Pulsed nuclear thermal rocket

A pulsed nuclear thermal rocket is a type of nuclear thermal rocket (NTR) concept developed at the Polytechnic University of Catalonia, Spain, and presented at the 2016 AIAA/SAE/ASEE Propulsion Conference for thrust and specific impulse (Isp) amplification in a conventional nuclear thermal rocket.

[1] The pulsed nuclear thermal rocket is a bimodal rocket able to work in a stationary (at constant nominal power as in a conventional NTR), and as well as a pulsed mode as a TRIGA-like reactor, making possible the production of high power and an intensive neutron flux in short time intervals.

In contrast to nuclear reactors where velocities of the coolant are no larger than a few meters per second and thus, typical residence time is on seconds, however, in rockets chambers with subsonic velocities of the propellant around hundreds of meters per second, residence time are around

and then a long power pulse translates into an important gain in energy in comparison with the stationary mode.

The gained energy by pulsing the nuclear core can be used for thrust amplification by increasing the propellant mass flow, or using the intensive neutron flux to produce a very high specific impulse amplification – even higher than the fission-fragment rocket, wherein the pulsed rocket the final propellant temperature is only limited by the radiative cooling after the pulsation.

A rough calculation for the energy gain by using a pulsed thermal nuclear rocket in comparison with the conventional stationary mode is as follows.

This energy may be written as where: On the other hand, the energy generated in the stationary mode, i.e., when the nuclear core operates at nominal constant power is given by where: Also, for the case of cylindrical geometries for the nuclear fuel we have and the linear power given by [2] Where: Therefore, the energy ratio between the pulsed mode and the stationary mode,

Typical average values of the parameters for common nuclear fuels as MOX fuel or uranium dioxide are:[3] heat capacities, thermal conductivity and densities around

, and the temperature drop between the center line and the cladding on

energy amplification by pulsing the core could be thousands times larger than the stationary mode.

More rigorous calculations considering the transient heat transfer theory shows energy gains around hundreds or thousands times, i.e.,

are typical in the technology for production of amorphous metal, where extremely rapid cooling in the order of

The most direct way to harness the amplified energy by pulsing the nuclear core is by increasing the thrust via increasing the propellant mass flow.

Increasing the thrust in the stationary mode -where power is fixed by thermodynamic constraints, is only possible by sacrificing exhaust velocity.

The attainment of high exhaust velocity or specific impulse (Isp) is the first concern.

times more energy than the stationary mode, and then the fraction of prompt neutrons or

][citation needed] could be equal or larger than the total energy in the stationary mode.

Because fast neutrons created in fission events have very high neutron temperature (2 MeV or 20,000 km/s on average), they are capable of exchanging very large amounts of kinetic energy.

Neutrons also exchange kinetic energy much more readily with nucleons of similar mass, so low molar mass propellant can absorb most of it while the heavy atoms in fuel are mostly unaffected.

This allows temperatures to be obtained in the propellant that are higher than in the fuel, potentially by orders of magnitude, enabling Isp far beyond what a standard nuclear thermal rocket is capable of.

times more energy than the stationary mode, the Isp amplification is given by Where: With values of

amplification attainable makes the concept specially interesting for interplanetary spaceflight.

There are several advantages relative to conventional stationary NTR designs.

Because it directly uses the fission fragments as a propellant, it can also achieve a very high specific impulse.

from fission fragments is unwanted energy and must be continuously evacuated by a heat removal auxiliary system using a suitable coolant.

[1] Liquid metals, and particularly lithium, can provide the fast quenching rates required.

This implies a large dedicated heat transfer surface.

For instance, the use of standard control rods in a single or banked configuration with a motor driving mechanism or the use of standard pneumatically operated pulsing mechanisms are suitable for generating up to 10 pulses per minute.

However, for pulsations ranking the thousands of pulses per second (kHz), optical choppers or modern wheels employing magnetic bearings allow to revolve at 10 kHz.

[8] If even faster pulsations are desired it would be necessary to make use of a new type of pulsing mechanism that does not involve mechanical motion, for example, lasers (based on the 3He polarization) as early proposed by Bowman,[9] or proton and neutron beams.

A sequence for a stationary-pulsed-stationary maneuver for a pulsed thermal nuclear rocket. During the stationary mode (working at constant nominal power), the fuel temperature is always constant (solid black line), and the propellant is coming cold (blue dotted lines) heated in the chamber and exhausted in the nozzle (red dotted line). When amplification in thrust or specific impulse is required, the nuclear core is "switched on" to a pulsed mode. In this mode, the fuel is continuously quenched and instantaneously healed by the pulses. Once the requirements for high thrust and specific impulse are not required, the nuclear core is "switched on" to the initial stationary mode.
Pulsed nuclear thermal rocket unit cell concept for I sp amplification. In this cell, hydrogen-propellant is heated by the continuous intense neutronic pulses in the propellant channels. At the same time, the unwanted energy from the fission fragments is removed by a solitary cooling channel with lithium or other liquid metal.