Future of Ramjet and Scramjet Engine with options of using Nuclear Energy

Submitted By :-

Bhupendra Khandelwal Project Engineer,

Aerospace Engineering Department,Indian Institute of Technology Bombay

Powai, Mumbai, India - 76

Introduction

What is Ramjet Engine?

Ramjets are simplest air breathing engines available. They compress the air and expand it because of its shape and movement through the air.

Ramjet Engine does not require any moving parts like compressors and turbines so it is very easy in construction, any minute changes can be done very easily.

Schematic of Ramjet

Source:Mattingly

Propulsion engines work on Newton’s third law of motion.

The action component is produced either by burning of fuel or by addition of heat using nuclear energy and expanding the hot gases, which produces the reaction component, i.e., Thrust.

Principle

S

ComponentsDiffuser Fuel Injection System

Combustor & Flame Holder Nozzle

Working

Why Ramjet ?

Simpler Design: Moving parts like compressors and turbines are not required.

Lower cost makes it an attractive propulsion device.

Very High speeds can be achieved by these engines ( Upto Mach8).

Speed range of Ramjet Engine

NUCLEAR RAMJET AND SCRAMJET ENGINE

The velocity difference between the entrance and exit is increased by heat addition to the air stream flowing through its body.

Chemical reactions caused by burning fuel in the combustion chamber is standard method for heat addition in the ducted air stream. But …

Thus, I could use nuclear energy produced from nuclear reactions - fission or fusion – using a nuclear reactor.

Why Nuclear Energy ?

The basic components like diffuser, nozzle, etc. remain same. However, additional components used in a nuclear ramjet and scramjet engine will be

Reactor Core

Radiation Shield

Heat Exchanger

Components

The general concepts for this kind of propulsion device is similar to those of the traditional liquid propulsion systems. A propellant is heated in the rocket chamber, thereby raising its stagnation enthalpy, and exhausted through a converging-diverging nozzle to achieve supersonic exit flow.

The NTR replaces the chemical combustion in a liquid propulsion system with a nuclear reactor.

COMPARISON OF CHEMICAL AND NUCLEAR FUEL

• The maximum practical velocity of a Oxygen and Hydrogen engine is about 4,550 m/s, and so energy per kilogram is, by substitution, 10.4 MJ/kg.

• For comparison the energy contained in 1 kg of pure U-235 is 79.3*106 MJ

NOZZLE ASSEMBLY

The design of an appropriate nozzle for the NTR will not differ significantly from their design for use on current chemical systems. A traditional converging-diverging nozzle can be used to accelerate the flow to supersonic exit velocity.Expansion ratio in chemical engines 80 Expansion ratio for Nuclear powered engines 300, so this way thrust coefficient is much closer to theoretical maximum, and this will contribute to an increase in exhaust velocity.

REACTOR CORE SIZE

If pure U235 is available then it can become critical at a diameter of 3.47 cm. The volume of this cylinder is 264cm3 , and mass is 4.95Kg. Pure U235 is unlikely to be available, and a real reactor will use enriched uranium, nevertheless containing 50-90% U235.

Therefore, the main issues in deciding on the reactor core size are likely to be related to the availability of enriched uranium, and engineering challenges, rather then energy content.

RADIATION SHEILDThis necessitates the construction of an effective radiation

shield between the engine and the crew compartment. For neutron attenuation, light materials such as lithium-hydride can be used. For gamma-rays, however, a dense material (such as tungsten) is required. The radiation shield tends to be a major investment of mass and may not be necessary in cargo type applications. Although radiation shield are necessary between the propulsion system and the crew compartment, a more significant radiation problem is created by cosmic rays. Therefore, even a chemical system will need to shield on missions of any appreciable length.

HEAT EXCHANGER

Improved performance of the nuclear engine, in terms of exhaust velocity is solely on raising the operating temperature of the heat exchanger; there is more then adequate power available, to generate useful thrust. Heat from heat exchangers is carried away by propellant (air). Graphite has relatively high thermal conductivity so we can use graphite for manufacturing heat exchangers.

PRINCIPLE OF NTR(Nuclear Thermal Rocket) USED YET

Few tests which had been done on the above principle.

NAME TEMP. POWER COMMENT

KIWI B4E 1,980K 937MW Operated for 8 min at full power

Phoebus 2A 2,310K 5,000MW Operated for 12 min, including restart.

PEEWEE 2,550K 503MW Small test reactor. Exhaust velocity 8,450m/s

NRX A2 1,100MW Exhaust velocity 7,600m/s

NRX A6 2,342K 1,100MW 60 min of operation at full power.

ROVER and NERVA Program Results Both the ROVER and NERVA programs were technical

successes. The decision to terminate the programs in 1972 was made on the basis on public policy choices and not on the merit of the technology.

The results of these programs were to demonstrate that a nuclear thermal rocket engine could be developed to meet the objectives of structural integrity, restart capability, predictability, controllability, and reliability. At the programs end, a NERVA-1 design was produced that would have met the program objectives.

POTENTIAL APPLICATION OF NUCLEAR ENGINE

A chemical rocket, using liquid hydrogen and liquid oxygen, gives 4.55km/s of exhaust velocity, requiring 10kg of propellant per kg of pay load. A nuclear engine can give an exhaust velocity of 9km/s requiring 2.4kg of propellant per 1kg of payload. An ion engine can produce much high speeds(20km/s), but thrust developed by ion engines is very less (fraction of Newton), while nuclear engine can generate a thrust of hundreds of kilo-Newtons.

PUBLIC POLICY

An area of particular concern for NTR development is the shape of public policy. Certainly nuclear thermal propulsion has technical challenges. Public policy, however, is perhaps the greatest of the difficulties facing efforts to deploy a NTR system.

1. To protect workers and the public against "unreasonable" exposure to radiation and toxic materials.

2.To protect the Earth and local space environment against risk of "significant" alteration.

3. To protect the mission against nuclear system failure.

To respond to the most common fears of the public, NTR designs should withstand the following specific hazards:

The worst-case pressure gradient associated with the most credible scenario for detonation of the liquid and/or solid rocket propellant.

The worst-case temperature due to flame from the detonation.

The worst-case credible combination of pressure gradients, temperature, and vibration due to range safety destruct of launch vehicle during ascent.

SAFETY ISSUES WITH THE DESIGNES YET

Radioactive particles were coming out of the engine.

Public was not aware.

Proper sealing materials were not available.

Nuclear fission or fusion energy can be used for heating instead of chemical reaction in the combustion chamber.

Nuclear Energy Powered Ramjet

Figure shows a schematic of scramjet engine using nuclear energy for heat addition instead of combustion of fuel. Air enters through the diffuser at high velocity and the speed is reduced increasing the pressure at inlet to combustion chamber. The speed is still supersonic where heat is directly added to the flowing air-stream which is expanded through convergent – divergent nozzle designed for supersonic or hypersonic flight.

Working

COMPARISION OF VELOCITY

Normal combustion ramjet. Nuclear energy powered ramjet.

Operating conditions of both, chemical combustion ramjet and nuclear powered ramjet, were same in the simulation.

Dimensions of both engines were also same.In nuclear powered we have used heat exchangers for

heating the propellant.Both the engines are giving comparable velocity and

thrust.Velocity in both the engines we are getting around

1,100m/s.

The Heat exchanger used to carry out the heat addition is of a plat type heat exchanger , as this reduces the blockage in flow , hence reducing the unnecessary shocks and expansion waves in the combustion chamber.

It was also observed that the jet issuing out at the exit of the nozzle in the nuclear powered ramjet was much more circular and wider occupying the entire cross section of the diverging part of the nozzle.

Hybrid ramjet and scramjet engine with heat addition by both nuclear energy and chemical reactions.

Hybrid Ramjet & Scramjet Engine

HYBRID NUCLEAR THERMAL REACTOR

Although this paper does not specifically address this concept, the area of hybrid propulsion system should be introduced as a viable solution.

The combination of nuclear thermal reactors with chemical and/or electric propulsion systems is a major area of research.

Aircraft.Target Drones.Anti aircraft guided missiles.Space applications.

Applications

Conclusion•This report has attempted to cover the more significant design issues and concepts in the field of nuclear thermal propulsion. This technology, in whatever form it finally takes, appears necessary to meet our nation's commitment to the goals of the Space Exploration Initiative.

•It is clear from the presented material that there are significant advantages to developing NTRs. The rich history of analysis and testing of NTR concepts and designs provides a foundation upon which current design concepts are based. The future of this particular technology rests in adequate funding for the continuation of research and the commencement of construction and testing phases for the more advanced designs.

•In addition, a major effort to educate the public must be made to avoid a public policy mandate to end the NTR program without an opportunity to test and deploy. The role of safety in all phases of development must be emphasized.

•In the government and industry segments closely connected with the effort to explore space, there are a large number of professionals who believe strongly that our ability to begin serious exploration and settlement of our solar system depends on this nuclear propulsion program. From the data available, it seems clear that this sentiment is correct.