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Nuclear Rocket Cycles: A Brief Primer

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I recently began a class on nuclear rocket propulsion, and one of the first topics covered was various nuclear rocket cycles. I'll do my best to explain them using amazing MS Paint drawings and words.




The first is the hot bleed cycle. In this cycle, some propellant does not go through the reactor, but is instead shunted off in a different direction. This is mixed with some of the propellant that has passed through the reactor, but not out the rocket nozzle, creating a relatively hot stream of propellant. This propellant is passed through a turbine, which then powers the fuel pump. After passing through the turbine, the propellant is exhausted overboard (on some designs this can be used for attitude control). Since the propellant that has passed through the turbine is at lower temperature than that which has passed through the reactor, some efficiency is lost. The NERVA design from the 1960s/1970s utilized the hot bleed cycle.


The cold bleed cycle is similar, except no propellant from the reactor is used to power the turbine. As a result, the propellant passing through the turbine is colder, thereby reducing turbine efficiency. However, this does have the advantage of producing less thermal stress on the turbine components. However, since the mass flow through the turbine is larger, the cold bleed cycle is less efficient than the hot bleed cycle.


The expander cycle cleverly avoids propellant wastage by passing all the propellant used in the turbine back into the reactor. This avoids expending propellant in the relatively low temperature turbine exhaust, and means that the expander cycle NTR has a higher specific impulse than the hot or cold bleed cycles.

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What is the reasoning behind the cold-bleed cycle?  Who the hell is able to make a nuclear reactor that heats up LH2 and a nozzle that can reasonably collimate such exhaust, but isn't able to make turbine blades that can withstand a reasonable TC/TH?


A Nazi Germany that somehow got functioning nuclear program, but is still as shitty as ever at turbines?


I don't think a cold bleed reactor has ever made it to the build stage (we had hot-bleed in the 60s), but theoretically if you really wanted to you could make one.

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  • 2 weeks later...

Just to prevent this section of the forum becoming the Colli and Unstart Nuclear Jargon Corner, here's what's going on:



nuclear thermal rocket or NTR is a rocket where the propulsive gas (which is called reaction mass) that shoots out the nozzle is heated by a nuclear reactor instead of being heated by a chemical reaction as in existing, chemical rockets.  There are several potential advantages of nuclear thermal rockets over chemical rockets.


One often overlooked advantage is that the reaction mass of a nuclear thermal rocket can be relatively benign and inert.  Chemical rocket fuels are potentially nasty shit.  There was an incident in a missile silo in Alabama where a technician dropped a wrench, and the wrench landed just wrong such that it pierced the very thin skin of an ICBM.  The rocket started leaking hypergolic fuel, which then exploded.  Oh, did I mention that this was one of them nuke-tipped ICBMs?


The reaction mass in an NTR doesn't have to be chemically reactive.  For thermodynamic reasons the ideal NTR reaction mass is liquid hydrogen, which isn't exactly the best behaved stuff in the universe, but at least it doesn't explode when you throw a wrench at it.  Completely inert reaction mass could be used, albeit at some loss of performance.


The other major advantage of an NTR is that nuclear reactions are fuckloads more powerful than chemical reactions.  Per kilogram, fissioning uranium provides 1.5 x 106 times more energy than burning fuel.  Harnessing this energy in an effective way is a challenge, however, and one of the big problems is powering the massive turbopumps the feed fuel to the rocket nozzle.  The diagrams Unstart posted show three different ways of potentially doing that.

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Getting this thread vaguely back on topic, there are analogous differences in liquid propellant chemical rockets.  In some rockets a portion of the propellant is burned to spin a turbine and vented overboard; this is called a "gas generator" cycle.  In others, the cold side of the turbine gas is used in the main nozzle.  This is "staged combustion," and it's part of the reason the RD-180 has such good ISP for a LOX/kerosene rocket.

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