Can Nuclear Rockets Ever Compete With Chemical For Thrust?

[Spacescifi]

 

Just wanted to know... for scifi purposes.

 

Here is what I think which you can correct or add to as needed.

Nuclear rockets are more efficient, meaning it takes less propellant to reach a given distance... in vacuum. Yet they have less thrust than chemical rockets, which are the means we use to even reach vacuum. I am aware that NTTR attempts to increase thrust with airbreathing, but in space there is no air so that's only for launch to orbit.

What I want to know is, if you want high thrust momentarily to get out if a jam in space with a nuclear engine, do you have to have a separate chemical rocket engine for that?

Or can you do it by adding redundant nuclear reactors or by modifying the nuclear or chemical mix any conceivable way?

No antimatter this time.

 

My original idea was to just to add redundant small nuclear reactors in the hope that configuring it somehow will increase overall thrust.

My reasoning is that nuclear reactors are more user friendly than project Orion, and thus more likely to be used if spaceships become more common, especially by mature civillian industry.

What do you think?

 

Edited April 18, 2020 by Spacescifi

[sevenperforce]

No need to add redundant small reactors. Just pump in heavier propellant.

Nuclear engines have constant temperature so all you have to do is change the mixture. Pump water through the engine for high thrust and low(er) specific impulse; mix in liquid hydrogen to increase specific impulse at the cost of lower thrust, go to pure hydrogen to maximize specific impulse.

A LOX-afterburning nuclear thermal rocket (LANTR) is a very good design. Your main engine is a pure-hydrogen NTR, but you have LOX injectors downstream that you can use to add mass and energy. The LOX burns with the hot hydrogen, raising temperature even hotter than in the chamber and adding reaction mass to boost thrust tremendously. It would be fairly easy to build a LANTR-based SSTO, if the normal hurdles could be overcome.

A 1994 study looked at a LOX/LH2 variable-mixture nuclear thermal rocket. It found that a pure-hydrogen rocket gets 941 s with a T/W ratio of 3. Inject an equal mass of LOX downstream, and your T/W jumps to 5 while only reducing your specific impulse to around 770 s. You can raise the LOX/LH2 mixture ratio as high as 7 for a specific impulse of 514 seconds and a T/W ratio of more than 13. 

https://space.nss.org/lunar-base-studies-1994-lantr-and-lunox/

[Bill Phil]

Apparently Project Rover found that DUMBO reactors could compete with chemical TWRs, but the higher complexity meant that it would be more difficult to develop and more likely to fail in flight. 

However, with modern modelling software and material advances a modern DUMBO-like NTR could probably be built. And with good performance at that.

Though it probably wouldn't be used to launch unless it could be well shielded.

Though maybe a Titan III like vehicle could work - use SRBs to get the DUMBO core to a high altitude to reduce the radiation exposure on the ground and some velocity. Of course the risk of a reactor in flight would still be present in the event of a launch failure. Liquid boosters may be a better bet though.

[Spacescifi]

37 minutes ago, sevenperforce said:

No need to add redundant small reactors. Just pump in heavier propellant.

Nuclear engines have constant temperature so all you have to do is change the mixture. Pump water through the engine for high thrust and low(er) specific impulse; mix in liquid hydrogen to increase specific impulse at the cost of lower thrust, go to pure hydrogen to maximize specific impulse.

A LOX-afterburning nuclear thermal rocket (LANTR) is a very good design. Your main engine is a pure-hydrogen NTR, but you have LOX injectors downstream that you can use to add mass and energy. The LOX burns with the hot hydrogen, raising temperature even hotter than in the chamber and adding reaction mass to boost thrust tremendously. It would be fairly easy to build a LANTR-based SSTO, if the normal hurdles could be overcome.

A 1994 study looked at a LOX/LH2 variable-mixture nuclear thermal rocket. It found that a pure-hydrogen rocket gets 941 s with a T/W ratio of 3. Inject an equal mass of LOX downstream, and your T/W jumps to 5 while only reducing your specific impulse to around 770 s. You can raise the LOX/LH2 mixture ratio as high as 7 for a specific impulse of 514 seconds and a T/W ratio of more than 13. 

https://space.nss.org/lunar-base-studies-1994-lantr-and-lunox/

 

Thank you very much! I will use that.

 

I suppose if I want maximum thrust with the least ISP I could use liquid mercury... for when the ship is trying to escape hostile fire.

As it is right now my scifi spaceships only will use rocketry for hyperspace travel and when a portal is open  (hyperspace is a vast tunnel network with massive rooms), since a tweak of the hyperdrive allows for constant acceleration. When the portal is open they must rely on rocketry since the tweak won't work.

How does the hyperdrive constant acceleration work? Right before a hyperspace portal opens it creates a highly magnetic point in space. Spaceships can project the magnetic point in front of them, essentially being pulled along as long as they stall the portal until they choose to open the portal.

Thus, ships getting up close and personal is far more likely in hyperspace than normal space.

Edited April 18, 2020 by Spacescifi

[magnemoe]

1 hour ago, Bill Phil said:

Apparently Project Rover found that DUMBO reactors could compete with chemical TWRs, but the higher complexity meant that it would be more difficult to develop and more likely to fail in flight. 

However, with modern modelling software and material advances a modern DUMBO-like NTR could probably be built. And with good performance at that.

Though it probably wouldn't be used to launch unless it could be well shielded.

Though maybe a Titan III like vehicle could work - use SRBs to get the DUMBO core to a high altitude to reduce the radiation exposure on the ground and some velocity. Of course the risk of a reactor in flight would still be present in the event of a launch failure. Liquid boosters may be a better bet though.

NTR are most relevant as upper stages anyway because its here you want high ISP, you could run an NTR on heavier propellant but then it would not have very good ISP anymore. SSTO are stupid unless you have something like Sabre or better and only makes sense with reuse. 

An NTR powered reusable SSTO would be expensive to operate because of the reactor and crashes would create fallout. 
You can much easier handle the radiation in orbit its no cities or nature to radiate only space. 

Single use NTR makes lots of sense for the first versions anyway, however they only makes sense if you want to push something heavy like an base in moon orbit or an ship to Mars. Or you want to push something medium weight very fast with an smaller engine. 
This could also let you let the stage do later tests if you have spare fuel. And this is the problem with NTR, an lack of missions. Does anybody doubt Russia would had any issue making them if they could get NASA money for deep space probe missions?

However at this time I think NTR is an dead technology, using an reactor and an ion drive or other better engines are probably smarter and cheaper. Its much cheaper to develop an nuclear reactor to use in space as you are just making an small nuclear reactor. who can work in any orientation. 

[Guest]

Actually, an NTR is a pretty robust thing. You are overstating the dangers they pose. NERVA was pretty much blown up (as a test) with no serious radiation release. I think we are more than capable of topping a 60s era engine's safety record. A crashed reactor would likely be recovered, refurbished and reused. This stuff is expensive. That is, if it didn't land deep in the ocean, as if would if launched from the Cape.

A nuclear SSTO would be expensive to operate, period, like all nuclear reactors, but it'll be far more economical than throwing them away. A methane NTR would allow a performance in excess of that of LH2, with propellant that is only mildly cryogenic, no oxidizer, and thrust no worse than a comparable chemical engine. Lower TWR, yes, but still enough for an SSTO. 

NTRs make sense for human spaceflight, because they have a lot of thrust compared to other technologies, while maintaining good Isp. Ion engines are only ever going to be good enough for probes, thrust is not high enough. Advanced nuclear-electric rockets could be easier to manage (reactor can power other stuff when not thrusting), but combined cycle NTRs provide similar advantages. 

[Spacescifi]

1 hour ago, Dragon01 said:

Actually, an NTR is a pretty robust thing. You are overstating the dangers they pose. NERVA was pretty much blown up (as a test) with no serious radiation release. I think we are more than capable of topping a 60s era engine's safety record. A crashed reactor would likely be recovered, refurbished and reused. This stuff is expensive. That is, if it didn't land deep in the ocean, as if would if launched from the Cape.

A nuclear SSTO would be expensive to operate, period, like all nuclear reactors, but it'll be far more economical than throwing them away. A methane NTR would allow a performance in excess of that of LH2, with propellant that is only mildly cryogenic, no oxidizer, and thrust no worse than a comparable chemical engine. Lower TWR, yes, but still enough for an SSTO. 

NTRs make sense for human spaceflight, because they have a lot of thrust compared to other technologies, while maintaining good Isp. Ion engines are only ever going to be good enough for probes, thrust is not high enough. Advanced nuclear-electric rockets could be easier to manage (reactor can power other stuff when not thrusting), but combined cycle NTRs provide similar advantages. 

 

Is not minimag orion just a magbetic nozzle with fusile or fissionable pellet propellant?

Pretty much beats all other nuclear tech as an orbit to orbit craft?

[tater]

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170003378.pdf

Current work Mike Houts and his people are doing:

 

[sevenperforce]

1 hour ago, Dragon01 said:

A methane NTR would allow a performance in excess of that of LH2, with propellant that is only mildly cryogenic, no oxidizer, and thrust no worse than a comparable chemical engine. Lower TWR, yes, but still enough for an SSTO. 

A LOX-afterburning methane-based nuclear-thermal rocket would be hella thrusty. Jump off the pad like an SRB; burn to orbit with an efficiency exceeding an RL-10.

[Bill Phil]

38 minutes ago, Spacescifi said:

 

Is not minimag orion just a magbetic nozzle with fusile or fissionable pellet propellant?

Pretty much beats all other nuclear tech as an orbit to orbit craft?

Mini Mag Orion isn’t an NTR. NTRs are actually a much more well understood technology as well. 

[Guest]

24 minutes ago, sevenperforce said:

A LOX-afterburning methane-based nuclear-thermal rocket would be hella thrusty. Jump off the pad like an SRB; burn to orbit with an efficiency exceeding an RL-10.

Not as much as you'd think. The main reason LANTR works so well is that LH2 isn't very dense. When you add oxygen, mass flow goes up. An oxidizing reaction would be a plus, but a relatively minor effect, especially since it would occur inside the nozzle. With methane, you don't have mass flow problems in first place, since it's reasonably dense. Oxygen would not give a significant performance bonus here, if you want to switch gears you can just dump more methane straight into the reactor. It will cool down, but that by itself wouldn't harm the nuclear reaction, but will trade Isp for thrust.

LH2 NTRs in general have the same problems as hydrolox engines, only magnified by the fact all of their mass flow is hydrogen. This is also the source of the "NTRs have low TWR" misconception.