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. 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
10 − 2 s {\displaystyle 10^{-2}s} to : 10 − 3 s {\displaystyle 10^{-3}s} 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.
Statement of the concept A rough calculation for the energy gain by using a pulsed thermal nuclear rocket in comparison with the conventional stationary mode is as follows. The energy stored into the fuel after a pulsation is the sensible heat stored because the fuel temperature increase. This energy may be written as
E pulse = c f M f Δ T {\displaystyle E_{\text{pulse}}=c_{f}M_{f}\Delta T}
where:
E pulse {\displaystyle E_{\text{pulse}}} is the sensible heat stored after pulsation,
c f {\displaystyle c_{f}} is the fuel heat capacity,
M f {\displaystyle M_{\text{f}}} is the fuel mass,
Δ T {\displaystyle \Delta T} is the temperature increase between pulsations. 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
E stationary = χ l l t {\displaystyle E_{\text{stationary}}=\chi _{l}lt}
where:
χ l {\displaystyle \chi _{l}} is the linear power of the fuel (power per length of fuel),
l {\displaystyle l} is the length of the fuel,
t {\displaystyle t} is the residence time of the propellant in the chamber. Also, for the case of cylindrical geometries for the nuclear fuel we have
M f = π R f 2 l ρ f {\displaystyle M_{f}=\pi R_{f}^{2}l\rho _{f}}
and the linear power given by
χ l = 4 π κ f ( T f − T s ) {\displaystyle \chi _{l}=4\pi \kappa _{f}(T_{f}-T_{s})}
Where:
R f {\displaystyle R_{f}} is the radius of the cylindrical fuel,
ρ f {\displaystyle \rho _{f}} the fuel density,
κ f {\displaystyle \kappa _{f}} the fuel thermal conductivity,
T f {\displaystyle T_{f}} is the fuel temperature at the center line,
T s {\displaystyle T_{s}} is the surface or cladding temperature. Therefore, the energy ratio between the pulsed mode and the stationary mode, N = E pulse E stationary {\displaystyle N={\frac {E_{\text{pulse}}}{E_{\text{stationary}}}}} yields
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