Propulsion Division Progress Report for April 7, 1998
Propulsion Specialists
Jawanza Marsh and Bob Winata
Introduction
The goals for the current report were:
Propulsion Study for Odysseus I
The propulsion division was asked to consider chemical rockets for use in the Odysseus I cargo mission. An analysis was conducted to compare the mass of propellant needed for a chemical rocket to that needed for a nuclear rocket. The astrodynamics specialists report shows that about six months would be needed for Odysseus I to reach Mars. Based on the earlier research conducted in chemical rockets, a chemical rocket would not be able to reach the velocity changes needed without using an enormous amount of propellant.
Based on earlier research, chemical rockets provide smaller specific impulses than nuclear rockets. Nuclear rockets produce much more energy per unit mass of fuel than chemical rockets. The energy-producing medium is separate from the thrust-producing propellant in nuclear rockets. Also, because there is no chemical combustion needed in a nuclear rocket, a separate oxidizer is not necessary.
Below is a table comparing the amount of propellant needed for a chemical rocket and the amount needed for a nuclear rocket. Because the mass of Odysseus I isnt finalized, different values of the mass of Odysseus I as a percentage of Odysseus II were used. The data given were:
DU1 = 3.620 km/s
DU2 = 0.931 km/s
(Isp)chem = 350 s (average specific impulse given for chemical rockets)
(Ueq)chem = 3.433 km/s
(Ueq)nuclear = 9.320 km/s
The velocity increment values were obtained from the astrodynamicists last report. Ueq for the nuclear rocket was obtained from propulsion divisions third report The propellant masses were calculated using
DU = Ueq*ln(Mo/(Mo-Mp)) (1)
where Mo is the total mass of the ship and MP is the propellant mass.
|
Odysseus I mass as percentage of Odysseus II |
Total chemical propellant mass (tons) |
Total nuclear propellant mass (tons) |
|
75% |
165 |
44.1 |
|
80% |
176.8 |
46.9 |
|
85% |
188.1 |
49.9 |
|
90% |
196.3 |
52.1 |
|
95% |
209.8 |
55.7 |
|
100% |
223.1 |
58.7 |
Table 1. Comparison of propellant weights using chemical or nuclear rockets.
Table 1 shows that as the mass of Odysseus I is increased, the mass of chemical propellant increases much faster than the propellant mass of the nuclear rocket. This is because chemical rockets use propellants with higher molecular weights than the propellants of nuclear rockets. Therefore, if the mass of Odysseus I is to be kept within reasonable limits, it would better to use nuclear propulsion for Odysseus I just as Odysseus II.
Multistaging
The propulsion division was also asked to study the possibility of using multistage rockets for Odysseus II and/or Odysseus I. In a multistage rocket, each stage is separated and discarded after the fuel in that stage has been consumed. The successive discarding of each stage reduces the weight of the fuselage and increases the mass ratio of the rocket. For example, in a three-stage rocket, the mass ratio is multiplied by three if the mass ratios of the stages are the same. By increasing the number of stages, the speed of the rocket is increased. These are good advantages of multistage rockets, however it would not be a good idea to use multistage rockets for either mission if they are both to use nuclear propulsion because there are still ethical questions about leaving any type of nuclear material in space.
Goals
The goals for next time are: