I’d heard of SpinLaunch before, and wondered what sort of launch system they were planning. Details were sparse, and they still are.
Secretive Startup SpinLaunch Breaks Ground on Satellite-Flinging Test Flight Facility
SpinLaunch has begun building the facility where it will test out its radical new satellite-lofting strategy.
What is that strategy? Checking their 2018 press release:
“…SpinLaunch’s innovative use of rotational kinetic energy to revolutionize the smallsat market.”
OK, spin and rotational. They’ll spin the sat around in a circle, getting it up the speed, and let it fly off at a tangent. The concept goes back as far as David taking out Goliath with his sling. What kind of sling is the interesting part. They aren’t saying, and I think they need to.
Offhand, I’d break it down to two possibilities.
1. Mechanical: an actual sling, an electrically-driven centrifuge scaled up for satellites.
2. An electrically-powered mass driver (think the space catapult from Heinlein’s classic The Moon Is A Harsh Mistress). But arranged in a circle, instead of linearly.
There is a third theoretical possibility I’ll touch on later.
Trying to guess at performance parameters can give us clues as to what they’re going to do. From the article above, we know they have ten acres of land. From this article and this one, we know SpinLaunch is claiming an ejection velocity of 5,000 MPH.
First, let’s pretend the facility being built now is going to achieve that 5,000 MPH target, just for giggles.
Ten acres gives a maximum 660 foot diameter circle; 330 foot radius. Just for the thought experiment, an object whipped around a radius=330′ circle to 5,000 MPH would experience over 5,000 g in centrifugal forces. (I’m not even going to try to guesstimate direct 0-5000 acceleration, since it depends on how fast they spin up.)
That’s gonna be hard on the spacecraft and payload, but it’s also going to be hard on a mechanical centrifuge arm which is also experiencing that force. The arm will have to be pretty strong just to keep itself together, much less accelerate the craft. I think we can rule out mechanical.
The problem is, a circular mass driver/accelerator is going to impart that same centrifugal force. As you whip the package through those solenoid drive coils, the force is going to try to push the package out the side of the coils’ path. 5,000+ g is a lot, and the coils will have to be strong just to keep the package confined. Every bit of energy going into confinement isn’t going into acceleration.
But the current build is presumably a scaled down tech demonstrator. Let’s say its target ejection velocity is only 500 MPH. Now we get centrifugal force down to 50 g. That’s a little more reasonable, and survivable for the payload.
But if you scale up the launcher to get 5,000 MPH ejection, while holding centrifugal force to that same 50 g, you need a circle more than twelve and a half miles in diameter, with centrifuge arms over 6 miles long.
Mechanical is right out.
Now look at the spacecraft being launched. You’re going to put your little rocket in a circular mass driver and subject it to 50 g in centrifugal force.
Conventional vertical launch rockets are designed to launch at around 3 to 15 g (lower for crewed missions, higher for tough, inanimate cargo), but the loading is essentially on the vertical axis of the craft. A rocket launched by the SpinLauncher is going to need a little redesign. No. A lot of redesign.
The rocket will have to be shooting through the driver coils nose first, so that when it finally exits on the correct tangent it will be cutting through air nose first. Otherwise — hitting the air at hypersonic speed — it will be destroyed.
But that means the rocket will be experiencing high g loading from centrifugal force laterally. And it still needs the vertical strength for the conventional rocket-propelled part of its journey to space. The structure will have to be beefed up massively, and that will come at the expense of payload.
So why the hell the focus on rotational kinetic energy? Why a freaking circle?
Why not take that mass driver, and stretch it out in a straight line like everyone else (except for particle physics researchers working with the mass of individual atoms)? The only acceleration forces would be on the axis of the craft (OK, Earth’s gravity, too, but that’s minor compared to the launch). We design for that all the time: aircraft.
And without the centrifugal force, the mass driver doesn’t need to waste energy compensating for that — at the expense of velocity — holding the rocket in coil path.
And hold it in that path you must. If a rotational system loses its grip on the package, it’s going to fly off at some random tangent. Hitting a house a couple of counties away at 5,000 MPH isn’t going to endear you to neighbors or your insurance company. A linear launcher would only put folks downrange at risk. There’s a reason the ground-based launcher in my novel Net Assets was linear.
A rotational launch system seems to make no sense. At least at this scale: there’s always the skyhook concept, but that requires tech I don’t think we have yet, and it precludes the use (absent beamed energy) of the ground-based power generation SpinLaunch touts. Nor am I sure how the FAA, ICAO, and every military in the world would react to that cable whipping through their skies.
So, SpinLaunch; what are you doing? Please share some information to show this is more than vaporware meant to suck money out of gullible investors.