By Dr. Gary L. Deel, Ph.D., J.D.
Faculty Member, Wallace E. Boston School of Business
After what feels like decades of relative dormancy, humankind is once again looking to the stars. There are exciting plans on the horizon to build new space stations and colonize moons and planets in our local solar system. As public and private space efforts are gearing up for these new missions, space engineers are also thinking up new and innovative ways of getting us into space.
One novel idea is to use spinning wind-up launchers that would essentially fling spaceships into orbit. Consider, for example, the new startup company SpinLaunch. This company was founded in 2014 and is developing a system that essentially spins a spacecraft up to tremendous speeds for launch.
Its facility consists of a vertically oriented circular chamber with a long, rotating arm that spins around in the chamber like a hand on a clock. A spacecraft is then mounted to the end of the rotating arm, and it begins spinning faster and faster. When the desired velocity is reached, the spacecraft is released from the arm and jettisoned out through an exit port in the top of the chamber. And off it goes.
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Spin Launchers Provide a Boost in Velocity to Spacecraft
Now, at this point, I must be fair to the creators of SpinLaunch and acknowledge that their design does not call for the rotational centrifuge to provide all of the velocity necessary for a spacecraft to reach orbit. Instead, the spin launcher simply assists by providing a boost in velocity to spacecraft.
Once the spacecraft is jettisoned upward, it ignites onboard rockets that propel a spacecraft the rest of the way up and out of the atmosphere. So in this sense, SpinLaunch is intended to be a boost to space launches. It will reduce fuel consumption and chemical energy production requirements.
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Is It Possible to Build a ‘Pure’ Spin Launcher without Chemical Rocket Propulsion?
But the idea of spin launchers got me thinking. Could you build a “pure” spin launcher that flings spacecraft all the way into orbit without the need for chemical rocket propulsion at any point? If so, how fast would it need to spin? And what could you launch with it?
These questions are exciting to ponder, and if such a thing were physically possible, there would be several potential advantages. First, one of the hardest parts of space exploration is the safe, effective operation of chemical rocket launch vehicles. Many of the tragic fatalities endured in human spaceflight to date – including both the Challenger and Columbia Space Shuttle disasters – are directly attributable to chemical rocketry and its dangers. A launch system that doesn’t require any rockets or volatile rocket fuel would likely be safer for the human occupants of spacecraft.
Second, a pure spin launch concept might allow for unprecedented reusability and frequency of spacecraft launches. For years, launch vehicles were completely destroyed and/or abandoned on launch, and it was necessary to build a new rocket for every new launch.
Elon Musk’s company SpaceX, however, changed all that with the development of the first-ever recoverable first stage boosters. Today, SpaceX Falcon 9 rocket first stages routinely return back to Earth so that they can be reused. In addition, SpaceX just recently broke its turnaround record for back-to-back flights with a Falcon 9 booster that was launched, recovered, refurbished and reflown within just three weeks’ time.
But a pure spin launcher could take reusability and launch frequency to a whole new level. In theory, spin launchers could be used to launch spacecraft mere minutes apart from one another.
The frequency of the launches would only be limited by the minimum time needed to wind down the launch apparatus, reload it and wind it up again. The implications for future space launches are significant.
And with a pure spin launcher there’s nothing flammable or explosive – no rocket engines to malfunction, no volatile supercooled liquid fuel to cause dangerous icing issues, and no inextinguishable solid rocket fuel to burn a hole in something. Simply put, there’s nothing to blow up, burn up, freeze or otherwise fail catastrophically.
The Catch to Pure Spin Launches
There must be a catch to pure spin launches, right? Yep. There always is. While a pure spin launcher offers the prospect of serious advantages for safety and launch frequency, it would be severely limited in what it could actually launch.
Why? Because the centrifugal forces generated inside the wind-up chamber of such a facility would be far too extreme for virtually any kind of payload.
We can calculate the relative centrifugal force (RCF) with a simple formula: RCF = 1.12 x Radius (cm) x (RPM/1,000)2 where RCF equates to the relative centrifugal force felt by the rotational momentum of the spacecraft.
We know that the radius of SpinLaunch’s concept will be 150 feet (45.72 meters). That figure can be used for our thought experiment.
As far as rotations per minute (RPMs), it is necessary to first know the target velocity to calculate the RPMs. But orbital velocity varies with altitude. For example, at 200 kilometers (km) in altitude (considered a low Earth orbit), the orbital velocity is approximately 7.8 km per second.
However, at 1,500 km (much higher up) the orbital velocity is only 7.12 km per second. A velocity figure like 7.5 km per second could be used as a middle-of-the-road number. Also, it’s worth noting that any spacecraft wanting to reach orbit at this speed would have to leave the surface of the Earth at a faster speed to counter for the atmospheric drag it would experience on the way up, but we can ignore that factor for now in the interest of simplicity.
To convert velocity to RPMs, it’s necessary to know the circumference of the circle that the spacecraft will be traveling inside the spin launcher chamber. Assuming that a spacecraft is at the very end of the launch arm and therefore at the edge of the circle, the circumference can be calculated with this formula:
C = 2?r
C = 2(3.14)(45.72)
C = 287.12 m
Now that the circumference of the launch spin facility is determined, it’s possible to divide the average velocity figure (7.5 km/s) by the circumference to determine how many rotations per second the spacecraft would complete each second at orbital launch speeds:
7,500 m/s / 287.12 m = 26.12 rotations per second
And now that the rotations by second have been calculated, we can simply multiply by 60 to get our rotations per minute:
26.12 rotations per second * 60 seconds = 1,567.20 RPM
Finally, with this number we can complete our equation to calculate the gravity forces (G-forces):
RCF = 1.12 x Radius (cm) x (RPM/1,000)2
RCF = 1.12 x 4,572 cm x (1567.20/1,000)2
RCF = 1.12 x 4,572 cm x (1.5672)2
RCF = 1.12 x 4,572 cm x 2.4561
RCF = 1.12 x 4,572 cm x 2.4561
RCF = 12,576.8 G
So at an orbital velocity of 7.5 km/s, any spacecraft inside a pure spin launcher would be moving at a speed of 12,500Gs! What does this number mean? Well, one G is the gravity that we feel here on Earth.
So if you weigh 200 pounds, for example, then that number is your weight under one G of gravity (one Earth’s gravity). But as the Gs go up, so does your relative weight and the pressure you feel pushing against your body (either downward or outward, as the case may be).
You might wonder about the average G forces that astronauts feel in chemical rocket space launches. Astronauts typically experience around three to four Gs of force during a spacecraft launch. So a 200-pound astronaut feels, temporarily, like he weighs a very uncomfortable 600 or 800 pounds.
But this physical effect is only temporary during the ascent into space. And we know humans who are in good physical condition can survive this experience.
But how much centrifugal pressure would that 200-pound astronaut feel inside a pure spin launcher at 12,500 Gs? The answer is about 2.5 million pounds!
The Forces Generated by Spin Launchers Would Not Be Practical for Humans or Delicate Space Instruments
So first and foremost, humans could never utilize spin launchers for manned missions. I would feel terribly sorry for any humans who might ever dare to try it as they would undoubtedly be turned into puddles of goo by centrifugal forces.
This point is important because, while spin launchers could end up being safer and more reliable than using chemical rockets, they would not be a replacement for chemical rockets as far as humans are concerned. So any reliability benefits are limited strictly to the safety of other payloads.
Granted, we don’t want rocket explosions killing astronauts, but we also don’t want them destroying expensive satellites or telescopes, either. So that’s something, right? Not really.
For the same reason a pure spin launcher would not be viable for humans, it would also not be viable for any kind of sophisticated electronics or technology. Think about all your electronic devices at home. Which of them do you think would survive 2.5 million pounds of pressure? The correct answer is: none.
And this principle is doubly true for space electronics. A lot of what we send up into space consists of very sensitive instruments and sensors. Even subtle jostling around can risk damaging these delicate space instruments. It goes without saying that applying the weight of 200 African elephants on top of them is a total non-sequitur.
So what would a pure spin launcher that incorporated no chemical rocket launch assistance be good for? Not very much, it turns out. The centrifugal G forces are just too high for just about anything except maybe solid steel or concrete, which aren’t in high demand in outer space.
And remember, we are ignoring the other very big problem of atmospheric drag on the way up. Beyond merely slowing a vehicle down, launching out of a spin launcher and into surface-level density atmosphere at orbital speeds would be like smacking into a brick wall made of air.
Most spacecraft would implode or crumple up like tin cans upon such a collision after ejection. So the atmospheric obstacle is yet another reason why a pure spin launcher would not work.
This, I suspect, is why the creators of SpinLaunch only intend for their technology to provide an initial boost to launch vehicles – presumably at much lower RPMs and therefore much smaller G forces.
According to the SpinLaunch website, “The velocity boost provided by the accelerator’s electric drive results in a 4x reduction in the fuel required to reach orbit, a 10x reduction in cost, and the ability to launch multiple times per day.”
It is still likely that the centrifugal forces would preclude any human payloads. But it’s unclear what kinds of other cargo the G forces anticipated by SpinLaunch will actually allow. Much will depend on the RPMs and peak velocities of the system.
And as an important physics note, it’s worth pointing out that the larger the diameter of the spin launcher, the smaller the centrifugal forces felt at different velocities. So larger spin launchers might be better in some ways. But a larger spin launcher would also be more expensive and still does nothing about the atmosphere problem.
But SpinLaunch does at least seem to project the reusability benefits we talked about previously. This aspiration is predicated on the assumption that the launch apparatus will be mechanically reliable and not self-destruct at high speeds.
Nonetheless, the creators of SpinLaunch seem to have a viable concept in mind and I am excited to see it in action. I wish them much luck in development and hope to see this idea playing a role in many non-human space launches in the years to come.
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