Institute of Atomic-Scale Engineering

 

South Pole Accelerator

By Forrest Bishop

This is an overveiw of a previously unpublished spacecraft launch system originally conceived and designed in 1981.

 There are three major concepts which distinguish this proposal. Peer review to date suggest they may be original, taken individually as well as in the aggregate.

The first is to install a vertical launcher near the South Pole, perhaps on Mt. Erbus. The principle reason is to have launch windows several hours long for mutually rendevousing projectiles. Launch windows for Lunar delivery are days long, whereas the "window" for other missions is continuously open.

The second concept is to boost to nearly Earth escape velocity, to allow the wide variety of non-planar, near free trajectory maneuvers that can be performed at the limits of the Earth's sphere of influence. When a slow moving body (in the Earth frame) is in this region, the Sun's gravitational field strength is of the same order as the Earth's. Depending on the time of year, and owing to the Earth's axial tilt, this means the body is either closer or further from the the Sun than is the Earth, but traveling at Earth's nominal orbital velocity. In addition, the body is out of the plane of the ecliptic. The net force acts in general (and with some nudging) to move the body toward the plane of the ecliptic, while imparting a tangential velocity component (relative to Earth). This means that the payloads can be transfered to Lunar intercept orbit, or even to an equatorial plane orbit, with very minimal additional delta V. The traditional apogee burn of Two-Body analysis is replaced by one or more smaller "nudging" burns. An analogous maneuver was recently accomplished by the Japanese spacecraft Muses 1.

 The third idea is to launch many small projectiles (a few hundred grams to a few kilograms each) in rapid order. This is done to keep the bore of the accelerator small (a few centimeters), and to minimise the peak demand on the power plant. The ratio of mass to cross section for these bodies is so low that single ones would not be able to pass throught the atmosphere. A high launch rate is required to establish and maintain a vertical corridor of rarified gas (and plasma). This reduces the kinetic energy lost by the projectiles due to atmospheric drag. The ablation rate is problematic.

A group of these payloads is assembled while en route to the Earth's field limit, which takes several days. This is to permit applying the nudging burns to a packet, rather than to each individual payload.

The following subsystems are descibed breifly:

  • Electromagnetic accelerator (mass driver) design and installation.
  • Projectile/payload types and design.
  • Modeling the atmospheric corridor.

(1) Electromagnetic Accelerator

There is a large body of work, both theoretical and experimental, on the design and construction of these kinds of machines. The launch velocity is about escape velocity (11.1 km/s) plus on the order of 1 km/s to overcome atmospheric drag.

In choosing an accelerator, there is a trade-off between the average acceleration and the length of the accelerator. Something over 6000 g's cuts the length to about 1.5 km.

The launch tube is a vacuum vessel with a diaphragm or iris at the exit. This is to assist in starting up the accelerator. After it has begun operating, the projectiles keep the launch tube swept out, more or less. Vacuum pumps might assist in this. Injecting gaseous hydrogen at some places along the tube lowers the average molecular weight of whatever gasses remain, thus reducing drag and heat transfer. Conditions at the top are a problem. There may be a requirement for active cooling as well as active cancellation of noise and vibration.

The upper section has the usual steering and speed adjustments for the projectiles. The exit velocity vector is very slightly canted from the vertical. This reduces the danger from possible returning projectiles (during initialization), as well as permitting an installation site some distance from the actual Pole.

 Sustaining a launch rate in excess of 10 Hertz takes some very special handling equipment. If it hangs up for more than a few hundred milliseconds, the flight corridor has to be re-initialized.

The loading equipment at the base of the accelerator has to cool the reaction coils below the superconducting critical temperature, charge them with a circulating supercurrent, and load (clips of) them into the launch tube. Somewhere along this line they are placed in vacuo. Part of this technology may come from the bottling equipment industry, and part from military systems.

 (2) Projectile design

The size and acceleration limits the kinds of payloads. The most valuable commodity this system could launch is probably water ice. Other materials, fuels, and such could also withstand the rigors of launch.

The themal protection system could be either insulative, ablative, and/or magnetohydrodynamic.

 Using frozen hydrogen has a distinct advantage besides thermal protection. As it blows off during the atmospheric passage, it and its reaction products reduce the average molecular weight of the gas/plasma in the flight corridor. Water ice can also be used here, to lesser effect. When the accelerator first starts up, projectiles consisting mostly of these substances would initialize the corridor.

By allowing the supercurrent (for projectiles with integral coils) to continue circulating after the projectile clears the accelerator, its induced magnetic field would deflect the plasma component of the wake. This may cause more drag and other problems than it's worth, so provision is made to quench the supercurrent for atmospheric passage.

 Some arrangement of auxillary magnetics allows the projectile to be spun up about its long axis during acceleration, for spin stabilization.

To facilitate collection, it may be desirable to make the projectiles in two parts connected by a fiber. Sometime after leaving the atmosphere, the two pieces separate. The angular momentum of the spun projectile is transfered to this new configuration, resulting in a flat spin state roughly perpendicular to the (earth reference) flight vector. The effective capture cross section is increased as the square of the fiber length.

It is possible to include an integrated, miniature flight control system in each projectile. Ground or space based radar can track the projectiles, and issue serial numbered instructions to individual members. On board Quartz Rate Sensors, silicon micromachined accelerometers and valves operate a miniature reaction control system for course correction and rendezvous. This would obviate the need for the flat spin idea above.

(3) Modeling the atmospheric corridor

This is a difficult system to properly model. The pulsed nature of the aerodynamics in the ground reference frame makes time invariant solutions suspect. The radial variation of density and specific heats disallows ordinary heat diffusion equations. Aerothermochemicaldynamic effects add their own non- linearities, as does radiative energy transfer.

 Prior work indicates the feasibility of successfully launching single projectiles of a few kilograms through the atmosphere at 10-15 km/sec, with about 10% mass loss. The rarefaction should allow better performance than this, however the additional heating incurred may offset the gains.

In addition to the unanswered technical issues, there are the usual items of environment, politics and finance.

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