Introduction

Interstellar starships will not spring fully able and formed from nothing. There will probably be a long series of precursor missions, done with vehicles of ever expanding capabilities. What might be the precursor missions to Interstellar flight? What are the capabilities required and how do they compare to those of an Interstellar starship?

Precursor technologies

Table 1 shows some possible technologies for precursor missions. The only technology that has actually been used is solar electric propulsion (SEP) with hall effect thrusters or similar ion thrusters(1). The technology that is required for Interstellar exploration requires a specific impulse that is more than 2 orders of magnitude higher(2).

Project Icarus(4) proposed precursor missions named Pathfinder, Starfinder Mk1 and Starfinder Mk2(5). Precursor missions using 4 grid ion(6) or solar sails have also been proposed(7).

The first high capacity outer planet explorer would probably use nuclear electric propulsion (NEP). This requires the development of a nuclear reactor designed for space operation. The Kilopower(7) reactor concept is the one that is the closest to being built, with some elements having undergone ground testing. Kilopower could provide 1 to 10 kW of power with an efficiency of about 25%, for up to 15 years of operation. This is sufficient for perhaps very small probes, but for a program leading up to Interstellar Starships, lacks ambition. The Megapower concept, an evolution of the Kilopower reactor, would offer up to 2 MW of power to supply an ion drive. Using hydrogen, a specific impulse in the order of 40 000s would be possible. A 2 MWe reactor combined with a 10 000 to 100 000 ISP 4 grid ion drive would be capable of most of the precursor mission described here. In addition to the actual missions, 4 grid ion engines could be used in the Firefly design as injectors(9). This would complete a design circle for this technology, as they were originally designed for efficient injection of nuclear fuels into Tokamak reactors.

4Grid ion nuclear electric probe

A 4-grid ion nuclear electric probe would be a very capable and scalable technological platform for outer planet exploration and the first stages of Interstellar exploration.

Early fusion powered probe

The most critical item to start on the development path is not the thruster, it is the power source. Deep space exploration is unlikely without a space nuclear reactor. An alternative path might be beamed solar power sails, as proposed by Project Starshot().

Going small: project Tintin

Nuclear electric of fusion powered rockets are large vehicles that require a number of new technologies and substantial spending. They are big science. Is there another way? In 2012 Icarus Interstellar proposed another development path, project Tintin₍₂₅₎, based on the concept of cubesats and a number of small vehicles with short development cycles. The power source would be a tiny Radioisotope Thermal Generator (RTG), and the drive a Field Emission Electric Propulsion (FEEP) ion drive, with an ISP that should be scalable from an early 5 000s to 15 000s and perhaps beyond. Tintin would evolve from an first proof of concept mission to the Moon to missions aimed at more distant targets, increasing in capacity as technology advances. Progress all the way to the capacities of an interstellar probe would depend on the development of some type of small scale fusion device.

REFERENCES

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4. Long, K.F., et al, “Project Icarus: Flying Closer to Another Star”, JBIS, 62, pp.403-414, 2009.

5. Project Icarus: Exploring the Interstellar Roadmap using the Icarus Pathfinder and Starfinder Probe Concepts R. W. Swinney; K. F. Long; A. Hein; P. Galea; A. Mann; A. Crowl; R. Obousy (2012), JBIS, 65, 244-254

6. Bramanti, C., D. Izzo, T. Samaraee, R. Walker, and D. Fearn. "Very high delta-V missions to the edge of the solar system and beyond enabled by the dual-stage 4-grid ion thruster concept." Acta Astronautica 64, no. 7-8 (2009): 735-744.

7. Bramanti, Cristina, Roger Walker, Orson Sutherland, Rod Boswell, Christine Charles, David Fearn, Jose Gonzalez Del Amo, and Marika Orlandi. "The Innovative Dual-Stage 4-Grid Ion Thruster Concept-Theory And Experimental Results." In 57th International Astronautical Congress, pp. C4-4. 2006.

8. https://en.wikipedia.org/wiki/Kilopower

9. Freeland, R. M. "Plasma Dynamics in Firefly's Z-pinch Fusion Engine." JBIS 71 (2018): 288-293.

a. Practical Interplanetary Propulsion Group, 3rd NFFP Briefing, AIAA, Ronnie Nader, Stephanie Thomas ,Brent Freeze & John Fuller August 27, 2020

b. Icarus precursor missions : K.F.Long, A.Hein, P.Galea, , A.Mann, A. Crowl, R. Obousy, A. C. Tziol, R.W.Swinney and as. in JBIS, 65, ,p244-254 july/aug. 2012.

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21. Icarus Interstellar, Project Tintin

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