MITEE - A Miniature Nuclear Propulsion Engine
Chemical rockets have reached their limits. The hydrogen/oxygen chemical rocket can only deliver a specific impulse of about 450 seconds. The specific impulse is a measure of the thrust delivered by the rocket per unit mass of propellant consumed. Increasing the specific impulse to 1000 seconds, which is possible with Nuclear Thermal Propulsion, enables the spacecraft velocity to be over 2 times greater than that for a hydrogen/oxygen rocket, for the same mass of propellant consumed.
Sidebar 1 compares the IMLEO (Initial Mass in Low Earth Orbit) for hydrogen/oxygen rockets with those for nuclear rockets as a function of the velocity increase (Delta-V) imparted to the spacecraft, starting from its initial location in Low Earth Orbit. The IMLEO increases exponentially with Delta-V, causing the practical limit on Delta-V to be ~10 kilometer per second for hydrogen/oxygen rockets, and ~22 kilometers per second for nuclear rockets. The greater Delta-V capability of nuclear rockets allows space missions, particularly to the Outer Planets (see The MITEE Express to Jupiter and Beyond) to be carried out in much shorter time with much smaller launch vehicles. The spacecraft trajectories are direct, and do not require multiple gravity assists from flybys of Jupiter and other planets.
In addition to faster, cheaper mission, nuclear rockets enable unique new missions not possible with chemical rockets, such as sample return from Europa and Pluto, a Pluto orbiter, etc.
Plus Ultra has developed a new and unique, very small, very lightweight nuclear rocket concept, termed MITEE (MInature ReacTor EnginE). MITEE is described in detail in 3 reports: MITEE-1, the initial design; MITEE-2, are advanced design; and MITEE-3, a report on the NASA-SBIR program carried out on MITEE.
As illustrated in Sidebar 2, the MITEE nuclear engine consists of a close-packed assembly (typically 37) beryllium pressure tubes. Each pressure tube contains an outer annular cylinder of 7LiH moderator, and an inner annular rod of perforated tungsten 235UO2 metal matrix composite fuel sheets. Cold hydrogen propellant flows downwards at ~100 K along the outer surface of the 7LiH moderator, then radially inwards through the moderator and the tungsten - UO2 fuel sheets. The hydrogen propellant emerges from the final fuel sheet at 3000 K, and then flows longitudinally down through a central hot gas channel to the exit nozzle at the end of the pressure tube.
The MITEE engine is similar to the Particle Bed Reactor (PBR) engine which underwent development for defense applications (see Nuclear Thermal Propulsion), except that it is smaller and lighter, uses multiple pressure tube construction instead of a single pressure vessel, and tungsten -UO2 metal matrix fuel sheets instead of a packed bed of small HTGR type fuel particles. The main features of MITEE are summarized in Sidebar 3.
The mass of the MITEE reactor is very low, ~100 kg, and the weight of the complete engine only about 140 kg. This order of magnitude reduction in mass over previous engines like NERVA makes MITEE very attractive for planetary science missions where high V performance and very low engine mass are critically important to mission success, and to keeping mission cost within acceptable limits.
MITEE has a very strong technology base. Its tungsten -UO2 fuel was demonstrated to have >3000K lifetime capabilities in the 710 reactor program in the 1960's. Fuel region power density capability of 30 megawatts per liter was demonstrated in the PBR program, 3 times greater than the value called for in MITEE.
