Trjtdtk

I don't know if it has ever been considered by anyone.

In my view, this is not a good idea for at least the following reasons:

1. It is equivalent to mechanically throwing things retrograde. See this video for an overly simple example. This is obviously not a good way for propulsion, as the specific impulse is very low. Let's talk just about the impulse $$p=mv$$ here, where $m$ is the "reaction mass", i.e. the mass of the material that's being ground, the object being throw backwards, or chemical propellant. $v$ is the velocity of the reaction mass relative to the spacecraft. The velocity $v$ of the sparks is in the order of a few m/s (same velocity as the edge of the grinding wheel. With chemical propellants, it is a few km/s. So, for the same amount $m$ of reaction mass that you carry, classical propulsion gives you a factor of about 1000 more impulse than grinding.


2. The produced momentum is kind of stochastic. As can be seen in your graphic, the sparks form a cone instead of a straight line. While the upward and the downward motions statistically cancel each other, their vertical components are a waste. While admittedly this also applies to chemical rocket engines (and ion thrusters?), just throwing some stuff overboard would be more efficient in this respect.

Still, I like this question for thinking out of the box. On a side note, reading the title reminded me of this passage of J.D. Clark's book "Ignition!":

F.A. Tsander in Moscow [...] had suggested that an astronaut might stretch his fuel supply by imitating Phileas Fogg. When a fuel tank was emptied, the astronaut could simply grind it up and add the powdered aluminum thus obtaining to the remaining fuel, whose heating value would be correspondingly enhanced!

I think this was actually tried, but found not to work well because the Aluminium particles take too long to combust, i.e. they continue to burn after they have left the combustion chamber. (Some?) Solid rocket propellants are based on Aluminium, though, but that's different.

Edit: The recent news about dangerous space debris made me think about this question again. Thus, I'm throwing in a third disadvantage of grinding propulsion: it would produce tonnes of untrackable, high velocity particles with stochastic motion characteristics. Sure, one could take some care as to minimize the expected hazard. But with thousands of additional satellites being placed in orbit within the next decade, it might be dangerous to use grinding propulsion in Earth orbit.

The forces involved in spinning a wheel at high speeds are enormous. At 1600 km/h rim speed, the wheels on the Bloodhound SSC experience 50,000 G. Even the slightest imbalance (from, say an abrasive particle coming loose) would be catastrophic.

Good for you, for thinking outside the box! Fearlessly pursuing new ideas is where any new breakthrough comes from.

But rocket exhaust moves at thousands of meters per second -- supersonic speeds. Recalling the formula relating acceleration to velocity for circular motion, a=v^2/r. So, given a velocity of 3,000 meters per second and a wheel radius of, say, 1 meter, the acceleration at the rim would be roughly 9,000,000 m/s^2, or 900,000 times Earth gravity. And angular velocity, v=Rw, or 9,000,000 radians per second. I think you would have trouble spinning the wheel up to that kind of speed, and I think you would have trouble finding a wheel material that wouldn't fly apart.

Propulsion comes from acceleration of a reaction mass.

In this case the grinding wheel serves two purposes:

1. separate small bits of the workpiece from bulk at a slow and roughly uniform rate


2. accelerate those bits by mechanical friction, in a similar way that a tennis ball launcher uses a matched pair of counter-propagating spinning wheels to shoot a box full of tennis balls one at at time in a controlled direction and speed.

As @Muze points out using a matched pair of counter-propagating spinning wheels would be important in spaceflight as well.

Step 1: requires a large amount of work and there's no reason to do that in space. You can produce the particle on the ground, so your propellant "tank" would be a feed system dispensing pellets or powder. They could be suspended in a fluid for easier feed and to avoid electrostatic clumping.

If you have to produce it in space, for example if you are reusing your lower stage as a reaction mass Horace style (Monty Python reference), then you can grind or otherwise form at a lower speed first with a separate wheel. Particles could be remelted in order to make them spherical, and then size-sorted for the following step.

Step 2: would conceptually be accomplished with a mechanism similar to that of a tennis ball launcher. Two counter-propagating wheels with the particle feed introduced in the small gap between the two wheels' surfaces. The particles would have to be monodipserse, meaning all of a fairly uniform size, and slightly smaller than the gap for good friction. Either the particles or the wheels would have to be elastically compressible enough so that there's a good grip for acceleration, and yet the surfaces should not be readily damaged during the process.

You could also very slightly incline the wheels such that if you had a sorted range of particle sizes they can all be introduced at the appropriate gap width.

However, as @Greg points out and then @ Thorondor demonstrates quantitatively, getting your wheels to spin at mach 10 or faster (for a (mass-)specific impulse or Isp of say 300 or so) is a real materials problem.

One possibility for propellant would be a maximal concentration (essentially HCP) liquid suspension of latex or polymer spheres which can be obtained highly monodisperse with sufficient money. If you don't have a lot of room for propellant, then perhaps metallic or metal oxide or nitride nanospheres can be produced by pyrolysis.

Horiba Scientific

GIF from Tennis ball machine DIY - part 1 a better view of a similar mechanism can be seen in Tennis Tutor Ball Machine mechanics in operation.

Even if feasible materials-wise, I suspect energy efficiency would be horrible. To spin something fast, where is that (rotational) energy going to come from? If you use an electric motor, you could just modify it to make a plasma drive and use it (more) directly. In your device a lot of that energy gets turned into heat (of the wheel) by friction. Presumably you could recycle that heat to a large extent, but the stuff to do that is already adding another layer of complexity.

The big challenge will be the forces on the outside of the wheel, ripping it apart.

We have to pin the numbers to something, and the easiest number to pin down would be the angular velocity. Zippie style uranium enrichment centrifuges operate around 1,500 revolutions per second, so they make a good benchmark. (Some turbochargers go faster, up to 4,800 revolutions per second, but they are rather small diameters, so easier to manufacture). 1500 rev/sec is about 9500 rad/sec. Since $v=romega$, we can solve for the radius of the wheel, $r=fracvomega$. Let's target a rather low velocity: 500m/s. Chemical rockets have exhaust velocities in the 2500-4500m/s range, but we can target a lower rate because we can always fuel up later. That pegs us as needing wheels on the order of 50cm in radius.

The exact structure of a Zippie style centrifuge is a closely guarded secret, but Wikipedia gives a notional centrifuge size of 20cm or smaller. This means our wheel is going to be withstanding forces roughly four times that of a nuclear enrichment centrifuge. Now mind you those centrifuges are carefully managed, sealed in vacuum chambers and levitated on magnetic bearings. The forces of accelerating mass with the edge of one of these wheels is going to be far greater. More importantly, they will be off axis, which is always tricky for a centrifuge.

So the material properties we need push the limits of what is used in enrichment centrifuges, just to get to one tenth of the ISP of a rocket. So the real question will be, are there mission plans where 1/10th of a rocket's ISP is useful, but where other technologies are insufficient.

A huge amount of energy would be wasted in the grinding process. Only a tiny fraction would be converted to kinetic energy: the rest would be wasted as heat, plus your grinder would wear down and you would have to have all sorts of complicated mechanisms to keep the thing running.

You could reduce the wear and the energy required to grind off the particles by using a weaker material.

But then you could just decide to not grind the particles off in the first place and throw powder out the back using a spinning wheel without the need to grind it.

But then you can just ionize the powder, eject the particles with an electric field and get rid of the wheel completely. It would be a lot more efficient.

Then you could replace the powder with xenon for more efficiency.

And now you have an ion drive and we already have those.

There are two considerations: one is ISP, which is discussed in several of the answers. The other is energy efficiency - how many joules of input energy end up converted to kinetic energy in the expelled propellant mass.

If you consider a typical rocket, energy efficiency is surprisingly high - IIRC something like 70% of the available chemical energy is converted to kinetic energy of propellant mass ejected along the thrust axis (the remainder is waste heat radiated away or components of propellant velocity perpendicular to the thrust axis).

Using a power plant e.g. nuclear to create electricity to drive a motor involves multiple conversion steps, each of which incurs losses. To begin with, a nuclear reactor itself creates heat which has to be converted to mechanical work (at best about 40% efficient IIRC) which is then converted to electricity (maybe 80%-90% efficient) and then to mechanical (again maybe 80%-90%).

Using a nuclear reaction to heat a propellant which you directly expel (nuclear rocket) gets you back to the same sort of energy efficiency as chemical rockets, but higher ISPs because you can potentially achieve higher temperatures.