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Repulser Engines

by Antti Lahtinen

During the early days of TNE, there was a discussion on GDW- Beta list about using tractor and repulsor devices as atmospheric or water thruster engines. The basic idea was to use tractor to accelerate large volume of air or water towards the vehicle so that the vehicle would also be pulled forwards at the same time. An attractive feature in this kind of propulsion system would be its low environmental impact, since some people did not like the idea of using HEPlaR drives on anywhere near biosphere.

There is very little information about tractor/repulsor devices in FF&S, so I made some assumptions.

1) The gravitic attraction or repulsion field generated by tractor or repulsor is not a narrow beam but rather like the field caused by directional radio antenna. That is, the field may resemble a stretched "teardrop". The effect gradually drops off at the edge of the field.

This prevents from using a tractor as a direct-damage weapon. If the tractor was a narrow full-effect beam, it could be used to tear off pieces of the target. ("That man, with green shirt. Aim the tractor at his left eyeball.")

2) The field shape can be changed by the user (within limits, as with a directional radio antenna).

A narrower field could be used to affect one target at long range, while wide field could affect several targets at short range.

3) The field strengths are cumulative and opposing fields cancel each others.

Tractors placed on different walls of a cargo hold could manipulate cargo containers; a carrier ship with tractors on turret extenders could manoeuver shuttles into landing pads.

4) Any force has an opposing force. When a vehicle with tractor pulls a target towards itself, the vehicle is also pulled towards the target.

With these assumptions I made some calculation to check how well tractor performs when used as atmospheric thruster. Here the tractor is used as a normal mass-flow thruster to pull air towards the aircraft (as propeller or rotor) or through a hole in a the hull (as ducted fan or lift inducer).

Using the values from FF&S1 p.59 (or table 151 in FF&S2) with thrust formulas, a tractor thruster appears to be large and expensive, but has better thrust/power ratio than HEPlaR. The following tables show engines for constant values.

Constant Volume
Engine Volume (m3) Mass (Ton) Power (MW) Price (MCr) Thrust (MN)
TL-10 HEPlaR 1 1 10 0.01 2
TL-12 Tractor 1 1 0.03 0.1 0.03

Constant Power
Engine Volume (m3) Mass (Ton) Power (MW) Price (MCr) Thrust (MN)
TL-10 HEPlaR 0.1 0.1 1 0.001 0.2
TL-12 Tractor 33.333 33.333 1 3.333 1

Constant Price
Engine Volume (m3) Mass (Ton) Power (MW) Price (MCr) Thrust (MN)
TL-10 HEPlaR 100 100 1000 1 200
TL-12 Tractor 10 10 0.3 1 0.3

Constant Thrust
Engine Volume (m3) Mass (Ton) Power (MW) Price (MCr) Thrust (MN)
TL-10 HEPlaR 0.5 0.5 5 0.005 1
TL-12 Tractor 33.333 33.333 1 3.333 1

When the thrust and power values of a 1 m3 TL-12 tractor are examined with mass-flow formula, the results are:

Induced mass flow: 15000 kg/s
Velocity of mass flow: 2 m/s
Volume of mass flow: 12755.1 m3 (air density is ~1.176 kg/m3)
Equivalent rotor area: 6377.6 m2
Equivalent rotor diameter: 90 m

These values suggests that the tractor has similar effect on air as a rotor with 45 meter long weightless and dragless blades. In the same way, the 1 MN tractor performs as a rotor with 130 meter blades.

If an air vehicle (air-raft) used tractor as thruster engine, the environmental effects would be similar as with a helicopter. Turning the tractor to another direction would cause similar effects as tilting the rotor.

Example Design of a TL-12 Air Raft

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