Megawatt electric facility converts 0.6 kilo-Mg per year of water into cryofuel. Once every comet period this facility
could convert the entire 2000 Mg of fuel returned by 4 tankers into cryofuel.

Performance and Payback Parametrics

What would we use for propulsion? Would we use water, which has about half the performance of cryofuel, to
bring back payloads from the comet? Or would we risk the complexity of a water splitting unit at the comet? Which
would bring back the most payload?

Analyses showed (Zuppero 1991 & 1992) that for missions from periodic comets, or from any object energetically
closer, steam rocket propulsion propulsion delivers 10 times more than any competitor.

A very fortunate accident of nature allows the simplest to perform better than the intuitive, more complex.
Cryofuel has higher specific impulse, and should return more fuel to Earth orbits. But the systemthat considers only
how much is launched from Earth versus how much propellant resource is returned shows that the inefficiency of
propellant use is more than compensated by the amount of resource transported. The key is the nearly unlimited
availability of water.

Figure 10 shows a simple tanker designed to use water as propellant, a low mass nuclear reactor to heat the water
for the rocket engine: The tanker delivers water ice as payload. Further analysis showed that the waste heat from a
rather small (20 kW) electric generator sent for the first, exploratory mission was sufficient to melt and purify all the
propellants needed for an manned mission scenario, or about 36,000 Mg of water.

The key questions to ask are:
* How much propellant is used coming back from comet?
The analysis needs to examine the payback of the propellant delivery process.
* How does propellant use trade off against the risk and shortened nuclear reactor life associated with operating
the reactor hotter, where the rocket performs better?
A low risk approach would operate the reactor at temperatures where the reactor fuel has been successfully tested
for long life. Powell (1992) reports that just such (Silicon Carbide) fuel elements have been tested for commercial
use at 1200 Celsius (C). If there are impurities in the water we might need to operate colder.

* How much performance degradation do we suffer if we operate the reactor at 500 C or 800 C?

Near infinite life is important because the payback ratio is linear with reactor life. Running the reactor hotter
shortens its life. We want to use the tanker rocket engine about ten to thirty times, so we want long life. Therefore
we need to know at what temperature to operate.

Delivery Process

Suppose we can use the thermal rejected heat from the first , exploratory extraction mission to a comet to produce
water propellant. How much of this propellant could we return to Earth? How much would the tanker use to deliver
this propellant?

The answer depends on how much performance we can pack into the tanker rocket engine. This performance is a
function of how hot we can run the reactor with water in it, and of how many megawatts we can pack into a given
mass of nuclear rocket reactor .The colder the rocket runs, the poorer the delivered payload at Earth. And fewer
megawatts packed into the rocket engine result in gravity losses, which also reduces the payload returned to Earth
orbit HEEO.

According to TABLE 1 and FIGURE 11, a tanker with a 800 MW thermal engine operated at 800 C uses 4581
Mg of propellant to return 500 Mg to HEEO. Notice that the higher power rocket engine results in less gravity
losses. The braking maneuver at HEEO to cause the capture requires a high power thrust. A higher power engine
produces higher braking thrust. This results in lower gravity losses. An extractor would need to produce this 4581
Mg propellant and 500 Mg payload water, or 5081 Mg s. How long would this take?
A realistic extractor rejecting ("wasting") 1 MW thermal and run for 5400 hours, as in ISU (1990), would melt
and purify8.8 kilo-Mg of 1 C water per year. This assumes that the entire 2.2 Megajoules per kilogram is not
recovered on condensing the 1 C steam back to 1 C water. This is a very conservative assumption, but saves
launched mass at the expense of nuclear generated, low temperature heat. Later, this 1 Megawatt is shown to be the
design point for the first system sent to the comet.

The extractor would take about 1.73 years to produce this. Running for 4.2 years, the period of the comet, the
extractor would produce 36.96 Kilo- Mg s. If each of 4 tankers takes 5081 Mg s, this is enough for7 tankers. This
would result in 7 x 500 Mg s, or 3500 Mg at HEEO.