4TH EUROPEAN CONFERENCE FOR AEROSPACE SCIENCES (EUCASS) THE NTER: A PROPOSED INNOVATIVE PROPULSION CONCEPT FOR MANNED INTERPLANETARY MISSIONS C. Dujarric European Space Agency, HQ, 8-10 rue Mario Nikis, 75738 Paris Cedex 15, FRANCE A. Santovincenzo, L. Summerer European Space Agency, ESTEC, Noordwijck, Netherlands Abstract proposed at an IAF conference in 1999.[1][7] Following a slow maturation process, a new concept was defined which Conventional propulsion technology was submitted to a concurrent design (chemical and electric) currently limits the exercise in ESTEC in 2007. The pre- possibilities for human space exploration to definition work made clear that, based on the neighbourhood of the Earth. If farther conservative technology assumptions, a destinations (such as Mars) are to be specific impulse of 920s could be obtained reached with humans on board, a more with a thrust of 110kN. Despite the heavy capable interplanetary transfer engine engine dry mass, a preliminary mission featuring high thrust, high specific impulse analysis using conservative assumptions is required. showed that the concept was reducing the required Initial Mass in Low Earth Orbit The source of energy, which could in compared to conventional nuclear thermal principle best meet these engine rockets for a human mission to Mars. A requirements is nuclear thermal. However patent was filed on the concept. Because of the nuclear thermal rocket technology is not the operating parameters of the nuclear core, yet ready for flight application. The which are very specific to this type of development of new materials, which is concept, it seems possible to test on ground necessary for the nuclear core will require this kind of engine at full-scale in close loop further testing on ground of full-scale using a reasonable size test facility with safe nuclear rocket engines. Such testing is a and clean conditions. Such tests can be powerful inhibitor to the nuclear rocket conducted within fully confined enclosure, development, as the risks of nuclear which would substantially increase the contamination of the environment cannot be associated inherent nuclear safety levels. entirely avoided with current concepts. This breakthrough removes a showstopper for nuclear rocket engines development. Alongside already further matured activities in the field of space nuclear power sources The present paper will disclose the NTER for generating on-board power, a low level (Nuclear Thermal Electric Rocket) engine investigation on nuclear propulsion has been concept, will present some of the results of running since long within ESA, and the ESTEC concurrent engineering exercise, innovative concepts have already been and will explain the concept for the NTER Copyright © 2011 by C. Dujarric, A. Santovincenzo and L. Summerer. Published by the EUCASS association with permission. Symposium on Propulsion Physics, Session on Nuclear Propulsion for Exploration on-ground testing facility. Regulations and and without effective means to influence safety issues related to the development and decisively their fate, especially when facing implementation of the NTER concept will non-nominal situations. It is mandatory to be addressed as well. give them a propulsion mean on which they can rely to implement their decisions to Introduction restore their nominal mission or initiate a safeguard alternative and monitor the effect The present paper focuses on inter-orbital of their action within short delay. propulsion, i.e. the propulsion needed to escape earth orbit and conduct space Nuclear propulsion state of the art exploration, including return to Earth. For such missions the optimisation of the inter- Nucleo-thermal rocket propulsion was orbital propulsion is a key driver for the developed both in the U.S. and in Russia overall mission cost, as the slightest aiming to satisfy the simultaneous need for performance gains on the interplanetary high thrust and high specific impulse. transfer engine has huge impacts on the size of the required Earth departure means. The nuclear propulsion development efforts lasted in the U.S. from 1955 until 1972. The typical advantage of chemical Twenty rockets and furnaces were propulsion is to provide a high spacecraft successfully ground tested in the frame of acceleration for a low engine mass, while the KIWI and NERVA programs. The electric propulsion may be preferred for its NERVA-NRX/XE, which featured Uranium higher specific impulse whenever thrust can carbide fuel core coated with Zirconium be applied for a long time without carbide, was the ground-qualified model the detrimental effect on the payload. R&D closest to a flight model. strives to mitigate the shortcomings and pushes the limits of each type of propulsion. However even taking into account the anticipated progresses of these technologies, a part of the long-term space exploration needs may lie beyond the physical limits of those conventional propulsion means. Human exploration of space beyond the Figure 1: the NERVA NTR engine Moon orbit generates mission constraints The nuclear thermal propulsion which neither chemical nor electric development efforts started also in 1955 in propulsion can satisfy. The time of the Russia but lasted until 1989. Several engines interplanetary travel must remain (RD-0140, RD-0411 and RD-410) were sufficiently short to keep the space radiation designed in CIS and their nuclear fuel effects to an acceptable level. The crew elements based on ternary carbide twisted endures not only a cumulated radiation dose ribbons were successfully tested, however proportional to the duration of their travel at not as part of an engine system. first order approximation, but also takes a proportionally cumulated risk of being hit More recent conceptual studies were by a solar flare or meteorite. Furthermore, conducted in the U.S., which are not yet on a long interplanetary ballistic trajectory, tested at engine system level. The core the crew cannot be left in a waiting mode technology investigations led to the Page 2 C. DUJARRIC, A. SANTOVINCENZO, L. SUMMERER: THE NTER: A PROPOSED INNOVATIVE PROPULSION CONCEPT FOR MANNED INTERPLANETARY MISSIONS ceramic-metallic (CERMET) fuel in a fast ESA’s initial proposals to upgrade the neutron reactor. This solution offers better nucleothermal propulsion performance resistance to hydrogen corrosion and a lighter design, but further work is still Several ideas were already investigated in needed to reduce fission gas swelling effects the 1990’s, in particular by Aerojet in the in the fuel. U.S., to take advantage of the unlimited power available from the nuclear core to At system level, bimodal engines were increase the specific impulse of the engine investigated in the U.S. and in Russia to as compared to what is obtained by a simple deliver electric energy to the spacecraft heat transfer to the hydrogen. These studies during cruise for example to keep the were stopped in view of the complexity of hydrogen in liquid state, and also to mitigate the thermal machines involved, and their the hydrogen consumption during the consequent mass. nuclear core start-up/shutdown transients. Along these lines, ESA proposed its first Some conceptual design work on nuclear ideas in 1999, which consisted in using the propulsion also started in Europe with the electric power obtainable from a bimodal French MAPS program in 1995, but this engine to heat the hydrogen plasma effort did not result in any hardware supersonic exhaust by electric induction. fabrication or testing. [1][2] This nuclear inductive concept is recalled in Figure 2: 25% parahydrogen, 75 % orthohydrogen 75 b, 700 K 100 b, 563 K Caesium 68 MW seeding 150 MW catalysised para/ortho 2000 K 2200 K 95 b conversion Pc = 34 MJ/kg 1 bar, 75 K 60 b H=404 kJ/kg 20% ortho 90 b 53K 70 b Alternator 82 MW Inductive heating O2 (shown Impedance & Electric circuitry 100 bar operating loop : 70 MW into gas frequency deep cooling 5 MW acceleration matching parameters are with no Total enthalpy 44 MJ/kg oxygen Expansion ratio 250 feed) 3 b, 725 K Theoretical performance without O2 : Isp= 930 s , Thrust = 64 kNH2 slush 7 kg/s 100% parahydrogen H= -341 kJ/kg Figure 2: the nuclear inductive concept Page 3 Symposium on Propulsion Physics, Session on Nuclear Propulsion for Exploration One particular benefit of the idea was the The NTER concept synergy and efficiency gain, which was achieved by using as a cold source for the From 2000 till 2007 the nuclear inductive Brayton cycle the incoming cryogenic concept went through a slow maturation propellant. Variants of this concept included process during which the inductive heating mixed chemical/ nuclear inductive of the supersonic exhaust plasma was propulsion to increase the thrust when proposed to be replaced by conductive necessary, to the detriment of the specific heating of the hydrogen in the subsonic area. impulse. This change removed all uncertainties and most potential inefficiencies due to the non- The nuclear inductive concept offered some well mastered plasma physical behaviour. advantages, such as a high expected specific impulse (930s) despite a moderate core A major goal of this evolution was also to temperature (hydrogen temperature at the reduce the engine dry mass; therefore the nuclear core exit of 2200K). It produced transmission of energy from the Brayton large thrust (64kN) at the highest Isp, while cycle to the hydrogen had to be simplified. offering the possibility to further multiply In this respect, an innovative device called the thrust level by a factor 3 at the expense turbo-inductor, which is the object of a of a large specific impulse decrease (480s). European patent has been described.[3] This Since the additional energy was introduced new device reshapes the whole engine in the supersonic zone of the exhaust flow architecture, whose name becomes now the where the flow is already cooling down, no NTER (Nuclear Thermal Electric Rocket) part of the engine had to sustain a engine.