Sagar Final Report

8/3/2019 Sagar Final Report

1/24

NERVA

MMCOE Page 1

CHAPTER 1

1.1 PRINCIPLES OF ROCKET FLIGHT

A rocket in its simplest form is a chamber enclosing a gas under

pressure. A small opening at one end of the chamber allows the gas to

escape, and in doing so provides a thrust that propels in the opposite

direction. A good example of this is a balloon. Air inside a balloon is

compressed by the balloon's rubber walls. The air pushes back so that the

inward and outward pressing forces are balanced. When the nozzle is

released, air escapes through it and the balloon is propelled in the

opposite direction.

When we think of rockets, we rarely think of balloons. Instead, our

attention is drawn to the giant vehicles that carry satellites into orbit and

spacecraft to the Moon and planets. Nevertheless, there is a strong

similarity between the two. The only significant difference is the way the

pressurized gas is produced. With space rockets, the gas is produced by

burning propellants that can be solid or liquid in form or a combination of

the two.

One of the interesting facts about the historical development of

rockets is that while rockets and rocket-powered devices have been in use

for more than two thousand years, it has been only in the last three

hundred years that experimenters have had a scientific basis for

understanding how they work.

The science of rocketry began with the publishing of a book in 1687

by the great English scientist Sir Isaac Newton. His book, entitled

Philosophiae Naturalis Principia Mathematica, described physical principles

in nature. Today, Newton's work is usually just called the Principia. In the

Principia, Newton stated three important scientific principles that govern

the motion of all objects, whether on Earth or in space. Knowing these
http://inventors.about.com/library/inventors/blrocket.htmhttp://inventors.about.com/library/inventors/blrocket.htmhttp://inventors.about.com/library/inventors/blnewton.htmhttp://inventors.about.com/library/inventors/blnewton.htmhttp://inventors.about.com/library/inventors/blnewton.htmhttp://inventors.about.com/library/inventors/blrocket.htm


2/24

NERVA

MMCOE Page 2

principles, now called Newton's Laws of Motion, rocketeers have been able

to construct the modern giant rockets of the 20th century such as the

Saturn V and the Space Shuttle. Here now, in simple form, are Newton's

Laws of Motion.

1. Objects at rest will stay at rest and objects in motion will stay inmotion in a straight line unless acted upon by an unbalanced force.

2.Force is equal to mass times acceleration.

3.For every action there is always an opposite and equal reaction.


3/24

NERVA

MMCOE Page 3

CHAPTER 2

2.1 Space Settlement relevance

Even though modern chemical rockets have sent us to the Moon, to build large

space settlements we will need something more. Today, the high cost of

launching into space limits the number of people who go there. Advanced

propulsion, however, offers a chance to dramatically lower launch costs.

Futhermore, many of the ideas being proposed would allow us to move around

the solar system much quicker than we do today. Instead of months to Mars, itwould be weeks. Years to Jupiter and Saturn would be months. And centuries to

the stars would become years.

2.2 Nuclear Fission Rockets

Nuclear rockets have, in one way or another, been studied for the past fifty

years. Some of the earliest ideas for nuclear rockets came from the nuclear

physicists struggling to build the bomb at Los Alamos. These desgins evolved

into what are called solid core nuclear rockets. Solid core nuclear rockets work

by taking liquid hydrogen and pumping it to the reactor through a jacket

surrounding the rocket engine. This pumping process helps cool the rocket, and

it also preheats the liquid hydrogen. Hundreds of narrow channels pass through

the nuclear reactor. As the liquid hydrogen flows through these channels, heat

from the reactor changes the fuel into rapidly expanding gas. The gas flowsthrough the exhaust nozzle at speeds up to 35,400 kilometers per hour. A

diagramof a solid core rocket. Several tests were carried out during the 1960s

with nuclear rockets. The most well known of these are the Nuclear Engine for

Rocket Vehicle Applications (NERVA) tests. NERVA involved the Los

Alamos laboratory, Westinghouse, Aerojet, and other industrial partners.

Together, they were able to build and test a solid core rocket with a thrust of250,000 pounds, and a specific impulse (Isp) of 850 seconds, nearly twice the
http://settlement.arc.nasa.gov/teacher/lessons/contributed/thomas/Adv.prop/scntr.gifhttp://settlement.arc.nasa.gov/teacher/lessons/contributed/thomas/Adv.prop/scntr.gifhttp://settlement.arc.nasa.gov/teacher/lessons/contributed/thomas/Adv.prop/scntr.gif


4/24

NERVA

MMCOE Page 4

best Isp of chemical rockets (specific impulse is a "miles per gallon" for

rockets). Unfortunately, the public's fear of anything nuclear led to the

cancellation of NERVA in 1971. The field of nuclear rocketry would lie nearly

dormant for the next 10 years.

In 1982 there was an upsurge of support for nuclear rockets. The old NERVA

designs were dusted off just in time for Ronald Reagan's Strategic Defense

Initiative (SDI, or "Star Wars"). This time though, a new design emerged: the

gas core nuclear rocket. Gas core rockets use a gaseous nuclear fuel instead of

the solid graphite core used in NERVA. A gaseous fuel could attain tempertures

of several tens of thousands of degrees which would provide an Isp of 3000 to

5000 seconds. A gas core engine would allow manned missions to Mars to be

accomplished in half the time that is being considered now. Originally, nuclear

rockets were going to move and power large SDI space-based lasers. When it

was decided that conventional means would work just as well, nuclear rockets

were largely dropped from SDI. Only the small Timberwolf Project continued

looking into their uses.

Today, research into nuclear rockets proceeds at a slow but steady pace. Most of

the work is done at Los Alamos, where nuclear rockets originated. Most of the

problems that the scientists are trying to solve now are how to build a nozzle

and combustion chamber that can withstand the high temperatures of a gas core

rocket, and how to contain the uranium plasma that forms. If viable nuclearrockets are ever developed, they would have the possibility of opening the solar

system to manned space flight. A trip to Mars would easily be within our grasp.

We will have to wait and see.


5/24

NERVA

MMCOE Page 5

CHAPTER 3

3.1 PROPULSION SYSTEM OF FUTURE

During more than 40 years of spaceflight, a lot of things have changed. Today'sSpace Shuttle is a luxury ship compared to the Mercury capsules that carried thefirst American astronauts into space. Forty years ago, a lot of people might havehad a hard time believing that Americans and Russians would be living togetherin space on one Space Station. Space probes have visited every planet except

Pluto, and a mission there is currently being planned.

One thing that has changed very little, however, is the way rockets work. Whiledifferent fuels have been used, and current rocket engines are more high-techthan their early predecessors, the basic concepts involved are basically thesame. But, NASA researchers are currently working on a way to change that, aswell.

What should a future spacecraft engine be able to do? Certainly, one major goalwould be for it to allow spacecraft to travel through the solar system more

quickly than they can now. While a lot of things have changed in over 40 years,today's spacecraft are still traveling at about the same speed that John Glenn didwhen he became the first American to orbit the Earth in 1962. One possible wayto change that would be the VariableSpecific Impulse Magnetoplasma Rocket(VASIMR). Not only would VASIMR allowfor faster space travel, it would have somepretty incredible side benefits, as well. Forexample, NASA researchers believe that

VASIMR would be able to travel to Marsmuch more quickly than a contemporary


6/24

NERVA

MMCOE Page 6

chemical-powered rocket, and then, once there, to refuel on Mars for the returnflight to Earth. The VASIMR engine could also even help protect astronautsfrom the dangerous effects of radiation during their trip. In the less-distantfuture, VASIMR could even help keep the International Space Station (ISS) in

orbit without requiring extra fuel to be brought up from Earth.

VASIMR is a plasma-based propulsion system. An electric power source isused to ionize fuel into plasma. Electric fields heat and accelerate the plasmawhile the magnetic fields direct the plasma in the proper direction as it is ejectedfrom the engine, creating thrust for the spacecraft. The engine can even vary theamount of thrust generated, allowing it to increase or decrease its acceleration.It even features an "afterburner" mode that sacrifices fuel efficiency foradditional speed. Possible fuels for the VASIMR engine could includehydrogen, helium, and deuterium.

The use of hydrogen as the fuel for the VASIMR project has many sidebenefits, according to researcher Franklin Chang-Diaz. In addition to being thedirector of the Advanced Space Propulsion Laboratory, Chang-Diaz is anastronaut who has flown into space on seven missions, more than any otherNASA astronaut. "We're likely to find hydrogen pretty much anywhere we go inthe solar system," he said. What this means is that a VASIMR-poweredspacecraft could be launched with only enough fuel to get to its destination,such as Mars, and then pick up more hydrogen upon arrival to serve as fuel for

the return trip home. Another benefit of hydrogen fuel is that hydrogen is thebest known radiation shield, so the fuel for the VASIMR engine could also be

used to protect the crew from harmful effects ofradiation exposure during the flight.

Electrical power sources for the VASIMRengine could include such things as a nuclearpower system or solar panels. For long-rangeflights, Chang-Diaz said, the best option is

nuclear power. "Nuclear power is definitely amust if we're going to go to Mars," he said.This means that VASIMR could be integrated

with NASA's recently announced Project Prometheus proposal to developnuclear power generators for spaceflight.

While solar power would not be practical for flying farther out into the solarsystem, it could have applications closer to home. After VASIMR completessome additional earthbound testing, its designers hope for it to be tested in orbitonboard the International Space Station. A prototype engine would be mountedon the ISS's truss to demonstrate how it works. The prototype would draw its


7/24

NERVA

MMCOE Page 7

electrical power from the Station's solar panels. If the test were successful, theVASIMR prototype could be moved to a different part of the Station and usedto help keep it in orbit. While ISS orbits the Earth, atmospheric drag graduallypulls it closer to the Earth. Every so often, the Station has to be boosted back

into a higher orbit. This is done by a variety of means, but all of them requirefuel launched into orbit from Earth. The VASIMR engine, however, couldchange that, since it would use hydrogen, which is already generated as a wasteproduct on the Station. By using waste hydrogen and electricity from theStation's solar panels, VASIMR could maintain the ISS's orbit without requiringany additional fuel. "This is very exciting for us, because NASA would be ableto solve a very serious problem," he said.

Although VASIMR is still years away from being used in space, Chang-Diazsaid that it has already shown great promise during tests on Earth. So, it isentirely possible that the engine that will carry the first person to Mars isalready running in a laboratory on Earth.


8/24

NERVA

MMCOE Page 8

CHAPTER 4

4.1 Introduction to Chemical Propulsion

Each of these propellants have their own properties, such as the temperature

that they burn at and how much thrust they produce. Chemical propulsion

systems have been the mainstay in the world's space programs thus far. Due to

the large amount of propellant that these systems require, many new

technologies have been proposed and are currently being tested. A nuclear

thermal rocket engine uses approximately 50 percent less mass of propellant

than the theoretically best chemical engine [this upper boundary (based on

thrust produced / Chemical Propulsion is an extremely broad topic, as there are

hundreds of elements and efficiency) is near a limit with the Space Shuttle main

engine.A chemical rocket is self-contained, which means that it carries both fuel

and oxidizer internally, rather than obtaining oxidizer from the outside

atmosphere as a jet engine does. Because of this, it can be used for propulsion

outside of the atmosphere .

4.2 How does it work?

Thrust to propel a rocket is based on Newton's Third Law of Motion, which

states that for every action, there is an equal and opposite reaction. Inside the

combustion chamber, the fuel and oxidizer are mixed and burned. In the case of

the Space Shuttle, the main engines burn hydrogen and oxygen from the

external tank. The resulting expansion of burning gases creates pressure inside

the chamber. All forces inside the chamber other than directly up and down are

equalized by their opposite and equal force, preventing the rocket from moving

left or right. The upward thrust created by the burning gases cannot be equalized

because of the open nozzle at the end of the rocket. This forces the rocket

upward. The amount of thrust produced by the rocket engine depends on the

mass and velocity of the burning gases when they leave the combustion


9/24

NERVA

MMCOE Page 9

chamber [4]. The combustion chamber has a converging section to the

narrowest portion, which is called the throat. At this point, the passage begins to

grow wider. The nozzle diameter can be four or five times that of the

combustion chamber. In liquid-propelled rockets, regenerative cooling is often

used to help in heat transfer at the throat, since this is where the greatest amount

of heat is generated.

4.3 Basic Types of Chemical Propulsion Systems

Solid-Propellant Rockets:

Solid-propellant rockets consist of the payload (if there is one), and the rocketengine. The propellant charge is stored and burned in the motor [4].

Liquid-Propellant Rockets:

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 aninert 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 [4].

Hybrids:

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 .


10/24

NERVA

MMCOE Page 10

4.5Propellants

Early solid-propellant rockets used a mixture of 60 percent saltpeter, 15 percent

sulfur, and 25 percent charcoal for combustion. These are the same ingredients

that are used in black gunpowder, but in different proportions, which allows the

rocket propellant to burn slower. In November 1918, Robert Goddard improved

the design by using smokeless powder instead of black powder. Modern solid

propellants are synthetic rubbers with an oxidizer mixed in during

manufacturing [1]. They are good fuels, and can be handled more safely. The

addition of powdered metals such as aluminum can make this synthetic rubber

fuelpowerful .

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 burning

gases.


11/24

NERVA

MMCOE Page 11

4.5 Limitation of Chemical Rocket Engines

In chemical rocket engines1,2, such as the Space Shuttle Main Engine (SSME),

the chemical reaction between the hydrogen and oxygen releases heat whichraises the combustion gases (steam and excess hydrogen gas) up to high

temperatures (3000-4000 K). These hot gases are then accelerated through a

thermodynamic nozzle, which converts thermal energy into kinetic energy, and

hence provides thrust. The propellant and the heat source are one in the same.

Because there is a limited energy release in chemical reactions and because a

thermodynamicnozzle is being used to accelerate the combustion gases that do

not have the minimum possible molecular weight, there is a limit on the exhaust

velocity that can be achieved. The maximum Isp that can be achieved with

chemical engines is in the range of 400 to 500 s. So, for example, if we have an

Isp of 450 s, and a mission delta-V of 10 km/s (typical for launching into low

earth orbit (LEO)), then the mass ratio will be 9.63. The problem here is that

most of the vehicle mass is propellant, and due to limitations of the strength of

materials, it may be impossible to build such a vehicle to just to ascend into

orbit. Early rocket scientists got around this problem by building a rocket in

stages, throwing away the structural mass of the lower stages once the

propellant was consumed. This effectively allowed higher mass ratios to be

achieved, and hence a space mission could be achieved with low-Isp engines.

This is what all rockets do today, even the Space Shuttle. In spite of the

relatively low Isp, chemical engines do have a relatively high thrust-to-weight

ratio (T/W)2. A high T/W (50-75) is necessary for a rocket vehicle to overcome

the force of gravity on Earth and accelerate into space. The thrust of the rocket

engines must compensate for the weight of the rocket engines, the propellant,

the structural mass, and the payload. Although it is not always necessary, a high

T/W engine will allow orbital and interplanetary space vehicles to accelerate

quickly and reach there destinations in shorter time periods.


12/24

NERVA

MMCOE Page 12

CHAPTER 5

5.1 Introduction to Nuclear Propulsion

Nuclear thermal 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 interplanetary

operations.

5.2 History of Nuclear Thermal 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 operational

engine [5].


13/24

NERVA

MMCOE Page 13

5.3 Nuclear Thermal Propulsion

Since nobody has fully developed a nuclear thermal engine, information on

design is subject to change. This section will describe the theory of operation, aswell as some of the potential designs for the future.

In nuclear thermal propulsion a nuclear reactor heats a coolant to extremely

high temperatures and expels it out a nozzle, similar to those used in chemical

propulsion. In nuclear thermal 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.

Whe uranium atom is split, creating tremendous amounts of energy.

5.4Nuclear Thermal Propulsion vs. Chemical Propulsion

The final question is which type of propulsion to use. The advantages nuclear

propulsion offers

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

thermalpropulsion.

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.


14/24

NERVA

MMCOE Page 14

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 be

developed.

Development must be completed

Despite being near operational status in the 1970s, no nuclear engines have

ever been used. Therefore, before an engine can be used on a manned

Mars mission, the engine must complete development.

The good news for nuclear thermal propulsion is that the radiation problem has

already been solved, as described earlier. 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 thistechnology would be needed and preferred over conventional chemicalpropulsion in a manned mission to Mars. The lower weight, lower overall cost,lower fuel consumption, and greatly higher thrust production help make nuclearpropulsion a unanimous choice for interplanetary travel.


15/24

NERVA

MMCOE Page 15

CHAPTER 6

6.1 Nuclear fuel

nuclear energy. Nuclear fuels are the most dense sources of energy available.

Nuclear fuel in a nuclear fuel cycle can refer to the fuel itself, or to physical

objects (for example bundles composed of fuel rods) composed of the fuel

material, mixed with structural, neutron moderating, or neutron reflecting

materials. Nuclear fuel is a material that can be 'consumed' by fission or fusion

to derive

Most nuclear fuels contain heavy metal fissile elements that can be made to

undergo a nuclear fission chain reaction in a nuclear reactor. The most common

fissile nuclear fuels are Uranium 235 (235U) and Plutonium 239 (239Pu). The

actions of mining, refining, purifying, using, and ultimately disposing of these

elements together make up the nuclear fuel cycle.

Not all nuclear fuels are used in fission reactors. Plutonium-238 and some other

elements are used to produce small amounts of nuclear power by radioactive

decay in radioisotope thermoelectric generators and other atomic batteries.

Light nuclides such as 3H (tritium) are used as fuel for nuclear fusion.
http://en.wikipedia.org/wiki/Nuclear_powerhttp://en.wikipedia.org/wiki/Energy_densityhttp://en.wikipedia.org/wiki/Nuclear_fuel_cyclehttp://en.wikipedia.org/wiki/Neutron_moderatorhttp://en.wikipedia.org/wiki/Fissilehttp://en.wikipedia.org/wiki/Nuclear_fissionhttp://en.wikipedia.org/wiki/Chain_reactionhttp://en.wikipedia.org/wiki/Nuclear_reactorhttp://en.wikipedia.org/wiki/Uranium-235http://en.wikipedia.org/wiki/Pu-239http://en.wikipedia.org/wiki/Nuclear_reactorhttp://en.wikipedia.org/wiki/Plutonium-238http://en.wikipedia.org/wiki/Radioactive_decayhttp://en.wikipedia.org/wiki/Radioactive_decayhttp://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generatorhttp://en.wikipedia.org/wiki/Atomic_batteryhttp://en.wikipedia.org/wiki/Nuclidehttp://en.wikipedia.org/wiki/Tritiumhttp://en.wikipedia.org/wiki/Nuclear_fusionhttp://en.wikipedia.org/wiki/Nuclear_fusionhttp://en.wikipedia.org/wiki/Tritiumhttp://en.wikipedia.org/wiki/Nuclidehttp://en.wikipedia.org/wiki/Atomic_batteryhttp://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generatorhttp://en.wikipedia.org/wiki/Radioactive_decayhttp://en.wikipedia.org/wiki/Radioactive_decayhttp://en.wikipedia.org/wiki/Plutonium-238http://en.wikipedia.org/wiki/Nuclear_reactorhttp://en.wikipedia.org/wiki/Pu-239http://en.wikipedia.org/wiki/Uranium-235http://en.wikipedia.org/wiki/Nuclear_reactorhttp://en.wikipedia.org/wiki/Chain_reactionhttp://en.wikipedia.org/wiki/Nuclear_fissionhttp://en.wikipedia.org/wiki/Fissilehttp://en.wikipedia.org/wiki/Neutron_moderatorhttp://en.wikipedia.org/wiki/Nuclear_fuel_cyclehttp://en.wikipedia.org/wiki/Energy_densityhttp://en.wikipedia.org/wiki/Nuclear_power


16/24

NERVA

MMCOE Page 16

fig1.1Rocket fuel tank

fig1.2 radiation shield


17/24

NERVA

MMCOE Page 17

fig1.3rocketengine

fig1.4nuclear engine


18/24

NERVA

MMCOE Page 18

CHAPTER 7

Types of Nuclear Thermal Rockets

A nuclear thermal rocket can be categorized by the construction of its reactor,which can range from a relatively simple solid reactor up to a much more

complicated but more efficient reactor with a gas core.

7.1 Solid core

The most traditional type uses a conventional (albeit light-weight) nuclear

reactor running at high temperatures to heat the working fluid that is movingthrough the reactor core. This is known as the solid-core design, and is the

simplest design to construct.

ANERVAsolid-core design

The simplest of nuclear thermal rockets, solid core reactors are limited by the

melting point of the materials used in the reactor cores. Since the efficiency of a

rocket engine is related to the square root of the temperature of the working

fluid, the solid core design needs to be constructed of materials that remain

strong at as high a temperature as possible. Nuclear reactions can create much

higher temperatures than the temperatures the materials can withstand, meaning
http://en.wikipedia.org/wiki/NERVAhttp://en.wikipedia.org/wiki/NERVAhttp://en.wikipedia.org/wiki/NERVAhttp://en.wikipedia.org/wiki/File:NASA-NERVA-diagram.jpghttp://en.wikipedia.org/wiki/File:NASA-NERVA-diagram.jpghttp://en.wikipedia.org/wiki/File:NASA-NERVA-diagram.jpghttp://en.wikipedia.org/wiki/File:NASA-NERVA-diagram.jpghttp://en.wikipedia.org/wiki/NERVA


19/24

NERVA

MMCOE Page 19

that much of the potential of the reactor for very high temperatures is sacrificed.

Even more limiting is the cracking of fuel coatings due to the large temperature

ranges (from 22 K up to 3000 K over the length of a 1.3m fuel rod), and the

necessity of matching coefficients of expansion in all the components. Using

hydrogen propellant, a solid-core design typically delivers specific impulses

(Isp) on the order of 850 to 1000 seconds, about twice that of liquid hydrogen-

oxygen designs such as the Space Shuttle main engine. Other propellants are

sometimes proposed, such as ammonia, water or LOX. Although these

propellants would provide reduced exhaust velocity, their greater availability

can reduce payload costs by a very large factor where the missiondelta-vis not

too high, such as withincislunarspace or between Earth orbit and Martian orbit.

Above about 1500 K hydrogen begins to dissociateat low pressures, or 3000 K

at high pressures, a potential area of promise for greatly increasing the Isp of

solid core reactors.

Immediately after World War II, the weight of a complete nuclear reactor was

so great that it was feared that solid-core engines would be hard-pressed [5] to

achieve athrust-to-weight ratioof 1:1, which would be needed to overcome the

gravityof the Earth on launch. The problem was quickly overcome, however,

and over the next twenty-five years U.S. nuclear thermal rocket designs

eventually reached thrust-to-weight ratios of approximately 7:1. Still, the lower

thrust-to-weight ratio of nuclear thermal rockets versus chemical rockets (which

have thrust-to-weight ratios of 70:1) and the large tanks necessary for liquid

hydrogen storage mean that solid-core engines are best used in upper stages

where vehicle velocity is already near orbital, in space "tugs" used to take

payloads between gravity wells, or in launches from a lower gravity planet,

moon or minor planet where the required thrust is lower. To be a useful Earth

launch engine, the system would have to be either much lighter, or provide even
http://en.wikipedia.org/wiki/Specific_impulsehttp://en.wikipedia.org/wiki/Specific_impulsehttp://en.wikipedia.org/wiki/Liquid_hydrogenhttp://en.wikipedia.org/wiki/Liquid_oxygenhttp://en.wikipedia.org/wiki/Liquid_oxygenhttp://en.wikipedia.org/wiki/Space_Shuttle_main_enginehttp://en.wikipedia.org/wiki/Space_Shuttle_main_enginehttp://en.wikipedia.org/wiki/LOXhttp://en.wikipedia.org/wiki/LOXhttp://en.wikipedia.org/wiki/Delta-vhttp://en.wikipedia.org/wiki/Delta-vhttp://en.wikipedia.org/wiki/Delta-vhttp://en.wikipedia.org/wiki/Cislunarhttp://en.wikipedia.org/wiki/Cislunarhttp://en.wikipedia.org/wiki/Cislunarhttp://en.wikipedia.org/wiki/Dissociatehttp://en.wikipedia.org/wiki/Dissociatehttp://en.wikipedia.org/wiki/Nuclear_thermal_rocket#cite_note-alvarez-4http://en.wikipedia.org/wiki/Nuclear_thermal_rocket#cite_note-alvarez-4http://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wikipedia.org/wiki/Gravityhttp://en.wikipedia.org/wiki/Gravityhttp://en.wikipedia.org/wiki/Gravity_wellhttp://en.wikipedia.org/wiki/Gravity_wellhttp://en.wikipedia.org/wiki/Gravity_wellhttp://en.wikipedia.org/wiki/Gravityhttp://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wikipedia.org/wiki/Nuclear_thermal_rocket#cite_note-alvarez-4http://en.wikipedia.org/wiki/Dissociatehttp://en.wikipedia.org/wiki/Cislunarhttp://en.wikipedia.org/wiki/Delta-vhttp://en.wikipedia.org/wiki/LOXhttp://en.wikipedia.org/wiki/Space_Shuttle_main_enginehttp://en.wikipedia.org/wiki/Liquid_oxygenhttp://en.wikipedia.org/wiki/Liquid_hydrogenhttp://en.wikipedia.org/wiki/Specific_impulse


20/24

NERVA

MMCOE Page 20

higher specific impulse. The true strength of nuclear rockets currently lies in

solar system exploration, outside Earth's gravity well.

7.2 Liquid core

Dramatically greater improvements are theoretically possible by mixing the

nuclear fuel into the working fluid, and allowing the reaction to take place in the

liquid mixture itself. This is the basis of the so-called liquid-core engine, which

can operate at higher temperatures beyond the melting point of the nuclear fuel.

In this case the maximum temperature is whatever the container wall (typically

a neutron reflector of some sort) can withstand, while actively cooled by the

hydrogen. It is expected that the liquid-core design can deliver performance on

the order of 1300 to 1500 seconds (12.814.8 kNs/kg).

These engines are currently considered very difficult to build. The reaction time

of the nuclear fuel is much higher than the heating time of the working fluid,

requiring a method to trap the fuel inside the engine while allowing the working

fluid to easily exit through the nozzle. Most liquid-phase engines have focused

on rotating the fuel/fluid mixture at very high speeds, forcing the fuel to the

outside due to centrifugal force (uranium is heavier than hydrogen). In many

ways the design mirrors the particle-bed design, although operating at even

higher temperatures.
http://en.wikipedia.org/wiki/Neutron_reflectorhttp://en.wikipedia.org/wiki/Neutron_reflectorhttp://en.wikipedia.org/wiki/Neutron_reflector


21/24

NERVA

MMCOE Page 21

7.3Gas core

Nuclear gas core closed cycle rocket engine diagram, nuclear "light bulb"

Nuclear gas core open cycle rocket engine diagram

The final classification is the gas-core engine. This is a modification to the

liquid-core design which uses rapid circulation of the fluid to create a toroidal

pocket of gaseous uranium fuel in the middle of the reactor, surrounded by

hydrogen. In this case the fuel does not touch the reactor wall at all, so

temperatures could reach several tens of thousands of degrees, which would

allow specific impulses of 3000 to 5000 seconds (30 to 50 kNs/kg). In this

basic design, the "open cycle", the losses of nuclear fuel would be difficult to

control, which has led to studies of the "closed cycle" or nuclear lightbulb

engine, where the gaseous nuclear fuel is contained in a super-high-temperature

quartz container, over which the hydrogen flows. The closed cycle engine

actually has much more in common with the solid-core design, but this time

limited by the critical temperature of quartz instead of the fuel stack. Although

less efficient than the open-cycle design, the closed-cycle design is expected to
http://en.wikipedia.org/wiki/Gas_core_reactor_rockethttp://en.wikipedia.org/wiki/Gas_core_reactor_rockethttp://en.wikipedia.org/wiki/Toroid_(geometry)http://en.wikipedia.org/wiki/Toroid_(geometry)http://en.wikipedia.org/wiki/Nuclear_lightbulbhttp://en.wikipedia.org/wiki/Nuclear_lightbulbhttp://en.wikipedia.org/wiki/Quartzhttp://en.wikipedia.org/wiki/Quartzhttp://en.wikipedia.org/wiki/File:Gas_Core_light_bulb.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_open_cycle.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_open_cycle.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_light_bulb.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_light_bulb.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_open_cycle.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_open_cycle.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_light_bulb.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_light_bulb.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_open_cycle.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_open_cycle.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_light_bulb.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_light_bulb.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_open_cycle.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_open_cycle.pnghttp://en.wikipedia.org/wiki/File:Gas_Core_light_bulb.pnghttp://en.wikipedia.org/wiki/Quartzhttp://en.wikipedia.org/wiki/Nuclear_lightbulbhttp://en.wikipedia.org/wiki/Toroid_(geometry)http://en.wikipedia.org/wiki/Gas_core_reactor_rocket


22/24

NERVA

MMCOE Page 22

deliver a rather respectable specific impulse of about 15002000 seconds (15

20 kNs/kg)


23/24

NERVA

MMCOE Page 23

CHAPTER 8

Conclusions

Nuclear thermal propulsion is the best nearterm 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, and restarting this program

will cost $3 - $5 billion. 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.


24/24

NERVA

CHAPTER 9

Referencess

1. Dewar, James and Bussard, Robert, "The Nuclear Rocket: Making OurPlanet Green, Peaceful and Prosperous", Apogee Books, Burlington,

Ontario, Canada, 2009

2. Dewar, James. "To The End Of The Solar System: The Story Of TheNuclear Rocket", Apogee, 2003

3. ^Wade, Mark."RD-0410".Encyclopedia Astronautica.http://www.astronautix.com/engines/rd0410.htm. Retrieved 2009-09-25.

4. "Konstruktorskoe Buro Khimavtomatiky - Scientific-ResearchComplex / RD0410. Nuclear Rocket Engine. Advanced launch vehicles".

KBKhA -Chemical Automatics Design Bureau.

http://www.kbkha.ru/?p=8&cat=11&prod=66. Retrieved 2009-09-25.

5. ^Alvarez, Luis, "There Is No Obvious Or Simple Way To Use AtomicEnergy For Space Ships",U.S. Air Services, January 1947, pp. 9-12
http://j/project/Nuclear_thermal_rocket%20referancwe.htm%23cite_ref-astronautix1_2-0http://j/project/Nuclear_thermal_rocket%20referancwe.htm%23cite_ref-astronautix1_2-0http://www.astronautix.com/engines/rd0410.htmhttp://www.astronautix.com/engines/rd0410.htmhttp://www.astronautix.com/engines/rd0410.htmhttp://en.wikipedia.org/wiki/Encyclopedia_Astronauticahttp://en.wikipedia.org/wiki/Encyclopedia_Astronauticahttp://en.wikipedia.org/wiki/Encyclopedia_Astronauticahttp://www.astronautix.com/engines/rd0410.htmhttp://www.astronautix.com/engines/rd0410.htmhttp://www.kbkha.ru/?p=8&cat=11&prod=66http://www.kbkha.ru/?p=8&cat=11&prod=66http://www.kbkha.ru/?p=8&cat=11&prod=66http://www.kbkha.ru/?p=8&cat=11&prod=66http://www.kbkha.ru/?p=8&cat=11&prod=66http://en.wikipedia.org/wiki/Chemical_Automatics_Design_Bureauhttp://en.wikipedia.org/wiki/Chemical_Automatics_Design_Bureauhttp://en.wikipedia.org/wiki/Chemical_Automatics_Design_Bureauhttp://www.kbkha.ru/?p=8&cat=11&prod=66http://www.kbkha.ru/?p=8&cat=11&prod=66http://j/project/Nuclear_thermal_rocket%20referancwe.htm%23cite_ref-alvarez_4-0http://j/project/Nuclear_thermal_rocket%20referancwe.htm%23cite_ref-alvarez_4-0http://en.wikipedia.org/wiki/U.S._Air_Serviceshttp://en.wikipedia.org/wiki/U.S._Air_Serviceshttp://en.wikipedia.org/wiki/U.S._Air_Serviceshttp://en.wikipedia.org/wiki/U.S._Air_Serviceshttp://j/project/Nuclear_thermal_rocket%20referancwe.htm%23cite_ref-alvarez_4-0http://www.kbkha.ru/?p=8&cat=11&prod=66http://en.wikipedia.org/wiki/Chemical_Automatics_Design_Bureauhttp://www.kbkha.ru/?p=8&cat=11&prod=66http://www.kbkha.ru/?p=8&cat=11&prod=66http://www.astronautix.com/engines/rd0410.htmhttp://en.wikipedia.org/wiki/Encyclopedia_Astronauticahttp://www.astronautix.com/engines/rd0410.htmhttp://j/project/Nuclear_thermal_rocket%20referancwe.htm%23cite_ref-astronautix1_2-0

Documents

Sagar Final Report