It’s fair to say that there’s really no phase of spaceflight that could be considered easy. But the case could be made that the most difficult part of a spacecraft’s journey is right at the very beginning, within the first few minutes of flight. At this point the vehicle’s booster rocket will be fighting with all its might against its own immense propellant-laden mass, a battle that it’s been engineered to win by the smallest of margins. Assuming the balance was struck properly and the vehicle makes its way off of the launch pad, it will still need to contend with the thick sea-level atmosphere as it accelerates, a building dynamic pressure that culminates with a point known as “Max q” – the moment where the air density imposes the maximum structural load on the rocket before quickly dropping off as the vehicle continues to ascend and the atmosphere thins.
While the vast majority of rocket launches have to contend with the realities of flying through the lower atmosphere, there are some exceptions. By launching a rocket from an aircraft, it can avoid having to power itself up from sea level. This allows the rocket to be smaller and lighter, as it doesn’t require as much propellant nor do its engines need to be as powerful.
The downside of this approach however is that even a relatively small rocket needs a very large aircraft to carry it. For example, Virgin Orbit’s LauncherOne rocket must be carried to launch altitude by a Boeing 747-400 airliner in order to place a 500 kg (1,100 lb) payload into orbit.
But what if there was another way? What if you could get all the benefits of starting your rocket from a higher altitude, without the cost and logistical issues involved in carrying it with a massive airplane? It might sound impossible, but the answer is actually quite simple… all you have to do it throw it hard enough.
Getting Revved Up
It might sound like science fiction, but that’s exactly what startup SpinLaunch is currently working on in the New Mexico desert. The plan is to use their mass accelerator, essentially a vacuum-sealed centrifuge, to spin a small rocket up to a velocity of 8,000 km / h (5,000 mph). The vehicle will experience up to 10,000 Gs before it’s carefully released at the precise moment that will allow it to exit the centrifuge skyward through a rapidly-actuating airlock.
The rocket would then coast to an altitude of approximately 61,000 meters (200,000 feet), at which point it would ignite its first stage engine. From that point on the flight would progress more or less like a traditional rocket launch, with the payload ultimately being accelerated to a nominal orbital velocity of 28,200 km / h (17,500 mph). The big difference would be the cost, as SpinLaunch estimates each launch could cost as little as 500,000 USD.
Currently, SpinLaunch is running tests on a one-third scale centrifuge that has a diameter of 33 m (108 ft) and forgoes the complex high-speed airlock for a simple sheet of thin material that the test projectile smashes its way through when released. This naturally means the centrifuge loses its vacuum upon release, but that’s not really an issue this early in the game; maintaining vacuum will only become important when the system is fully operational, and is intended to help maintain a rapid launch cadence as the massive centrifuge chamber won’t need to be repeatedly pumped down.
So far they have flung passive projectiles to a reported altitude of “tens of thousands of feet”, but that’s a long way from reaching orbit, much less space. The key to making this system work is developing a rocket that can not only withstand the immense g-forces it will undergo while being spun up to speed, but also be able to guide itself during the coast phase before engine ignition using either control surfaces. It should also go without saying that such a rocket only has one chance to get it right – should the engine of a traditional booster rocket fail to light at T-0, the launch can be scrubbed and the vehicle reconfigured to try again. But there’s no do-overs when the vehicle is already flying through the air.
SpinLaunch seems confident they can solve the engineering issues involved, but the fact remains that a similar project was undertaken as a joint venture by the United States and Canada in the 1960s, and things didn’t exactly go to plan.
The Need for Speed
Technically the High Altitude Research Project (HARP) got its start in the 1950s when ballistic engineer Gerald Bull got it into his head that with a large enough cannon you should be able to shoot a payload directly into space. But anyone familiar with Jules Verne’s From the Earth to the Moon knows that the idea is much older than that. Conceptually it makes a certain degree of sense, and it’s not as if humanity hasn’t spent hundreds of years perfecting gunpowder weapons anyway.
The HARP cannon was built by welding together 16-inch naval gun barrels, and mounted in such a way that it could be raised into a near vertical position. Barbados was selected as the primary test site as its relative proximity to the equator theoretically meant projectiles fired eastward would receive a boost to their velocity due to the Earth’s rotation. Starting in 1962, a series of launches were conducted that saw the cannon fire Canadian-made Martlet sounding rockets of roughly 1,800 mm (70 inches) in length.
Early flights carried research payloads that not only studied the performance of the cannon itself, but also observed upper atmosphere and near-space conditions. Updated versions included solid rocket motors that were designed to ignite after the rocket had coasted for about 15 seconds in an effort to increase their velocity and maximum altitude. The ultimate goal was to develop a multi-stage rocket that could carry a small 23 kg (50 lb) payload to an altitude of approximately 425 km (264 mi).
By the time HARP ended in 1967, the cannon had successfully fired more than 200 Martlet rockets, some of which reached an apogee as high as 180 kilometers (112 miles). With a per-launch cost of just $ 3,000 USD, or roughly 27,000 in 2022, it remains one of the most cost-effective means of delivering a payload above the 100 km Kármán line that marks the internationally recognized boundary of space.
Unfortunately, despite considerable effort, HARP was never able to develop a Martlet rocket that could successfully accelerate itself beyond the initial velocity at which it was fired from the cannon. Because of this, none of the rockets were able to reach orbit, and fell back down to Earth – often not far from the cannon itself.
The primary issue was the inability to develop a rocket engine that could survive the 12,000+ g’s each rocket was subjected to when fired from the cannon. So while HARP was technically a successful space launch program, it was limited to suborbital research flights which became less scientifically valuable as the more traditional rocket programs spearheaded by NASA began to mature.
Exploring New Opportunities
Of course, just because the HARP engineers couldn’t design a rocket engine that could survive high g-forces in the 1960s doesn’t mean SpinLaunch can’t do it. It would hardly be the first time a small startup achieved something the entrenched aerospace industry had deemed to be impossible. The company is also clearly aware of the challenge, as they’ve recently released videos explaining that a large portion of their research right now is going towards exploring the effects of the centrifuge environment on various rocket and spacecraft components.
But the fact remains that there are many challenges ahead for SpinLaunch. History tells us that the development of the engine won’t come easy, but there’s truly no precedent for building a mass accelerator of the scale that would be required to hurl their vehicle into the upper atmosphere. One also cannot ignore the reality that the cost of spaceflight is already dropping precipitously thanks to commercial competition between providers such as SpaceX, Rocket Lab, and Astra. A launch price of $ 500,000 would have been revolutionary 20 years ago, but today isn’t far off from where the market is headed anyway.
That said, all signs point to an exciting new era in space exploration ahead, and it’s not outside the realm of possibility that SpinLaunch could find its greatest success away from Earth. For instance, a SpinLaunch accelerator on the Moon would have a far easier time hurling vehicles into orbit without an atmosphere to contend with. Given NASA’s goal to establish a long-term presence on and around the Moon, a system that could cheaply loft payloads from the lunar surface would likely be in high demand.
One could also imagine a small centrifugal satellite launcher mounted to a future space station to dispense CubeSats and other payloads with limited internal propulsion. It might sound far fetched, but keep in mind that the Japanese JEM Small Satellite Orbital Deployer (J-SSOD) currently in use on the International Space Station uses a simple spring-loaded mechanism to push the spacecraft out of their storage racks. A small mass accelerator that allows the spacecraft operator to select the velocity and even departure angle for their craft would be a clear improvement over the current state-of-the-art.
The fact is, we simply don’t know what the future holds for SpinLaunch. Their current technology demonstrator is impressive for what it is, but at the same time, is so far removed from what would actually be required to achieve their goals that it’s hardly an indicator that the company is on the right track. Only time will tell if they can succeed where others have failed, or if their mass accelerator will join HARP as just another interesting footnote in the long history of spaceflight.