Nuclear Propulsion vs. Chemical Propulsion
I would like to start of by tell the chemical side. After that i will state the nuclear side. in conclusion I will put the two together to find out
with is better is terms of cost, speed, weight, fuel, and degree of safety.
Chemical propulsion is a very broad topic. There is hundreds of elements and compounds that can be used as fuel. Chemical propulsion systems have been
the mainstay in the world's space programs this far. But as the chemical propulsion systems consume large amounts of fuel other technologies are
being looked at such as nuclear which used 50% less mass of propellant then the best chemical engine.
A chemical rocket is self contained unlike jet engines which used the air outside to be driven. This lets the chemical rocket go to space where their
is no atmosphere.
How deos it work?
Chemical rockets used Newtons 3rd law. For every action there is a equal and opposite reaction. So when the fuel under high pressure is push through
the rockets nossle it gets a equal reaction that thrust the rocket forward. First though the fuel is mixed with oxidizer in the combustion chamber
where the fuel burns and gets pushed out at high velicoty.
Three Basic Chemical Propulsion Systems
There are three basic chemical propulsion systems.
Solid-propellant rockets consist of the payload (if there is one), and the rocket engine. The propellant charge is stored and burned in the motor.
Liquid-propellant rockets contain two main tanks, one containing the fuel, and the other holding the oxidizing agent. In a small liquid-propelled
rocket, the fuel and the oxidizer can be pressurized and forced into the rocket engine with an inert gas. However, in larger rockets, this process
would make the tanks too heavy. Therefore, between the tanks and the rocket engine are pumps that produce the required delivery pressure. The pumps
required are driven by a gas turbine and are high-capacity to manage the large amount of propellant used.
In a hybrid rocket the fuel is solid, and the oxidizer is liquid. The liquid is carried in a pressurized container above the fuel, which burns outward
from a center hole. This system combines the advantages of solid propellant and liquid oxidizer. The solid is easier to handle and the liquid allows
for the regulation of the rate of burning.
Early solid-propellant rockets used a mixture of 60 percent saltpeter, 15 percent sulfur, and 25 percent charcoal for combustion. Modern solid
propellants are synthetic rubbers with an oxidizer mixed in during manufacturing. They are good fuels, and can be handled more safely. The addition of
powdered metals such as aluminum can make this synthetic rubber fuel more powerful.
Liquid-propelled rockets originally used gasoline as fuel, then more recently used ethyl alcohol and kerosene. Burning ethyl alcohol with liquid
oxygen was a problem because the low boiling point of alcohol creates considerable evaporation losses. From this, hypergols were discovered. A
hypergolic propellant ignites spontaneously when the fuel and oxidizer are brought together, eliminating the need for an ignition source. The most
efficient fuel is liquid hydrogen, which is used for the US Space Shuttle. This fuel source, however, is rather difficult and dangerous to handle due
to its high flammability.
Statistics vary for different propellants and rocket engine systems. The amount of thrust produced primarily depends on the mass and velocity of the
Intro to Nuclear Propulsion:
Nuclear propulsion is a broad topic to describe several new engine designs that could begin to come available in the new millennium. The most common
are the NERVA (Nuclear Engine for Rocket Vehicle Application) designs developed and tested in the 1960s. Primary focus of this section will be given
to the NRX series engine developed by this program. The reason is that this was the most developed engine and to rebuild and develop a flight ready
engine is minimal in terms of cost and time. This section will describe the history of the nuclear thermal propulsion engine, examine how it works and
comment on the current status. Nuclear electric propulsion will not be looked at in detail because it is not as powerful, and thus not very useful in
History of Nuclear Propulsion
In 1960, the NERVA program was begun to develop an engine for possible use in the Apollo program. The program lasted 11 years and was terminated in
1971. During its lifetime, the NERVA program developed two separate engines. The first was the NRX. This engine was rated at 1100 – 1500 MW of power
output with 75,000 lbs of thrust. The other engine was the Phoebus engine. This was a much more powerful engine rated at 4500 MW of power and 250,000
lbs of thrust. Although both designs were tested, the NRX was further developed. By 1971, a fully integrated engine complete with LH2 turbopumps,
valves and nozzles, was tested at simulated altitude. At the time the program was terminated, focus had begun to shift to developing a fully flight
In nuclear propulsion a nuclear reactor heats a coolant to extremely high temperatures and expels it out a nozzle.. In nuclear propulsion the nuclear
reactor takes the place of chemical energy released in the combustion of LH2 and LO2 propellants. The reactor core consists of Uranium Carbide fuel
enclosed in a graphite matrix. When active, the uranium atom is split creating tremendous amounts of energy.
Nuclear Thermal Propulsion vs. Chemical Propulsion
The final question is which type of propulsion to use. The advantages nuclear propulsion offers include:
Shorter mission time
The time for a manned mission to Mars using nuclear thermal propulsion is 200 days. This is one third the 600 days required for chemical propulsion.
This reduced time is due to the drastically increased thrust given by the nuclear thermal propulsion.
Lower operating costs
Before a Mars mission is undertaken, all of the mass for the payload, engine and most notably, fuel, must be placed into Earth orbit. Using all
chemical propellant requires 46 additional launches (compared to nuclear thermal propulsion) to get all of the fuel into Earth orbit where it can be
utilized for a Mars mission. With a cost of $.15 billion per launch, $6.8 billion is saved using nuclear.
Despite these advantages, some disadvantages must be overcome before an engine is even considered:
Radiation dose to crew.
A nuclear engine will give off tremendous amounts of radiation. Before use of these engines is even considered, an effective shielding mechanism must
Development must be completed
Although NASA has played with nuclear propulsion it is still in its early stage so the develment should be complete before nuclear propulsion can be
used on manned mission and other missions.
The good news for nuclear thermal propulsion is that the radiation problem has already been solved. A combination of shields can protect the crew,
exposing them to only a 10 REM dose. By comparison, US civilians are to never exceed a 150 REM dose and military personnel are not to exceed 500 REM.
Despite the disadvantages of nuclear propulsion, it is obvious that this technology would be needed and preferred over conventional chemical
propulsion in a manned mission to Mars. The lower weight, lower overall cost, lower fuel consumption, and greatly higher thrust production help make
nuclear propulsion a unanimous choice for interplanetary travel.
Nuclear thermal propulsion is the best near–term method for powering a Mars mission. The engine offers lower costs and quicker travel times. In the
1970s, a nuclear thermal engine had been thoroughly tested. Finally, the complex shielding, necessary for any nuclear device, has already been
designed and additional concepts are on the drawing board.
By contrast, chemical propulsion is bulky, heavy, expensive and slow. Despite using it for several decades, continued use could potentially raise
costs to unaffordable levels. While many different materials can be used as propellants, the more effective materials are usually quite expensive, and
the amount of propellant needed greatly increases this cost. This large amount of propellant needed also adds a very significant amount of weight to
the rocket, decreasing its potential payload. The current state of chemical propulsion is near a maximum, as the main engine on the Space Shuttle is
near the upper limit of the theoretically best chemical engine.
[Edited on 31-3-2004 by Russian]