Shortly before publishing my article on David Kipping’s TARS concept (Torqued Accelerator using Radiation from the Sun), I received an email from Centauri Dreams associate editor Alex Tolley. Alex had come across TARS and offered his thoughts on how to improve the concept for greater efficiency. The publication of my original piece has launched a number of comments that have also probed some of these areas, so I want to go ahead and present Alex’s original post, which was written before my essay got into print. All told, I’m pleased to see the continuing contribution of the community at taking an idea apart and pondering alternative solutions. It’s the kind of thing that gives me confidence that the interstellar effort is robust and continuing.

by Alex Tolley

Dr. Kipping’s TARS proposed system for accelerating probes to high velocity is both simple and elegant. With no moving parts other than any tether deployment and probe release, if it works, there is little that can fail during the spin-up period. There are improvements to the basic idea that increase performance, although this essay will suggest a more complex, but possibly more flexible and performant approach using the basic rotating tether concept.

First, a small design change of TARS to increase the rate of spin-up. The TARS design is like a Crookes radiometer, but working in reverse, with the mirror face of the sail experiencing a greater force than the obverse dark, emissive face. As the tethers rotate, the reflective face increases the spin rate, whilst the emissive face swinging back towards the sun acts as a retarding force. An easy improvement, at the cost of a moving part, is to have the sail reorient itself to be edge-on to the sun as it returns. This is illustrated in Figure 1 below. The rotation can be any mechanism that sequentially rotates the sail by 90 degrees after the tether is aligned with the sun, or other electromagnetic radiation source.

Figure 1. The simplified TARS system with the sail rotating around the tether to reduce the retarding force in the rotation phase.

There are other possibilities to tweak the performance, but at a cost of complexity and added mass.

However, I want to offer an alternative approach that solves some of the limitations of the proposed TARS system.

These limitations include:

• The propulsive force is very phase-dependent as the tether rotates.
• The rotation rate is dependent on the sail aerial density and size&lt
• The sails add mass to the tether and therefore increase the tether tension, requiring an increased taper
• The TARS rotation must be aligned with the radiation source, limiting the direction it can throw the payloads. This means that a target on an inclined plane to the planets, such as a comet or exoplanet, requires the TARS to take on an inclined orbit, limiting its flexibility.
• The asymmetric forces on TARS change its orbit.

These limitations can be alleviated by eliminating the sails and replacing the rotation with an electric motor, powered by a solar panel. The basic design is shown in Figure 2.

Figure 2. Basic design of a rotating probe launcher using motor-driven tethers.

The tether is powered by an electric motor that requires a counter-rotating wheel or tether (see later) to prevent the system from rotating. This is similar to the power equipment astronauts use in space. The tether is attached to the solar panel by a 3-axis joint to allow full control of the rotational plane of the tether. As the only loads on the tether are its own mass and the releasable probes, the amount of taper should be less than TARS, allowing longer tethers of the same material. The tethers can be flexible or stiff, depending on deployment preferences. Figure 2 shows a preferred arrangement where the tethers form a square, with cable stays to increase rigidity and offset bending during spin-up.

The tether would have 2 releasable probes and 2 small ballasts to maintain tension, or 4 probes. The probes can be released simultaneously in opposite directions, or in the same direction from 1-10 milliseconds apart, depending on the rotation rate. If released in the same direction, the system will tend to be pushed in the opposite direction as the probes released in the same direction would act as propellant, generating thrust in the opposite direction.

A variant would allow for 2 contra-rotating tethers. Because they are mechanically coupled to the same motor, this guarantees that they rotate in synchrony and eliminate the gyroscopic action of a single tether. This removes the need for a counter-rotating disc for the motor, but more importantly, for multiple payloads allows the rotation plane to be changed between payload releases, allowing for different target destinations for the probes to travel in. This would be ideal for a standby to target comets and objects coming from different orbital inclinations, as well as more detailed mapping of the solar system’s heliosphere.

Because the rotation is controlled by a motor, this provides more precise timing of the payload releases. Once the maximum rotation rate is reached, the motor can idle, and the system continue its orbit until the optimum probe[s] release position is achieved, for example when Mars is in opposition. This avoids the continual rotation rate increase of TARS that must release its probe[s] before the tethers snap.

So what sort of rotational speed can a motor provide? The maximum speed for a small motor is 100,000 rpm, or 1667 rps. A much lower speed is achieved by hard disk drives at 7200 rpm or 120 rps.

This translates to:

Because the rotation rate is so fast, any probe release must be timed with very high precision to ensure it travels on the correct flight path towards its destination. While not critical for some missions, encounters with small bodies such as interstellar objects (ISO) like 2I/Borisov will require very high precision releases.

Unlike TARS, the tethers can also be spun down, making the system reusable to reload the payloads. If multiple payloads can be released sequentially like a Pez dispenser, then these can be reloaded periodically when the payloads have been exhausted. With extra complexity, these cartridges of probes could be carried on the system, and attached to the tethers after the rotation has been reduced to zero, making the device relatively autonomous for long periods.

Lastly, because the rate of rotation acceleration is dependent on the motor and power available, the power can be increased with a larger solar array, and the motor torque increased with a larger motor. These are independent of the tether design, making any desired upgrades simpler, or like CubeSats, configurable on manufacture before launch.