Propulsive Development

To just about everyone in the world, it was clear that the technologies of propulsion in space available in the late 1990s were inadequate for getting around in space. They were sufficient for cislunar travel and just barely for travelling towards Mars or Venus. Further out however, they were only useful for unmanned probes as even with solid core nuclear engines several months were needed to go to Mars and Venus.

Although Martian exploration wasn’t exhausted at all and virtually every month new information was published by the media, the focus of the early 21st century also started to move to the outer planets. There still were these derelicts in orbit around Saturns moon Titan and everyone wanted to know more about them. At best, there had been a few short visits there by the American Mariner probes and some longer visits by the Soviet Saturn probes and the American Cronos probes.

Yet going there with solid core nuclear engines would need years and large spacecrafts and current designs did not have the Delta-v needed to go there. So everyone was looking for new ways to move through space, by developing new engines that were vastly superior to existing engines.

In the United States, scientists were looking into the Variable Specific Impulse Magnetoplasma Rocket, or short VASIMR.

By heating up a gas into a plasma state with a correctly designed antenna in a two stage process, it was possible to heat up the plasma to several million degrees. Depending on the amount of gas and energy used within the engine it was even possible to ‘switch gears’ of the engine, trading exhaust velocity for thrust and vice versa.

In high gear, the engine had an exhaust velocity of 300 km/s with a thrust of only 40 Newton, a little better than existing ion thrusters. In low gear on the other hand the exhaust velocity dropped down to 30 km/s, but increased the thrust up to 400 Newton.

To NASA this had several interesting implications. The engine in low gear could provide an initial thrust to leave a planetary sphere of influence, while the high gear allowed longer continuous acceleration towards a given target.

The travel times to Mars could be reduced to three to four months, those to Jupiter and Saturn to just two years, compared to five years with a nuclear engine.

During the early 1980s the theoretical design for the VASIMR engine had been developed at MIT, who at the time was working on magnetic mirror fusion. In 1995 the Advanced Space Propulsion Laboratory began working with the MIT magnetic mirror device and by 2002 tested the first 50 kWe version of the VASIMR engine under laboratory conditions.

By 2008 the ASPL had further refined and developed the system and was able to produce a 1 MWe version of the engine. In 2010 NASA was ready to test the first 5 MWe version of the VASIMR engine in space, using hydrogen as propellant.

While the engine needed a month to transport a 50 tonne payload to the Moon, it was seen as a success and NASA considered the engine to replace the nuclear powered tugs on the Earth-Moon route for any unmanned cargo delivery by 2012, especially as the VASIMR only needed twenty tonnes of hydrogen for the round trip, compared to over 200 tonnes by a conventional nuclear propulsion module.

A cluster of four 10 MWe VASIMR engines was considered to be used for the round trip to Mars by NASA. The US Space Force on the other hand expected to continue using nuclear propulsion, augmented by VASIMR engines to be able to use rapid accelerations for possible combat maneuvers and the VASIMR for continuous acceleration.

In the Soviet Union, engineers and designers prefered to use what they understood best, nuclear fission.

One of the big problems in using nuclear fission had been to keep the reactor from melting, limiting the potential temperature of the engine itself and its exhaust. It went even as far as that the solid core nuclear engine having a lower temperature when compared to a conventional hydrolox engine.

Glushko had already considered the next logical step, by not caring that the nuclear core was molten, or even gaseous. His early gaseous fission core engine, the RD-600, had exhaust velocities of 20 km/s and a thrust of nearly 2000 kN. It vastly improved on the solid core fission engine, but while possible useful, computer simulations later on showed that it would have been rather unstable and inefficient. The need for cooling the engine, alone would have prevented it from being used for more than one minute at a time and it would have lost too much of the fissionable uranium hexafluoride fuel.

The advances in superconductors in the mid to late 1990s however allowed the engineers and designers to improve on the original RD-600. The RD-650 was a gaseous core fission engine that used a magnetic field to keep the fission plasma in check and from contact with the engine core, vastly reducing the temperature problem of the engine. Additionally the magnetic containment field rotated, keeping the heavy uranium fuel inside the engine longer.

Compared to the RD-600, the RD-650 had an exhaust velocity of nearly 40 km/s, while the thrust remained at 2000 kN, doubling the engines efficiency. Much like conventional solid core fission engines however, the gaseous core engine remained only useful for impulsive thrust maneuvers and the engineers knew.

For longer duration thrust, Soviet Engineers worked on using the knowledge gained from the Polyus laser satellite. Using a laser to heat up hydrogen as propellant, it was possible to reach exhaust velocities of 50 km/s, but at a low thrust of about 1 kN.

By combining both systems, the Soviets had a system that was able to pull even with the VASIMR system of the Americans.

In Europe, the development of the next generation propulsion system was going in a different direction. There was no previous experience with nuclear engines and with the Drachenfels discovery they had access to a Quetzal aerospace craft.

Several investigations into the remains of the engine showed that it was most likely a fusion based propulsion system for both atmospheric and orbital travel. It also gave ESA a direction on where their research should go.

At the time there were only few ways to study an actual fusion reaction. One was the inertial confinement system of the Farnsworth–Hirsch fusor, the Z-pinch system of the American Z-machine and the Tokamak reactors like the JET or the NSTX.

The fusor was in its design simple, but the fusion reaction was too low in power to be of any use. The Tokamak was in its early stages of being useful as fusion device, yet hard to work into a fusion based propulsion system. That left the Z-Pinch fusion as the only workable solution of getting a fusion engine for the time being.

The first Z-Pinch fusion system for research into continuous Z-Pinch fusion was built at the Imperial College in London. Finished in 1997, the Z-Pinch Drive Assembly Test System still utilized conventional capacitors and Marx generators, but was able to sustain one fusion pulse per second for a minute.

When superconductors, and with them superconducting capacitors, became available, the ZPDATS was rebuilt to make use of the new materials and by 2002, it was able to sustain two pulses per second for several minutes, using only Deuterium as fuel, the maximum of how the system could be used on Earth.

The research allowed ESA to project a Z-Pinch fusion engine that could sustain ten pulses for several days on end. With an exhaust velocity of nearly 190 km/s, it was more effective than the VASIMR system of the United States and with each pulse having a thrust of 4 kN, a ten Hertz run could produce a thrust of about 40 kN.

Seeing the drive system as their ace in the hole, ESA did its best to keep the system a secret. The CIA and the KGB did get partial information about the engine, out of which NASA and the Academy of Sciences could only conclude that ESA would need years, if not decades to get around to utilize the system.

In China, the search went on into a different direction. Much like the Soviets, the Chinese were looking into using an enhanced version of nuclear fission to propel them into the outer solar system. And once again the United States provided them with an idea to use and improve on.

Rather than use a nuclear fission to heat up a working medium, they looked into directly using the hot nuclear fission products for thrust. Theoretically the Fission Fragment engine was able to reach exhaust velocities of fractions of lightspeed. While the low thrust could be seen as a downside, the engine needed very little fuel and as such allowed very high delta-v.

The original American design of using disks of fission material rotating through a block of just about subcritical fission material was seen as quite ineffective by the Chinese scientists and so they began searching for an alternative. By 2005, they had developed the first theoretical model into a model that used a ‘dusty plasma’ of weapons grade plutonium nanoparticles suspended within a magnetic field, inside a chamber made from a nuclear moderator, in this case a lithium hydride.

The dust underwent nuclear fission once it had reached criticality and could be contained by a strong magnetic field. The fission products were allowed to escape through the magnetic engine bell, while the dust particles remained trapped in the magnetic field of the reaction chamber.

The thrust remained low, but the exhaust velocity was still at about two percent of lightspeed with an incredibly low use of propellant.

What helped the Chinese was that they had access to a Quetzal aerospace craft, that was more intact than those of the Mexicans or the Europeans. Making use of this, they managed to identify the contra-grav system and built a first prototype system. In combination with the fission fragment rocket engine, it had the markings of allowing the Peoples Republic of China unparalleled access to the outer solar system, once they were able to build a spacecraft that was able to make use of their discoveries.

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