roughly 30 to 90 days. Typical total trip time will be around 430 days. Often, an opposition class mission will
necessitate the transfer ship crossing inside the orbit of Venus on return in order to catch up to Earth.
With the GCNR, a third type of mission can be considered -- the point-and-shoot. This is an opposition class
mission wherein the ship transits to Mars in a few months, stays from 30 to 60 days, and returns to Earth in a few
months. Total trip time is under nine months. This type of mission requires very high delta-V burns at all four
staging points - Trans-Mars Injection (TMI), Mars Orbital Insertion(MOI), Trans-Earth Injection(TEI), and Earth
Orbital Insertion(EOI). In order to be able to execute such a mission with a reasonable mass fraction of the ship in
orbit, the propulsion system must have a specific impulse of around 2000 seconds or higher.
The delta-Vs for a fast transit mission occurring in the Year 2011 are (courtesy of Michelle Monk at NASA/JSC)
6.4, 12.3, 15.3, and 14.7 Km/s for the four burns at TMI, MOI, TEI, and EOI respectively. Thus, the total delta-V
for all four burns is near 50 km/s. If the GCNR has a specific impulse of 3000 seconds, then just under 20% of the
total ship mass in LEO will be payload and structure-- the rest will be fuel. That is to say, that it will require 4 kgs
of fuel for every kg of payload to perform the entire mission. Alternatively, for a solid core nuclear rocket to achieve
these delta-Vs and perform this mission would require over 100 kgs-fuel per kg-payload, a mass fraction in LEO of
less than 1%. A chemically propelled system cannot perform the mission.
The fact that the mass fraction is of order 20% also allows another advantage of the GCNR - radiation shielding.
Depending upon the year of the mission, the dose to the crew in free space will range between 45 cSv/yr to 120
cSv/yr for years of solar maximum and solar minimum respectively. The total dose allowed by the International
Committee on Radiation Protection is about 200 cSv for a lifetime. This lifetime limit translates roughly into a
15% chance of developing a lethal condition. Annual levels for a radiation worker recommended by the ICRP are
near 5 cSv/yr, almost a factor of 10 below the levels present in free space. Because of the performance of the
GCNR, a layer of shielding material, probably water, could be placed around the transfer module to drastically reduce
the radiation levels experienced by the crew.
During the past several months, NASA has reexamined potential Mars mission scenarios. The baseline assumptions
in their Design Reference Mission (DRM) have been: 1) a solid core nuclear rocket for TMI, 2) aerobrake capture at
Mars, 3) previously positioned cargo mission to put the return ship, which uses chemical propulsion, into Mars
orbit, and 4) aerocapture at Earth. Total mission time is 900 days away from Earth. Total mass in orbit including
the three cargo missions is 659 metric tons. The mission profile includes a 6 month transit to Mars, a 536 day stay
on the surface, and a 6 month return flight.
In addition, NASA has examined an opposition class mission that would provide a 90 day stay on the surface. This
scenario had most of the same mission components as the DRM but had higher delta-Vs, one less cargo mission,
and the shorter surface stay.
A comparison of the DRM and the 90-stay missions with the potential GCNR has been made. The "full-up" GCNR
mission is substantially different than the NASA profiles in that it includes the following: 1) propulsive burns for
all four junctures - TMI, MOI, TEI, and EOI, so that it does not require the development of high performance
aerobraking; 2) 40 to 60 day stay on the surface; 3) orbit transfers are three to four months, i.e. very high delta-Vs
are acquired; and 4) inclusion of shielding against Galactic Cosmic Rays is optional.
The results of calculations show that the DRM mission could expose the crew to more than their allowable lifetime
limit of 200 cSv. The 90-day stay reduces that exposure by half. Alternatively, the full-up GCNR mission reduces
the exposure to 61 cSv without having any shielding mass in the transfer ship. Using a 25 cm water shield around
the transfer module results in a total mission dose of 22 cSv. The IMLEO for the missions is 659 mT for the
DRM, 609 mT for the 90-day stay, 460 mT for the unshielded GCNR fast mission, and 582 mT for the shielded
GCNR mission. Thus, for slightly less mass in orbit, the gas core rocket can perform a 9 month round trip
mission, allow 3 independent landing sites to be explored, carry a crew of 6 astronauts, and protect that crew from
the radiation in space.
GCNR TECHNOLOGY STATUS
Simultaneous with the Rover/NERVA program in the 1960s, the gas core concept was also investigated " . The
erosion and the temperature limitations of the graphite fuel experienced by the solid-core nuclear rocket led several
researchers to theorize on the feasibility of having a non-solid, or gaseous core. A gaseous core would allow far
higher temperatures to be achieved and, thus, far higher performance by the rocket. Specific impulses of several