 1g though.

https://en.wikipedia.org/wiki/Specific_impulse

Please read.

That's just a really, really ineffective rocket. A spring has nowhere near the energy density of chemical fuel.

Because the specific impulse of the spring is negligible when you’re moving at 1/10c and why would they send a 1g probe if they could accelerate 100kg to that speed? Why do you suppose doubling the weight would be free instead of making the system infeasible?

Can a gyro work in space? Probably not or all the satellites would be using it instead of gas/ionized gas based propulsion.

Gyroscopes are used in spacecraft / satellites in the same manner they are in aircraft, to measure changes in orientation.

Reaction wheels are used to make adjustments to orientation. See https://en.wikipedia.org/wiki/Reaction_wheel

Thrusters of all sorts can also be used, generally to maintain altitude in satellites, and more generally to provide thrust for space probes.

What would the "screw" push off of? That rotational force would need to go somewhere or be corrected, else the probes would just rotate. I guess a gyroscope could do that, but what you're describing just sounds... very roundabout, and in terms of force, a few kilos of propellant would have the same effect.

This is infeasible for the reason other have mentioned about specific impulse. But surely you can imagine a set of parallel boards with a coil spring between them and a set of cylindrical guide rods to prevent relative rotation between the boards. A motor fixed to one board turns a screw that engages with threaded nut on the other board, mounted on a thrust bearing, and guide bushing that allows a linear movement, but disallows the rotation degree of freedom. Think of the lead screw on a milling machine or lathe.

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