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THE NUCLEAR THERMAL ELECTRIC ROCKET ENGINE (NTER)

An innovative rocket propulsion system to propel inter-planetary spacecraft e.g. for a manned mission to Mars

Presented to the AIAA STTCIn Tullahoma, TNOn July 29th, 2010By Christian DUJARRIC ESA/LAU-PA

Manned Mars mission requirements impacting the inter-orbital propulsion

Manned interplanetary spacecraft will necessarily have a large dry mass as

they are sized for a large ΔV and crew autonomy for a long duration.

A propulsion system for manned interplanetary spacecraft must feature a high

specific impulse (>900s) to contain the spacecraft initial LEO departure mass

to a reasonable amount, thereby containing the exploration mission total cost.

The inter-orbital transfer time must be minimized to minimize the crew

cumulated radiation dose during cruise, their exposure to solar flare risks,

to preserve the crew health from weightlessness effects and to protect their

mental equilibrium from the consequences of inactivity, helplessness.

To deliver its impulse in sufficient short time, the propulsion system must be

capable of a large thrust (104-105N). On top of this it must have the capability

of several re-ignitions for circularization and return, and of course its

reliability must be proven by extensive ground testing.The Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 2

The Nuclear Thermal Rocket (NTR) engine

The Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 3

Its advantages are:

Very simple in its principle

High trust, high specific impulse

Already developed up to TRL 6

The development of the Nuclear Thermal propulsion has been stopped due to:

The indefinite postponement of the missions mandatorily requiring nuclear propulsionThe need may be revived in an international frame if manned Mars exploration is recognized as a dream of humanity which deserves to be turned into reality by our present generation.

The public fear for atomThis fear is now less irrational as nuclear power plants have become part of our daily life

A number of technical snagsThese technical difficulties are analyzed hereafter

The lack of political support and funding is a consequence of the above, as a worldwide funding of this mission would make it affordable

Nuclear thermal propulsion was designed to meet the previous requirements

The first technical snag identified

It was discovered during the NERVA ground qualification tests:

The nuclear core degrades fast due to a thermal expansion mismatch of the zirconium carbide coating with the graphite core matrix, which allows hydrogen to reach the graphite and to react chemically with the core. The implications of this problem are:

a limited core lifetime as the nuclear activity of the core is progressively modified by the loss of U235 carried out with the exhaust flow

nuclear pollution of the gas exhaust which hampers ground testing

A lot of effort was devoted in the U.S. to the resolution of this problem, mainly focused on finding a better coating. Several variants were designed and tested.

Significant progresses were made, but as far as the worldwide achievements are known in Europe, the problem of the core corrosion by hydrogen is today not totally resolved.


The difficulty to test on ground :A second technical snag for NTR

–Qualification testing on ground of the NTR engine, including endurance tests, is mandatory.– Open air engine exhaust testing as was done during the NERVA programme is no longer considered possible for environmental protection reason (especially knowing the present nuclear core protection coating defect)– Storage of the exhaust gas limits the duration of the test.– Testing in close loop is practically unfeasible because the H2 exhaust at a temperature of 2750 K would have to be cooled down to 20 K before re-entering the cryogenic pump of the nuclear engine.Test preparation in the Nevada desert

(during the sixties)


ESA’s suggestions for work-around solutions

How to increase the stagnation temperature of the exhaust beyond 2900K to further increase the engine

specific impulse while at the same time reduce the maximal operating temperature of the nuclear core

to improve the engine lifetime? How to keep the whole nuclear assembly operating in a range of

temperature where the ZrC coating is known to effectively prevent the core corrosion by hydrogen? How

to test such engine on ground?

The answer to the above contradictory requirements is not obvious, but the start of a paradigm shift is

made possible by the following observation: The conventional NTR concept does not make a complete

use of all the energetic resources available on board a spacecraft equipped with NTR. Without increasing

the hydrogen mass flow rate, if the already available cold source (the cryogenic hydrogen flow) and the

available hot source (the unlimited nuclear power) are effectively put at work, a thermal machine can be

installed which produces mechanical power. This additional power may be transformed into electrical

power, which may be re-injected in the exhaust flow, thereby increasing the engine specific impulse

without any need to increase the nuclear core operating temperature. A first patent was filed by ESA in

1999 (FR 2,788,812 and US 6,971,228) describing a nuclear inductive concept.The Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 6

Several thermodynamic cycles were analysed

Impedance &frequency matching

Caesiumseeding

Alternator 82 MW100 bar

Expansion ratio 250

Inductive heating loop : 70 MW into gas

acceleration

1 bar, 75 K

2000 K95 b

Electric circuitry

deep cooling 5 MW

Theoretical performance without O2 : Isp= 930 s , Thrust = 64 kN

2200 K

H=404 kJ/kg20% ortho

H2 slush 7 kg/s100% parahydrogenH= -341 kJ/kg

100 b, 563 K

3 b, 725 K

75 b,700 K

Pc =60 b

25% parahydrogen, 75 % orthohydrogen

150 MW68 MWcatalysisedpara/orthoconversion

Total enthalpy 44 MJ/kg

90 b53K

34 MJ/kg

70 b

O2 (shown operating parameters are with no oxygen feed)

This one seems today the most reasonable starting

point with respect to design feasibility


The thermodynamic cycle retained by ESA for further investigation

The NTER concept evolution is described by an ESA patent newly filed on the following basis:

The electrical energy is generated by a Brayton cycle using Helium heated by the nuclear core,

and cooled through a countercurrent heat exchanger with the incoming cryogenic hydrogen.

The introduction of the electric energy in the hydrogen exhaust flow is done by convective heating

of electric heaters in the plenum chamber upstream the throat (despite a higher performance

potential, the induction heating of the hydrogen supersonic plasma in the nozzle divergent has not

been retained as initial baseline because its feasibility and efficiency has still to be proven)

The transformation of the mechanical energy of the Brayton cycle into electric power in the

heaters is done through an innovative device called turbo-inductor which combines several functions

into one piece of engineering in order to reduce the propulsion system mass.

Hybridization of the concept with chemical propulsion brings no benefit for inter-orbital propulsion;

it may however be considered if the same engine is also used for planetary takeoff; then oxygen

could be injected in the chamber only during the takeoff phase to increase the engine thrust.


The latest evolution of the NTER concept

Liquid H2

H2 540K

He 690 K

H23250

K

Cryogenic heat

exchanger

He 75 K

H22550

KHe 2550K

H2 1500K

He 1900K

Small space heat exchanger

He430K

The Turbo-inductor

He430K

H2 62K

excitation

The latest NTER concept features:

–The conventional NTR circuit

–The Brayton cycle to produce added

energy which increases Isp

– The Turbo-inductor to inject the

added energy into the exhaust flow,

equipped with a cooling He bleed

– Additional heat exchangers to

circumvent the nuclear core

zirconium carbide coating

degradation problem

– An optional bimodal circuit to

manage the core start/stop thermal

transients and generate electric

power during cruise

(OPTIONAL)


The turbo-inductor innovative features

Several alternatively contra-rotating turbines stages are freely rotating on

their bearings. There is no mechanical link between turbine stages nor any

mechanical shaft power output, there is no need for stator stages.

Hot hydrogen is flowing through longitudinal refractory pipes surrounding

the turbines, which are fitted with an internal tungsten coating

Induction coils are installed at the tip ring of each turbine stage, having no

magnetic core (due to high temperature, and centrifugal forces)

Foucault currents are induced in the tungsten coating (both in the axial and

ortho-radial planes). These currents heat the coating by ohmic effect. The

heat is transferred convectively to the hydrogen.

The central tube carries cold He for cooling the bearings and other devices

The exit temperature (therefore the overall engine specific impulse) is with

this design mainly limited by the melting temperature of the tungsten coating

Even higher exit temperature could be reached if a combination of this

concept with our previous direct plasma induction concept is shown possibleThe Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 10

A work around solution for the nuclear core corrosion problem

Conventional NTR design features a graphite core matrix protected from the hydrogen flow by a zirconium

carbide coating. This coating is applied by chemical vapor deposition at 1500 K. Due to the thermal

expansion coefficient mismatch with the graphite, the coating cracks after fabrication during its cooling.

During the NTR operation, the degradation is moderate at the hydrogen entrance into the core because

cold hydrogen does not spontaneously react chemically with the graphite matrix. However, above 100OK,

hydrocarbons form which get mixed with uranium particles into the exhaust flow. Above 1500K, due to

the thermal expansion of the zirconium carbide, the cracks close again, and material creeping makes the

coating gas-tight. There is no degradation of the nuclear core where the temperature exceeds 1500K.

In ESA’s concept, the core coating is not in contact with hydrogen below 1500K in normal

operation. The cryogenic hydrogen is heated successively at 62K by pumping, then at 540K by the

cryogenic heat exchanger, then slightly heated when cooling the nozzle throat, then heated successively

by a heat exchanger with Helium, and finally by a heat exchanger with the nuclear core support structure.

The last two heat exchangers are dimensioned to bring H2 up to 1500K at the entrance of the core.

During the core startup and shut down, no H2 flow occurs as the bimodal Helium circuit can control the

core temperature evolution, so that no hydrogen is in contact with the nuclear core below 1500K.The Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 11

Ground facility principle enabling an environmental-friendly testing of the NTER

Liquid H2

He 690 KHe 75 K

H22550

KHe 2550K

H2 1500K

He 1900K

He430K

He430K

H2 62K

excitation

H2 540K H2 540K

RIVER WATER

Water pump

MechanicalPower input

H2 3250K

NUCLEAR CONFINEMENT WALL

Steam exhaust

Circulation pump

Mass flow rates must be tuned equal

The engine testing,

including long duration

tests and re-ignition tests,

is performed in closed

circuit within a

confinement wall.

Cooling is performed

through a heat exchanger

with external water. This is

made possible by the fact

that H2 does not need to

be cooled below 540K in

the closed loop.

The bimodal circuit can

be simulated and tested

separatelyThe Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 12

Advantages and drawbacks of the proposed NTER concept

Advantages

– High thrust, high specific impulse engine, versatile utilization possible (it could also support ISRU if the

engine is brought down to a planetary or asteroid surface; thrust augmentation with Lox is possible for takeoff)

– It can be qualified by testing on ground in environmental friendly conditions

– Improved nuclear core lifetime, increased nuclear core thermal margins are beneficial to reliability

and crew safety. The energy addition device (turbo-inductor) is intrinsically redundant.

Drawbacks

- Engine design complexity, engine dry mass, engine development cost

Balance between advantages and drawbacks:

– This balance shall be assessed at mission level only when the engine pre-design work will have

converged on a reliable dry mass & performance estimation. It is not yet time to draw a conclusion.

– The development cost must be assessed at the overall mission level, not only at the engine level

– The most adapted propulsion system certainly depends on the mission requirements. NTER looks

like an enabler for the heaviest missions. Manned missions are among heaviest missions.The Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 13

Next development steps

The development of an interplanetary spacecraft for a manned mission to Mars seems reasonable only in the frame of a worldwide cooperation. Worldwide technical and financial capabilities will be necessary to meet this challenge.

The propulsion system for such spacecraft will most likely require the use of nuclear power. ESA offers to further investigate the NTER concept. As a first step ESA offers to share this concept with its international partners and to commonly evaluate its feasibility taking into the technology which is available worldwide. Relevant know-how already exists in European industry to design and build the thermodynamic circuit. The nuclear part builds on knowledge existing worldwide.

After an international preliminary design phase proposed at Agencies level, it will be possible to decide, given the political priorities of the various potential contributors, whether to proceed with industrial development work in Europe and abroad.


CONCLUSION

The Nuclear Thermal Electric Rocket engine proposed by ESA is

potentially a major technical enabler for a manned exploration

mission to Mars.

This propulsion concept is offered to be deeper investigated at

worldwide Agencies level in order to assess its benefits as

compared to other propulsion options.

Worldwide technologies will be needed to contribute to the

development of a manned interplanetary propulsion system, and

many of the technologies relevant to the NTER concept are readily

available in Europe.


THANK YOU FOR YOUR ATTENTION

Christian DUJARRICSenior Launcher System Engineer ESA/LAU-PA

[email protected]

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