Philosophical Society of Washington

Minutes of the 2122nd Meeting

Speaker: Harold E. Gilreath, Principal Staff Research Engineer, The Johns Hopkins University Applied Physics Laboratory, Baltimore, MD
Topic: “Light-Gas Gun Satellite Launchers”

President Spargo called the 2122nd meeting to order at 8:15 p.m. on November 3, 2000. The Recording Secretary read the minutes of the 2121st meeting and they were approved as read.

The speaker for the evening was Harold E. Gilreath. The title of his presentation was “Light-Gas Gun Satellite Launchers”.

Would it be feasible, both technically and economically, to use guns work to launch satellites? The first discussion of the use of guns to launch space vehicles was in Jules Verne's From the Earth to the Moon published in 1865. In this remarkably prescient novel, Verne describes a large gun being used by the United States to send three men to the moon from the east coast of Florida. In 1930 Baron von Pirquet of Germany invented a distributed injection gun that would actually have been capable of launching a satellite. It eventually led to plans for the German V-3 weapon, which was not used before the end of World War II. It consisted of a 120 meter, eight-stage gun that would have been capable of launching 14 tons of shells per hour at targets hundreds of miles away.

Since World War II, the concept of inexpensively launching satellites from modified guns has been the focus of a number of feasibility studies and experimental projects for improving the design of both the launcher and the launch vehicle. The good points are that such launch systems are relatively simple since they do not require in-flight guidance, they have an order of magnitude higher payload capacity, they have much better operability in extreme conditions such as severe weather, and they are relatively inexpensive. Their bad points are that the usual designs are constructed and fired at a fixed inclination, they have a very high launch stress generating ranging from 20 to 30,000 G's, and they have severe trans-atmospheric flight heat and shock stress.

The High Altitude Research Project (HARP) was started in 1962 and lasted into the early 1970's. It was an primarily an atmospheric study using a device based on a 16 inch naval gun known as the Marlet 4 capable of launching a 1.5 km per second muzzle velocity shell. Later, DARPA set up the Orbital Express Program with the objective of producing a system of strategic and tactical maneuverable satellites. By being maneuverable the satellites could fly in formation or alter orbital coverage so their behavior would be evasive and unpredictable to an enemy. Smaller gun-launched satellites would be used to refuel, repair and upgrade the orbiting satellites. While the total specific cost depended strongly on the launch vehicle, by employing gun-launched satellites the servicing costs for the fleet of satellites is expected to be halved each year, extending Moore's Law into space.

The use of hydrogen in these guns is unavoidable. The maximum achievable muzzle velocity is determined by the speed of sound in the propellant gas. The speed of sound in a gas is inversely proportional to the square root of the average molecular weight of the gas (assuming adiabatic wave propagation). Therefore, the lighter the molecular weight of the gas the greater the muzzle velocity will be. With the muzzle velocity of these guns the projectile can leave the atmosphere in 10 seconds. To increase the target velocity they must be fired at a lower angle, but this also increases the atmospheric stress time. A muzzle velocity of at least 7 kilometers per second he is optimal for balancing the minimization of heat stress and boosting the orbital velocity. Using lower velocities would lower the stress but require multiple stages. One gun has been designed 5000 feet in length with 1 primary and 15 side injectors, and a launch angle of 22 degrees. Extremely fast valves and hydrogen heaters are key to the development of this system. Each shot would require $350,000 worth of hydrogen gas. To lower costs, as much of this gas is recovered as possible, however the turnaround time is increased to about 40 hours. Thermal protection is a design challenge; the design uses an ablation-cooled, carbon composite shell. The 1500 pound launch mass leaves the gun in 0.435 seconds and the atmosphere in about 300 seconds. The initial costs are $320,000 per service vehicle, $298 million for gun construction, $4 million for R&D, and $4 million per year for fixed operating costs. Projecting a 90% learning curve rate and a maximum launch rate of 300 per year, eventually a cost of less than $1 million per mission could be achieved. At that point, 86% of the cost would be in the launch vehicle, and more than 1 launch per week would make it economically. While such a mission might be technically and economically feasible, there remains the problem of high-G stress damage to structure and power systems.

Mr. Gilreath kindly answered questions from the floor. President Spargo thanked Mr. Gilreath for the society, and welcomed him to its membership. Joe Seifried spoke briefly on recruiting members. The President made the announcements about the next meeting, parking, and refreshments, and adjourned the 2122nd meeting to the social hour at 9:37 p.m.

Attendance: 31
Temperature: 15.1°C
Weather: cloudy

Respectfully submitted,
John S. Garavelli
Recording Secretary

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