Jules Verne returns: Will space guns provide low-cost access to space?by Bart Leahy
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| Human beings would not survive such a blast: accelerations for the payloads would reach in the neighborhood of 2,000 gravities. |
However Ben Joseph, a 25-year-old aerospace engineering graduate of MIT, and a team of students and professors are resurrecting Gerald Bull’s space gun for peaceful purposes, including a radically new kind of impulsive launch technology known as the “ram accelerator.” That does not mean Joseph and his colleagues in the company he founded, Ballistic Flight Group LLC (BFG), are shying away from military applications:
It is not a question of “if” the ram accelerator gets militarized, but “when” the ram accelerator gets militarized. Any technology that can impart significant kinetic energy in a short amount of time is going to find itself used in weapons applications. While our primary goal is to do commercial space launch with this technology, we nonetheless anticipate DoD [Defense Department] interest. We do not intend to shun this source of funding if this is what it takes to make a ram accelerator space launcher a reality.
With a lot of preliminary engineering done and experimental data already captured, BFG is now hunting for investors from the private and public sectors.
Concept of operations
So how would a gun-launched satellite system actually work? A typical artillery weapon uses a large explosive force at the base of a gun to propel a shell down a rifled metal tube, which is angled to provide the projectile with its trajectory. In the space gun launcher being promoted by BFG this cannon type of firing is merely the first step in the process. The main step is the ram accelerator, a technology invented and developed by faculty and staff at the University of Washington in Seattle, where Ben Joseph studied as an undergraduate. After a pre-launcher gun (e.g., light gas or gunpowder propelled) accelerates the projectile up to speeds of over 500 meters per second, the projectile enters the ram accelerator by passing through a breakable diaphragm and entering another tube, this one filled with a more volatile propellant, such as oxygen and methane. Because the projectile enters this second tube at supersonic speeds, it interacts with the tube wall to produce a ramjet–like effect inside the barrel. This ramjet effect forces the projectile to combust the fuel behind it, increasing its acceleration through the tube. The projectile exits the barrel with a muzzle velocity of around 8 kilometers per second. An upper stage rocket would circularize the trajectory of the payload (approximately one third of the projectile’s 2,000-kilogram mass) to a low Earth orbit of around 800 kilometers.
This is the point at which Jules Verne enters the real world. Human beings would not survive such a blast: accelerations for the payloads would reach in the neighborhood of 2,000 gravities. There is also the problem of noise. A payload launching at this velocity from Earth is going to produce an incredible amount of noise—nearly 150 decibels—the equivalent of having the noise of a Saturn 5 launch within a circle 20 kilometers across. For this reason, the accelerator would be better off on a high mountain somewhere, preferably with a lot of open space around it. And due to security considerations, the site would need to be located within the United States.
What makes the ram accelerator so appealing is its economic potential. BFG estimates that the accelerator could be built for an estimated $157 million, a price tag that includes the launch tube and its supports, the pre-launcher gun for initial acceleration, and propellant handling system for the oxygen, hydrogen, and methane gases for the ram accelerator portion of the launch system. This price tag is astonishingly low—cheaper than some expendable rockets—and it could be fired hundreds or thousands of times. Depending on the gun’s final muzzle velocity, prices for payloads could drop to nearly $500 per kilogram, a drastic reduction from current market prices.
Applications and implications
During Joseph’s presentation on the commerce track of the International Space Development Conference, he concentrated on the most obvious markets for the ram accelerator: commercial satellite launches. BFG has taken particular interest in the Iridium and Globalstar constellations, which were financial failures but technically viable. Those satellites are nearing the end of their service life, and the ram accelerator would reduce the costs launching new satellites to nearly one tenth of their projected value. At those prices, a large LEO constellation becomes financially competitive with a high-bandwidth satellite chain in geosynchronous orbit. This does not change the economics of human spaceflight, but it does represent the order-of-magnitude cost improvement NASA and the private sector has sought for over 20 years.
| “I think that if the environmental issues are sufficiently addressed, and people believe the ram accelerator is safe, we will get approval to build this facility no matter what the politics of the day,” said Joseph. |
Of course any satellites launched by the ram accelerator would require acceleration-tolerant electronics, much of which exists today, as well as new technologies like inflatable antennas. This type of hardware doesn’t last long—only about three years, on average—but the accelerator allows for many more launches per year, effectively canceling out the disadvantage of the satellites’ short lifespan.
Satellites are not the only potential market for the accelerator. As Joseph explained, “Its natural role is launching dumb bulk material: rocket fuel, water, oxygen, construction materials, solar panels, foodstuffs, etc.” And the most natural customers for these sorts of deliveries would be the International Space Station and future commercial space stations, such as those being developed by Bigelow Aerospace.
I asked Ben Joseph a lot of questions about the political implications of this device, such as the space gun’s connection to the late physicist-turned-arms-dealer, Gerald Bull, and the potential for political opposition to the accelerator. Like his presentation at ISDC, his answers were clear and well-thought-out.
On the topic of Gerald Bull:
The history of Gerald Bull after Project HARP is completely unrelated to our efforts. While his goals for gun-launched space-access were similar to ours, the means that he employed to fund his efforts after HARP do not fit the political realities of our time. I think this is immediately obvious to anyone reading about this man’s life, thus I am not concerned about any negative perception due to the recent history of super-guns.
As for any political opposition:
We hope that the ram accelerator will inspire excitement in the general public about the future of space. Cheap mass transport from Earth’s surface combined with inexpensive space habitats (Bigelow Aerospace) opens up a lot of interesting things in LEO and beyond…I think that if the environmental issues are sufficiently addressed, and people believe the ram accelerator is safe, we will get approval to build this facility no matter what the politics of the day. We as Americans say we like innovation, we like being the best in space, so I couldn’t see how being against the ram accelerator would help a politician.
There is a lot work to be done yet with the ram accelerator, but one cannot help thinking that if Jules Verne could see what Ben Joseph and BFG are doing today, he’d more than likely smile.