# Space Transport and Engineering Methods/Guns and Accelerators2

Note: Continued from sections A and B at Guns and Accelerators

## C. Light Gas Guns

Light gas guns are designed to reach higher muzzle velocities than combustion guns. These types of guns are also called hypervelocity guns since the projectile travels at more than Mach 5 (1500 m/s at sea level). In aerodynamics, velocities above Mach 5 are are called hypersonic. They reach higher velocities by using hot hydrogen (or sometimes helium) as the working gas. These have a lower molecular weight, and therefore a higher speed of sound than the combustion products of smokeless powder, liquid, or gaseous fuels.
Guns are strongly limited by the speed of sound of the gas they use, since pressure waves travel at the speed of sound. Therefore any gas at the back of the barrel no longer contributes to pushing a projectile once it reaches the local speed of sound, because the pressure waves cannot keep up. The gases near the projectile and moving at the same speed can still provide pressure, though. So the efficiency of the gun falls significantly, but does not stop entirely as the projectile goes supersonic. Light gases like Hydrogen do not generate high pressures and temperatures by themselves, as do combustion byproducts in standard guns. Therefore some external means are required to produce the hot gas. The various methods are listed in the following sections.
Scaling - Light gas guns have been used for hypersonic research for about 40 years. Examples are testing heat shield materials, and meteoroid impact damage. These do not require large projectiles, so a light gas gun large enough for practical use in space transport has not been built yet. There is no reason to think larger guns will not work. Higher muzzle velocities than needed for space launch have already been demonstrated in research guns. It is more a matter of designing them for low cost operation rather than research, and simple scaling of the parts. Large guns will in fact be more efficient than research guns:
* The region close to the barrel wall is called the boundary layer. This is where heat is lost to the cooler barrel, and friction is created between the moving gas and stationary wall. The boundary layer stays the same thickness regardless of gun size, so represents a smaller fraction in larger guns.
* Atmospheric drag is proportional to the area of the projectile, while it's mass is proportional to area x length. The mass thus grows faster than drag, and deceleration due to drag goes down with size. So drag induced velocity loss becomes a smaller factor for larger guns.
Larger guns also tend to have lower acceleration of the projectile. The muzzle velocity of this type of gun is mostly set by the speed of sound of Hydrogen, and the practicality of flying through the atmosphere at high speeds. So the muzzle velocity will not be greatly different in larger guns. The barrel length goes up with gun size, so the acceleration is lower to reach the same final velocity. That means the projectile structure can be lighter, and a wider range of cargo can be carried. In the limit of very large guns, it would be possible to carry humans, but such large guns will not be built for some time, and might be overtaken by better alternatives.
Comparison to other Launchers - Compared to rockets, weight is not an issue for a terrestrial mounted gun. Thus their parts can be closer to industrial grade than aerospace grade, and should be relatively inexpensive. Gun barrels are also simple compared to electromagnetic coil type accelerators providing the same force. Thus they should have a lower construction cost. Electromagnetic devices have a higher theoretical efficiency and maximum velocity than a gas gun, so the choice will depend on construction vs operations costs of real designs. Electromagnetic guns have a shorter history, so their capability and cost at large scale are more uncertain.
Gun/Rocket Optimization - When launching from Earth, the projectile encounters the atmosphere at high velocity, and drag tends to scale as velocity squared. Even with light gas guns, it is difficult to reach full orbit velocity (~8 km/s), and even at that velocity the orbit will intersect the ground, because that is where it started from. Thus any gun system needs at least some propulsion to raise the low part of the orbit. When optimized for cost and efficiency, the gun velocity generally ends up being roughly half of orbit velocity, with the balance provided by a rocket on the projectile or some other method. A rocket that only has to provide a fraction of orbit velocity will be much smaller and less expensive, and repetitive smaller launches lower the size even more. For example, daily gun launches compared to about six conventional rocket launches per year gives a factor of 60, and approximately 4 times lower rocket to payload mass ratio gives a total reduction of 240 in rocket size. The main cost advantage of hypervelocity guns thus comes from replacing the large and expensive conventional rocket with a much smaller one plus a reusable and relatively cheap gun.
Location - Due to their size, light gas guns will likely be built on a mountain with the correct slope, pointing east. This minimizes construction cost and reduces air drag from the higher starting altitude. An ocean platform for the gun could be aimed, but has the drawbacks of more atmosphere to fly through at sea level than from a mountain top, and dealing with salt water corrosion. When choosing a mountain, equatorial ones allow meeting a space station every orbit (approx 90 minutes), while other latitudes only allow meeting once per day, when the launch site rotates under the orbit plane.
Evacuated Barrel - All large guns will operate more efficiently if the projectile does not need to push a column of air up the barrel. So some method of sealing the end and pumping out most of the air is generally desired. In smaller guns something as simple as a plastic film has been used, and the projectile just punches through it. In larger guns a flap or flaps held in place by the pressure difference of the barrel vs outside air can be forced open as the residual air in the barrel gets compressed by the projectile. This sort of automatic opening is preferred to some mechanical valve or door. A failure of the mechanical device would lead to a spectacular collision with the projectile.
Muzzle Design - It is worth considering a silencer type device at the muzzle for several reasons. In addition to sound reduction, it can capture the Hydrogen gas to be recycled. Otherwise escaping hot Hydrogen will immediately burn in air, producing a muzzle flash. When the projectile goes from high acceleration in the barrel to deceleration in air, if the transition is too sudden that might cause damage to the projectile or cargo. A silencer can bleed off some of the Hydrogen, giving a smoother transition. The residual air in the barrel will also provide some degree of shock buffer as it piles up ahead of the projectile. Other ways to lower the shock are to put gas nozzles around the muzzle to add a stream of gas as a transition region past the muzzle, or to shape the barrel as a flaring cone at the end to taper off the acceleration. Finally, the on-board rocket on the projectile can be used to counter the drag deceleration.
Projectile Features - Filling the area behind the projectile with gas, known as base bleed, has been used previously to reduce drag and extend the range of artillery. The velocity of light gas gun projectiles is high enough that shaping the back end to reduce drag may not be practical, and so base bleed is an option. This can be combined with the on-board rocket engine by running it at low thrust merely to fill the trailing area, or at full thrust to provide acceleration even while clearing the atmosphere. The right answer will require detailed design and analysis.
Projectile Recovery - To lower cost, you would like to use your cargo projectiles multiple times. Fortunately, making them rugged enough to fire out of a gun tends to also make them rugged enough to survive re-entry without a lot of extra work. They already need some heat shielding to survive going up through the atmosphere at high velocity when launched. You just need some more shielding to also handle re-entry. The same guidance system that enables them to reach their cargo destination also can guide it to a landing point. The terminal velocity of the empty projectile after re-entry should be low enough that the projectile can withstand landing without any landing system. It was fired fully loaded at high gees out of the gun, after all. At most it might need deployable flaps or fins or a small parachute to get the landing velocity low enough.

#### 15 Pressure Tank Storage Gas Gun

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Description: The gas is simply stored in a pressure vessel, then adiabatically expanded in a barrel, doing work against a projectile. Heating the gas raises issues of storage at high temperature, but not heating the gas leads to low performance. The storage tank needs to be large relative to the barrel volume, otherwise the pressure drop during firing leads to lower performance. This leads to higher cost than other versions. These drawbacks lead to other versions of a light gas gun to be preferred over this simple one.
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References:
* Taylor, R. A. "A Space Debris Simulation Facility for Spacecraft Materials Evaluation", SAMPE Quarterly , v 18 no 2 pp 28-34, 1987.

#### 16 Underwater Storage Gas Gun

Alternate Names: Quicklaunch
Type:
Description: In a gas gun on land the amount of structural material in the gun is governed by the tensile strength of the barrel and chamber. In an underwater gun, an evacuated barrel is under compression by water pressure. The gas pressure in the gun can now be the external water pressure plus the pressure the barrel wall can withstand in tension, which is up to twice as high as the land version.
Other features of an underwater gun are the ability to store gas with very little pressure containment (the storage tank can be in equilibrium with the surrounding water), and the ability to point the gun in different directions and elevations.
The underwater gas gun consists of a gas storage chamber at some depth in a fluid, in this case the ocean, a long barrel connected to a chamber at one end and held at the surface by a floating platform at the other end, plus some supporting equipment.
Example Design In one version of this type of gun, the chamber is a made of structural material such as steel. An inlet pipe allows filling of the chamber with a compressed gas. A valve is mounted on the inlet pipe. An outlet pipe of larger diameter than the inlet pipe connects to the gun barrel. An outlet valve is mounted on the outlet pipe. This valve may be divided into two parts: a fast opening and closing part, and a tight sealing part. The interior of the chamber is lined with insulation. The inner surface of the insulation is covered by a refractory liner, such as tungsten. An electrical lead is connected to a heating element inside the chamber.
An inert gas such as argon fills the insulation. The inert gas protects the chamber structure from exposure to hot hydrogen, and has a lower thermal conductivity. An inert gas fill/drain line is connected to the volume between the chamber wall and the liner. A pressure actuated relief valve connects the chamber with a volume of cold gas. This cold gas is surrounded by a flexible membrane such as rubber coated fiberglass cloth.
In operation, the gas inside the chamber, the inert gas, and the water outside the chamber are all at substantially the same pressure. Thus the outer structural wall does not have to withstand large pressure differences from inside to outside. One part of the chamber wall is movable, as in a sliding piston, to allow variation in the chamber volume. The gas in the chamber is preferably hot, so as to provide the highest muzzle velocity for the gun. When the gun is operated, this gas is released into the gun barrel. In order to preserve the small pressure difference across the wall of the chamber, either the chamber volume must decrease or gas from an adjacent cold gas bladder must replace the hot gas as it is expelled. This arrangement prevents ocean water from contacting the chamber walls or hot gas. In the case of the sliding piston, the membrane collapses, with the gas formerly within it moving in behind the piston. In the alternate case, the membrane also collapses, with the gas formerly within it moving through a valve into the chamber.
The chamber has an exit valve which leads to the gun barrel. It also has gas supply lines feeding the interior of the chamber and the volume between the chamber walls. These lines are connected to regulators which maintain nearly equal gas pressures, which in turn are nearly equal to the ocean pressure. This allows the chamber to be moved to the surface for maintenance, and to be placed at different depths for providing different firing pressures or different gun elevations.
The muzzle of the gun is at the ocean surface, so elevation of the gun can be achieved by changing the depth of the chamber end. Since the gun as a whole is floating in the ocean, it can be pointed in any direction. Some means for heating the gas stored in the chamber is needed, such as an electric resistance heater. At the muzzle end of the gun, a tube surrounds the barrel, with a substantial volume in between the two. There are passages through the wall of the barrel that allow the gas to diffuse into the tube rather than out the end of the gun, thus conserving the gas.
At the muzzle of the gun is a valve which can rapidly open, and an ejector pump which prevents air from entering the barrel. In operation, the ejector pump starts before the gun is fired, with the valve shut. The valve is opened, then the gun is fired. In this way, the projectile encounters only near vacuum within the barrel, followed by air.
Status: Hypersonic guns have been in operation for about 40 years as research devices, with relatively small (5 kg or less) projectiles. Large or ocean-going guns have not been built.
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#### 17 Particle Bed Heated Gas Gun

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Description: Hot hydrogen gives good performance for a gun, but is hard to store. In this method it is generated when needed by flowing the Hydrogen through a chamber which contains refractory oxide particles. The particles are heated slowly (roughly 1 hour time period) by some type of heater near the center of the chamber. This sets up a temperature gradient, so the exterior of the chamber is relatively cool, and can thus be made of ordinary steels. When the hydrogen flows through the chamber, the large surface area of the particles allows very high heat transfer rates - so the heat in the chamber can be extracted in a fraction of a second as the gun fires. The gas flows inward from the periphery to the center of the heat exchanger and then to the barrel.
* Example of Gun Component Sizing: - The following calculations are intended to show the steps for getting the initial estimates of component sizes. It is very far from being a complete engineering design. Rather, it is the starting point for the design process.
    - Set as a design goal to match the 1990's Livermore SHARP hypersonic gun (5 kg at 3 km/s). From kinetic energy formula (KE = 0.5*M*v^2) we get 22.5 MJ projectile energy.
    - If average barrel pressure is 20.7 MPa (3000 psi), chosen as a reasonable value for high pressure pipe, then a 10 cm diam, 5 kg projectile (matching SHARP parameters) will see 162,600 N force, or 32,470 m/s2. To reach 3 km/s requires 92.4 msec, and a barrel length of 139 m. Allowing for pressure drop during operation, friction, efficiency, thermal, and other losses, assume actual barrel will be 200 m long. That assumption would be later replaced by a simulation to get a better estimate.
    - From simple cylinder volume formula (V = pi * r^2 * h ) the volume of the barrel will be 1.57 cubic meters. From the physical properties of Hydrogen as a gas, that volume would contain 4 kg of Hydrogen at 20 MPa and 2000K temperature. At the point the projectile leaves the barrel, the barrel is full of gas. At standard conditions (273K or 0C), the speed of sound for hydrogen is 1284 m/s. That goes up as the square root of the Kelvin temperature, so at 2000K it will be 3475 m/s, and so for this gun it will be slightly subsonic.
    - The specific heat of Hydrogen from 300 to 2000K is about 15 kJ/kg-K. Therefore by multiplying the known temperature rise (1700K), mass of H2 (4 kg) and specific heat, we need 102 MJ of energy for heating. To this we add the 22.5 MJ of projectile kinetic energy, which comes at the expense of the Hydrogen heat and pressure, for a total of 124.5 MJ.
    - To heat the gas during the launch, we don't want the aluminum oxide particles to drop more than 500K in operation. They otherwise would be cooling down too much to heat the last part of the gas. At 1300 J/kg-K, need then need about 200 kg of aluminum oxide particles. That is a rough value which would be refined by a thermal analysis of the heat exchanger.
    - The density of Aluminum Oxide is 3.9 g/cc as a solid. As grains in a heat exchanger with allowance for inlet and outlet volume and screens to keep the particles in place, assume the density is 1 g/cc. Therefore heat exchanger will be about 0.2 cubic meters in size.
    - The storage tank for the unheated hydrogen can be found from the physical properties at room temperature. The initial pressure will have to be higher than 20 MPa for the gas to flow towards the barrel. Assume it starts at 1.5 times the average barrel pressure and finishes at 0.5 times. Thus 2/3 of the gas goes into the barrel, and the initial gas to be stored is then 6 kg at 31 MPa. Under those conditions the density of Hydrogen is 25.8 kg/m^3, so the tank volume is 0.23 cubic meters. Very quickly releasing the gas from the storage tank turns it into a rocket engine, so a sturdy support is needed to keep the tank in place.
    - A fast (compared to the < 0.1 sec firing time of the gun) valve will be needed to open the tank and let the gas flow through the heat exchanger and barrel. Conceptually this can be like a car engine cylinder, where a spark plug sets off a detonation, which slides the valve piston from the closed to open position. When doing the design, you would first review existing valve hardware to see if a suitable one exists. If not, then a custom fast-acting valve would need to be designed.
Status: A small research gun of this type has been built at Brookhaven Natl. Lab.
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#### 18 Particle Bed Reactor Heated Gas Gun

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Description: Hot gas is generated by flowing through a particle bed type nuclear reactor. Gas expands against projectile, accelerating it. Light gas guns have been operated to above orbital velocity, and 1 kg projectiles have been accelerated to over half orbital velocity. This type of gun rapidly becomes less efficient above the speed of sound of the gas. As a consequence the working fluid is usually hot hydrogen. Conventional gas guns have used powder charge driven pistons to compress and heat the gas. This is not expected to be practical on the scale needed to launch useful payloads to orbit. One way to heat the gas is to pass it through a small particle bed nuclear reactor. This type of reactor produces a great deal of heat in a small volume, since the small particles of nuclear fuel have a large surface/volume ratio and can efficiently transfer the heat to working fluid. This gives the benefits of nuclear power for space launch, without the drawbacks of a flying reactor.
The particles are retained in the reactor by spinning the bed at high velocity, and the gas flow cools the external structure. The particles of nuclear fuel do not need structural strength, so can go to higher temperatures than a solid core reactor, leading to higher performance. The improvement, however, is not great over refractory metal heat exchangers, so the cost and political issues for a nuclear device probably outweighs the benefits.
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#### 19 Electrically Heated Gun

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Description: The working fluid is heated by electric discharge or microwave induction, then pushes against the projectile in the barrel. The limiting factor for a light gas gun is the speed of sound in the fluid. One way to heat the fluid to much higher temperatures is an electric discharge. If a high enough temperature is reached, the fluid becomes a plasma. Either magnetic fields or a sheath of cooler gas may be used to keep the plasma away from the walls and prevent damage. The challenge for this method is to deliver hundreds of MegaJoules for even a very small gun (by space launch standards) in a short period of time. Magnetic fields are not strong enough to contain the plasma at these pressures, so it will lose energy rapidly to physical chamber walls. Therefore the heating must be rapid, which in turn requires a large power source. For versions large enough to deliver payloads to orbit the power source needs to deliver 2 GigaJoules or more, and the power source ends up dominating the total gun cost.
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#### 20 Nuclear Charge Heated Gun

Alternate Names: Nuclear Cannon
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Description: As in the other light gas guns, the objective is to use hot gas to accelerate a projectile. In this version the heating is from a small atomic bomb in a large underground chamber filled with the working gas. This concept only makes sense in a situation where very large payloads need to be launched, due to the large minimum energy to make any nuclear device function. A large barrel leads off the chamber upward at an angle. A crossbar is set into the barrel near the chamber, and the projectile is attached to the crossbar with a bolt that is designed to fail at a pre-determined stress. This restrains the projectile until the operating pressure is reached. A small atomic bomb is suspended in the chamber and detonated to create lots of very hot hydrogen in a very short time.
There are several obvious issues with this concept. Even a small atomic bomb delivers too much energy in too short a time to easily manage. The challenge is to keep from destroying the gun itself. Another issue is radiation, which can harm the cargo, and get ejected out the barrel as fallout. Finally are the political restrictions on using atomic bombs of any purpose.
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#### 21 Combustion Driven Two Stage Gun

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Description: This is a type of two-stage gas gun. A cylindrical chamber contains a piston. On the back side of the piston high pressure gas is generated by combustion. This can be gunpowder or a fuel-air mixture. On the front side of the piston is the working gas, which is usually Hydrogen. The Hydrogen is compressed and heated until a valve or seal is opened. Then the working gas accelerates the projectile. The compression can result in theoretically unlimited temperature and pressure. In practice it is limited by the pressure capacity of the chamber structure, but this can be made very high. Therefore muzzle velocities have reached above Earth orbit velocity (8 km/s) for small projectiles. The drawback is the chamber must be several times the barrel volume to contain both the combustion driver gas and Hydrogen, which increases the cost for large versions.
Status: This type of light gas gun is the most common that has been built. They were first constructed in the 1960's or earlier. The largest gun of this type was the Lawrence Livermore Laboratory SHARP gun, which was used to test scramjet components in the early 1990's. It had a 10 cm x 45 m barrel and a 30 cm x 100 m long chamber.
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References:
* Light Gas Gun (Wikipedia article)
* Aviation Week & Space Technology, July 23, 1990.
* "World's Largest Light Gas Gun Nears Completion at Livermore." Aviation Week & Space Technology, August 10,1992.

#### 22 Gravity Driven Piston

Alternate Names:
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Description: In this method a sliding or falling mass is used to compress gas in a chamber. The gas is then expanded in a barrel. As in all light gas guns, hot Hydrogen gas is the best working fluid, and the question is how to generate it. In the previous method a piston is driven by combustion, much like an internal combustion automobile engine. Here a good part of the high pressure chamber is dispensed with by using gravity to drive a massive weight. If the gun is built on the side of a mountain, as is usually desired, the energy for launch is stored as potential energy in the weight. The falling weight rides on an air or lubricated bearing and slides down the mountain to the chamber. The piston seals the back end of the chamber and is held in place. The falling mass is designed smaller than the piston, so it does not need an accurate fit at the high velocity it impacts. The chamber then leads to a barrel containing the projectile, which accelerates upward.
One advantage of this method is the falling mass is not stopped instantly, but rather can continue compressing the gas gradually as the gun fires. The system of falling mass + working gas + projectile can be designed as a tuned coupled spring system to maximize energy transfer to the projectile. Another is the relatively low cost, since the falling mass operates in open air, and can be made from low strength materials such as concrete. A third is the mass can be raised as slowly as needed, so large power supplies are not needed. Lastly this method can be added as a "supercharger" to a particle bed type heater to increase performance. The gas is first heated to the practical limit of flowing through a heat exchanger bed, and then the weight driven piston provides additional compression heating. The question then becomes whether the additional performance is needed and justifies the added complexity of a dual heating and compression system.
Status: As of yet this is only a concept.
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## D. Electromagnetic Accelerators

This group uses magnetic fields to provide a force between the accelerator and a projectile. Electric accelerators of moderate length require high peak power for a short period of time. Hence inexpensive energy storage is very important for these concepts. Two places to look for inexpensive energy storage are (1) Magnetic fusion experiments, which also need short duration high power supplies, and (2) Inductive energy stores. The latter falls into subcategories of cooled normal conductors, and superconductors. Alternately the peak power can be lowered by making the device very long, but that then becomes a construction issue to build a device many kilometers in size.

#### 23 Railgun

Alternate Names: Electromagnetic Gun
Type:
Description: In this method, very high electric current supplied by massive rails is shorted through a plasma arc induced at the back end of the projectile. The plasma is accelerated by the Lorentz force interaction with the magnetic field produced by current. The plasma then pushes projectile along the rails. Given a sufficiently large power supply, it can be considered for Earth launch systems at lower accelerations than those proposed for weapon systems.
Status: This device was under intensive development for the Strategic Defense Initiative. A large gun was built at Eglin AFB in Florida and used a bank of thousands of car batteries wired in parallel as a power supply. Prototype railguns achieved high velocities, but the high currents and plasma produced rail erosion.
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References:
* Robinson, C. A. "Defense Department Developing Orbital Guns", Aviation Week and Space Technology, v 121 no 12 pp 69-70, 1984.
* Bauer, D. P. et al "Application of Electromagnetic Accelerators to Space Propulsion" IEEE Trans. Magnetics vol MAG-18 no 1 pp 170-5, Jan. 1982.

#### 24 Coilgun

Alternate Names: Mass Driver
Type:
Description: In this method a series of coils forming the gun react with coil(s) on projectile magnetically, producing linear thrust. Popularly known as a Mass Driver, it functions on the same principles as an electric motor. The concept was originally developed in connection with launching Lunar materials for space manufacturing. The low orbital velocity and vacuum on the Moon made it feasible. Accelerator designs with high efficiency (>90%) and high muzzle velocities (>8 km/s) have been proposed for use on Earth. This potentially leads to a transportation system whose operating costs consist mostly of electricity. Because of the need for large amounts of power to drive the device, the cost of that power supply tends to dominate overall system cost, so an electromagnetic accelerator makes sense for very high volume launch.
Like other launchers that generate high velocity near the Earth's surface, some method of dealing with atmospheric drag and heating is needed. Options include heat shielding on the projectile, a vacuum tunnel, or installing the launcher on high enough towers to avoid the atmosphere.
Status: Linear electric motors and magnetic levitation trains operate on the same principle as a coilgun. A coilgun is just a higher performance version. Prototype coilguns were built around 1980 and reached 1800 gravities acceleration, which is more than sufficient.
Variations:
    24a Quenchgun - The energy is stored in superconducting coils making up the gun. The circulating current is quenched ahead of the projectile either by heating the coil above the transition temperature of the superconductor, or by raising the field using a coil on the projectile. In either case the current stops flowing in the gun coil. Since the coils behind the projectile are off, while those ahead are still on, the net force will accelerate the projectile.
References:
* Nagatomo, Makoto; Kyotani, Yoshihiro "Feasibility Study on Linear-Motor-Assisted Take-Off (LMATO) Of Winged Launch Vehicle", Acta Astronautica, v 15 no 11 pp 851-857, 1987.
* Kolm, H.; Mongeau, P. "Alternative Launching Medium", IEEE Spectrum, v 19 no 4 pp 30-36, 1982.
* Kolm, H. "An Electromagnetic 'Slingshot' for Space Propulsion", Spaceworld pp 9-14, Feb. 1978.