An investment in an impractical technology
Summary of the impracticality of Spinlaunch
The New Mexico Spaceport (https://www.spaceportamerica.com/), funded by taxpayers, started with Virgin Galactic’s space tourism entity as its anchor tenant. It has gained other tenants, thought not yet economically sustainable; space tourism may start in late 2019.
One new tenant is Spinlaunch, a company from Sunnyvale, California (http://www.spinlaunch.com/). They’ve raised $40M from investors (https://www.bloomberg.com/news/articles/2018-06-14/this-startup-got-40-million-to-build-a-space-catapult) including Google Ventures (now GV) and Airbus Ventures for a speculative technology, which I shall describe shortly below. They propose to use a large spinning platform to launch satellites from the ground (which must be with a rocket to complete the boost).
The idea sounded preposterous to me, so I worked out the limitations, which I claim are solidly against this being practical or even possible. Join me now:
The basic technology proposed is:
- A vacuum chamber with a radius of about 50 m (Bill Gutman from Spaceport America let out the knowledge that my first estimate of 500 m was “an order of magnitude too high;” pushing on a completely unrealistic guess helped spring this information loose).
- Bill says it’s patented, but there’s only a patent application dated July 2018, US 2018/0194496 Al to Jonathan Yaney.
- I note that a patent says nothing about the practicality of an “invention.” Patent examiners are not allowed to decide on issuing a patent based on practicality. I note that Henry Latimer Simmons obtained patent 536,360 for a ludicrous invention to let one train pass over the top of another on one track.
- Placing the satellite with its rocket motor on the periphery and spinning up to a tangential speed of Mach 4-5, as Bill cites. Yes, it could not be to LEO (Low Earth Orbit) speed; of course, the satellite would burn up on launch here in the lower atmosphere.
- Upon launch a rocket engine ignites to reach the speed for attaining LEO.
- I’ll take the lower speed, Mach 4, about 1,320 m s-1 to give the least stressful conditions.
- At a radius r = 50 m and a speed v = 1320 m s-1, the centrifugal acceleration is very simply calculated as v2/r = 34,850 m s-2. That’s very closely 3,500 g! We’re talking about a satellite and its rocket engine withstanding this, including electronics
- Bill Gutman says that there are already military projectiles that get accelerated to 40,000 to 50,000 g – to get a muzzle velocity of 1,000 m s-1 in a 10-m barrel. The electronics are potted to withstand the acceleration (https://www.raytheon.com/capabilities/products/excalibur).
- Fine, but:
- (1) A satellite has to have folded solar panels and antennae. These cannot be potted, and I cannot imagine any folding and cushioning that doesn’t destroy the joints or the panels. The military projectiles only have to deploy small vanes to steer. (I also don’t know how their performance meets specs.)
- (2) To reach vLEO (calculations below), there has to be a rocket engine. It will have to be a solid propellant engine; the complex plumbing and pumps of a liquid-fueled engine could not possibly survive 3500 g.
- This engine should really be two-stage. An effective vLEO of over 8,000 m s-1 is needed, with a bit of thrust vectoring to go from horizontal to tangential in the trajectory, as well as to overcome drag in the initial part of the trajectory. I get an estimate closer to 9200 m s-1, not achievable with one stage with solid propellant; see below. The exact calculation of air drag would similar to the math from interceptors such as Nike or the more modern (and low-effectiveness) GMD. So, the additional speed needed is well over 6,700 m s-1.
- The classic rocket equation expresses the gain in speed (yes, let’s say speed, since direction is not specified and does change) is Δv = vex ln(m0/mf), where vex is the exhaust velocity as determined by the propellant type and m0 and mf are the initial and final masses of the rocket. I’ve written this up, too (https://science-technology-society.com/wp-content/uploads/2018/01/rocket_equation_in_free_space.pdf). We assume the loss of mass is that of propellant. Taking the final hull and payload (satellite) as having a mass of only 10% of the initial mass (90% burn), we get the logarithmic factor as ln(10)=2.3.
- Solid propellants have only a moderate vex, hitting about 2,500 m s-1. We get Δv=5,750 m s-1. Yes, I’d say that a second stage is necessary.
- (3) Can a solid-propellant rocket withstand the lateral acceleration? Of course, the rocket has to point up, so the rocket and payload are aligned perpendicularly to the radius. There is an enormous bending force exerted on the rocket body. The force also gets relieved almost instantaneously on launch, generating a change in acceleration called, appropriately, jerk. This sets parts of the launched item into sharp motion – like your innards if you’re in a high-speed traffic accident.
- (4) How big a satellite can be launched, given materials limitations? There are some small satellites, e.g., the CubeSats, but they have economical and reliable launches already on standard rockets. For more practical sizes, I’m not about to do the engineering calculations to estimate the stresses on the launch platform and the safety factor. This assumes that the payload and its own rocket survive, which I flatly reject, as above. I note that:
- The proposed device would only fit small rockets and satellites, of notably smaller dimension than the spinning platform. There is a mature technology and market for launching (and making) CubeSats (https://makezine.com/2014/04/11/your-own-satellite-7-things-to-know-before-you-go/). Maybe Spinlaunch is aiming (but wildly off) at slightly larger satellites.
- (5) The whole idea was to save energy and cost in launching satellites. There’s a lot of energy put into the launch mechanism, far more than the kinetic energy imparted to the (putative) rocket + payload. Maybe some could be recovered in electromagnetic braking…needing a significant amount of electrical storage and circuits to handle massive currents.
There are other niceties:
- Consider the extremely active timing needed to release the rocket + payload. Suppose we want a launch direction error not to exceed 1o, or 1/57 of a radian. To reach a tangential speed of 1320 m s-1 at a radius r = 50 m, one needs a rotation rate of 1320/50 = 26.4 radians s-1. That’s a bit over 1,500 degrees per second. The window is less than 1 ms wide.
- Safety: What’s the shield in case the launch mechanism fails, sending out shard at high speed? How about releasing the rocket + payload nearly horizontally by accident? You need a BFS, a big functional shield.
- How about the reaction of the spinning platform when the rocket + payload is released? That’s quite a jolt on the suspension. Maybe some engineers can address that, but not the fundamental no-gos (a neologism?) I’ve noted all through.
- This is a pipe dream or a scam, poorly thought out at the very best.
- Yes, Google Ventures, now known as spinoff GV, and Airbus Ventures are among the investors in this. I can only attribute their lack of due diligence to their lack of sufficient technical expertise – I think Google and Airbus, both technically solid, spun off the MBAs and not the engineers into their Ventures.
The rest of the analysis uses equations set by MathType in Microsoft Word. These don’t come through in the Mammoth docx converter plug-in to WordPress, so I link here to a PDF version of the rest of the analysis.