Nuclear Powered Space Flight #2
So how does this even work?
Hello everyone, Welcome to Part 2 of our 4 part series on Nuclear Thermal Propulsion. Here is the link to Part 1.
Welcome to our second “Stellar snippet”. Today we are going to get into the technicals of how NTP works, and why it is a better mode of propulsion. Let’s get right to it (Cant afford to waste characters here!)
There are 2 elements to consider: the propellant — hydrogen- and the nuclear reactor. While hydrogen is the lightest element, it is a gas. This means it must be liquefied at cryogenic temperatures and stored in specialized tanks. The hydrogen then is pumped from its tank through the rocket nozzle, which allows the nozzle to remain cool, to the nuclear reactor.
The nuclear reactor is a solid matrix of neodymium-coated graphite — the coating prevents any chemical reaction between the graphite and hydrogen- and uranium oxide — which, due to mass constraints, is enriched to bomb-grade levels. The fission of the uranium heats up the graphite, which in turn heats up the hydrogen flowing through the core and out through the nozzle, which drives the rocket forward.
This explanation doesn’t do justice to the scale of engineering of NTP, however, the additional complexities are beyond the scope of such a blog:
Now, let’s figure out, with a bit of technical talk, what makes this technology so special. We will be dipping our feet in rocket science… so buckle up!
Force is an “influence” that can cause a body to change velocity.
Thrust is a force.
Thrust in rockets = Mass flow rate * Exhaust velocity
a) Mass flow rate (kg/s) — The rate of mass of exhaust exiting the nozzle
b) Exhaust velocity(m/s) — The speed the exhaust exits the nozzle
The exhaust velocity is inversely proportional to the square root of the mass of propellant
The impulse(Imp) of an engine is its thrust multiplied by the time the engine is burning.
Specific impulse (Isp) is the impulse of the engine divided by the weight of fuel flow rate. (Think of it as the efficiency of the engine to turn propellant into thrust)
Reread that if you have to!
Lets start with chemical rockets. They produce thrust by combustion, resulting in a heavy propellant and a high mass flow rate, at the cost of a low exhaust velocity and a high weight of fuel flow rate: which offers thrust, but poor Isp.
Moving up to electric propulsion, which addresses the Isp problem with an extremely light fuel flow rate. However, while the engine is efficient, it faces the opposite problem of thrust: Its low mass flow rate severely limits its thrust output, which limits payload capacity (We talked in Part 1).
Enter NTP: Using hydrogen, NTP can achieve good thrust- with a high exhaust velocity- while maintaining a good Isp — due to the low weight of the fuel flow rate. This balancing act NTP plays allows it to outperform chemical rockets and electric propulsion in terms of efficiency and thrust output, thus making it an all-rounder and the ideal candidate for space flight.
Here is a visual comparison from the Instagram post!
That was tough! Thank you for your time and attention, and see you next week!
