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Calliban

Member

From: Northern England, UK

Registered: 2019-08-18

Posts: 3,793

Re: A revival of interest in Nuclear Thermal Propulsion?

Quaoar, that is an interesting question.  The pressure inside the bulb will change dramatically as the mixture heats up during startup.  It would need to be much less than 1bar under shutdown conditions.  The limit in total pressure that you have stated, would limit the chamber pressure to no more than 2bar.  Surging of gas outside the core is unlikely to be acceptable, as it would lead to reactivity swings.  So I think it is safe to say that whilst the light bulb could have excellent ISP, it is a relatively low thrust propulsion system.

Another problem with the light bulb is that rapid thermal transients would result in thermal gradients in the glass that would fracture it.  Heat up and cool down need to be slow.  It also raises questions of how the engine can tolerate large temperature differences between the propellant gas on the outside and fission core gas on the inside.  If the bulb is hot enough, it will become more ductile and less brittle, but loses a lot of tensile strength.  The general conclusion seems to be that propellant gas must be relatively diffuse to avoid high rates of heat transfer (and erosion) of the glass.  That implies a low chamber pressure, probably no more than a few mbar and low power levels overall.  All unanswered questions in my mind.  But this sounds like a low thrust propulsion system, with thrust levels similar to electric propulsion.

Last edited by Calliban (2021-04-14 07:34:56)

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"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

Quaoar

Member

Registered: 2013-12-13

Posts: 665

Re: A revival of interest in Nuclear Thermal Propulsion?

Quaoar, that is an interesting question.  The pressure inside the bulb will change dramatically as the mixture heats up during startup.  It would need to be much less than 1bar under shutdown conditions.  The limit in total pressure that you have stated, would limit the chamber pressure to no more than 2bar.  Surging of gas outside the core is unlikely to be acceptable, as it would lead to reactivity swings.  So I think it is safe to say that whilst the light bulb could have excellent ISP, it is a relatively low thrust propulsion system.

Another problem with the light bulb is that rapid thermal transients would result in thermal gradients in the glass that would fracture it.  Heat up and cool down need to be slow.  It also raises questions of how the engine can tolerate large temperature differences between the propellant gas on the outside and fission core gas on the inside.  If the bulb is hot enough, it will become more ductile and less brittle, but loses a lot of tensile strength.  The general conclusion seems to be that propellant gas must be relatively diffuse to avoid high rates of heat transfer (and erosion) of the glass.  That implies a low chamber pressure, probably no more than a few mbar and low power levels overall.  All unanswered questions in my mind.  But this sounds like a low thrust propulsion system, with thrust levels similar to electric propulsion.

The trick is that the hot uranium plasma fuel never touches the quartz wall because there's a cold neon vortex between them. But the problem is about the pressure difference between the chamber and the bulbs: I don't know how much pressure a quartz bulb can withstand but if the chamber pressure at full thrust is 500 atm, the pressure inside the bulb must be almost the same, and when chamber pressure decreases during the cool-down the pressure inside the bulb must decrease with the same rate, otherwise the bulb might explode or implode.

The reference design has a mass of 15800 kg, a thrust of 596.7 kN and a Isp of 1826 s with an expansion ratio of 545, a chamber temperature of 6667 K and a chamber pressure of 500 atmosphere.

Last edited by Quaoar (2021-04-14 09:40:05)

Calliban

Member

From: Northern England, UK

Registered: 2019-08-18

Posts: 3,793

Re: A revival of interest in Nuclear Thermal Propulsion?

The trick is that the hot uranium plasma fuel never touches the quartz wall because there's a cold neon vortex between them. But the problem is about the pressure difference between the chamber and the bulbs: I don't know how much pressure a quartz bulb can withstand but if the chamber pressure at full thrust is 500 atm, the pressure inside the bulb must be almost the same, and when chamber pressure decreases during the cool-down the pressure inside the bulb must decrease with the same rate, otherwise the bulb might explode or implode.

The reference design has a mass of 15800 kg, a thrust of 596.7 kN and a Isp of 1826 s with an expansion ratio of 545, a chamber temperature of 6667 K and a chamber pressure of 500 atmosphere.

Very impressive indeed if it can do all of those things.  Exhaust velocity 18km/s, thrust levels of of 60,000kg-force, for a system mass of 15,800kg.  Plausible delta-v of up to 100km/s.  Almost as good as the Epstein Drive.

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"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

SpaceNut

Administrator

From: New Hampshire

Registered: 2004-07-22

Posts: 29,431

Re: A revival of interest in Nuclear Thermal Propulsion?

Another article on The Pentagon Wants to Launch a Nuclear Thermal Rocket in 4 Years

Oldfart1939

Member

Registered: 2016-11-26

Posts: 2,451

Re: A revival of interest in Nuclear Thermal Propulsion?

Here's a link to a video discussing use of Nuclear Energy for space propulsion.

https://www.youtube.com/watch?v=MMLgJlJX0Rk

GW Johnson

Member

From: McGregor, Texas USA

Registered: 2011-12-04

Posts: 5,796

Website

Re: A revival of interest in Nuclear Thermal Propulsion?

As I have often said,  you go with what you have on-hand,  while also trying to create better.  That approach to development applies to just about any technology.  Not doing it that way is rather stupid,  actually.

You build NERVA as it was ready for flight test decades ago.  Get started with that,  using it where appropriate.  But,  work on bringing the upgrades to NERVA to be also ready for flight.  And,  get to work on the gas core concepts (far higher payoff,  but very far from flight-ready).

The same thing applies to pulse propulsion.  Start with the old Project Orion fission charge designs,  and make them flight ready.  Use those where appropriate (really large vehicles),  while also trying to create better charge and subsystem designs. 

GW

Last edited by GW Johnson (2021-04-24 10:04:33)

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GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

SpaceNut

Administrator

From: New Hampshire

Registered: 2004-07-22

Posts: 29,431

Re: A revival of interest in Nuclear Thermal Propulsion?

US military picks 3 companies to test nuclear propulsion in cislunar space

General Atomics, Blue Origin and Lockheed Martin each received contracts for the Demonstration Rocket for Agile Cislunar Operations (DRACO) program's first phase. While DARPA did not disclose the contract values in its announcement, media outlet Space News reported General Atomics received $22 million, Lockheed Martin $2.9 million and Blue Origin $2.5 million.

NTP systems use fission reactors that heat up propellants (such as hydrogen) to high temperatures, spewing the gas at high speed through nozzles for thrust. The thrust-to-weight ratio with NTP is about 10,000 times higher than electric propulsion systems, and propellant efficiency (also known as specific impulse) is anywhere from two to five times greater than conventional chemical rockets, DARPA officials wrote in a description of the DRACO program.

Mars_B4_Moon

Member

Registered: 2006-03-23

Posts: 9,776

Re: A revival of interest in Nuclear Thermal Propulsion?

Ion Propulsion Math

Also a good thread here http://newmars.com/forums/viewtopic.php?id=7182 'Nuclear Ion Propulsion' also discussed Vasmir and Iodine use in Thruster