THE NUCLEAR THERMAL ELECTRIC ROCKET ENGINE … Papers/THE... · hydrogen to reach the graphite and...
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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 Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 4
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)
The Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 5
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 Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 7
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 Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 8
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 Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 9
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.
The Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 14
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.
The Nuclear Thermal Electric Rocket Engine | Christian Dujarric | Tullahoma, TN| July 29th, 2010 | ESA/LAU | Pag. 15
THANK YOU FOR YOUR ATTENTION
Christian DUJARRICSenior Launcher System Engineer ESA/LAU-PA