Fusion tube propulsion

[DerpenWolf]

So in a book I read recently "World of Ptavvs" by Larry Niven there was a propulsion system that was pretty much a throw away strap-on fusion SRB booster that pretty much worked by having a small controlled fission reaction ignite (yes I know this is not a good word for this but bare with me) a fusion reaction that uses up all the fuel in the tube whilst it squirts exhaust out an end, I also vaguely recall magnetic fields being used to help contain the reaction. So do you think we could build something like this with modern(ish) technologies?

[How2FoldSoup]

Off the top of my head that just sounds like using a controlled nuclear explosion as thrust..Reaching critical mass and causing fission to create the conditions necessary for fusion using boosting materials is exactly how we get 100+ Kiloton bombs

[phoenix_ca]

Sounds awfully like a barely contained H-bomb.

I suppose it's possible, but I don't see how you could get a decent burn time out of it. Those chain reactions happen fast. No matter how strong your containment, that bang is going to be big and all in one go, more-or-less.

[meve12]

So in a book I read recently "World of Ptavvs" by Larry Niven there was a propulsion system that was pretty much a throw away strap-on fusion SRB booster that pretty much worked by having a small controlled fission reaction ignite (yes I know this is not a good word for this but bare with me) a fusion reaction that uses up all the fuel in the tube whilst it squirts exhaust out an end, I also vaguely recall magnetic fields being used to help contain the reaction. So do you think we could build something like this with modern(ish) technologies?

The "Nuclear SRB", as I read it, seems to assume a perfectly reflective tube lining. So no, not as Larry Niven envisioned.

[Tommygun]

Maybe something like this? It has all the elements you described, but probably in a way Niven didn't envision in 1966.

Edit: The more I look at that picture, the more it sounds like what Niven was describing.

http://www.newscientist.com/article/dn3294-nuclear-fusion-could-power-nasa-spacecraft.html

Edited May 30, 2014 by Tommygun

[sgt_flyer]

as always, the main problem with all these technologies, is providing the electric energy needed to sustain or contain the reactions we need better power sources !

[Northstar1989]

Sounds awfully like a barely contained H-bomb.

I suppose it's possible, but I don't see how you could get a decent burn time out of it. Those chain reactions happen fast. No matter how strong your containment, that bang is going to be big and all in one go, more-or-less.

You're right, it does sound an awful lot like a barely-contained H-Bomb...

So why contain it at all? Project Orion was a perfectly viable plan that simply utilized shaped warheads to direct the majority of the blast at a pusher-plate:

http://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29

The technology was nothing new- they even built test models with chemical explosive pellets. There's no reason to think we couldn't do it, if we had a use for putting a few thousand tons of payload into orbit... (my guess would be a massive Methane-Oxygen or Kerosene-Oxygen fuel depot, and supplies for dozens of manned interplanetary missions... Or enough solar power satellites and microwave transmitters to power the Continental United States for decades...)

Regards,

Northstar

[Rakaydos]

So why contain it at all? Project Orion was a perfectly viable plan that simply utilized shaped warheads to direct the majority of the blast at a pusher-plate:

Are you saying this would work like a shape charge fusion bomb? Orion style engines are horribly inefficent at using the bomb's energy, making energy containment "someone elses problem" and relying on capturing a fraction of the raw power of the weapon

Without a warhead as directed as Orion, this would need devices that are even more powerful, EMPing a hemisphere or more during it's orbital insertion maneuvering.

[meve12]

Are you saying this would work like a shape charge fusion bomb? Orion style engines are horribly inefficent at using the bomb's energy, making energy containment "someone elses problem" and relying on capturing a fraction of the raw power of the weapon

Without a warhead as directed as Orion, this would need devices that are even more powerful, EMPing a hemisphere or more during it's orbital insertion maneuvering.

With an ISP of 43,000 m/s? Hardly. Or did you miss the bit about nuclear shaped charges?

I might add that a 'conventional' NERVA has only an ISP of 8093 m/s at best.

[Rakaydos]

With an ISP of 43,000 m/s? Hardly. Or did you miss the bit about nuclear shaped charges?

I might add that a 'conventional' NERVA has only an ISP of 8093 m/s at best.

For a nuke? 43000 ISP -is- inefficent. the shape charge is more efficent than a pure inverse square, but it's still less than half the output of the bomb. (as the other half is required to launch the shape charge at the plate- every action has an equal and opposite, after all...)

It is, however, quite EFFECTIVE.

[Northstar1989]

For a nuke? 43000 ISP -is- inefficent. the shape charge is more efficent than a pure inverse square, but it's still less than half the output of the bomb. (as the other half is required to launch the shape charge at the plate- every action has an equal and opposite, after all...)

It is, however, quite EFFECTIVE.

Actually, it's the majority of the charge that's directed at the plate. Over 70% of the energy if my memory serves me correctly. The charge is not launched at the plate, so that's a completely invalid argument- it's dropped from the Orion itself, and detonated just below the vehicle's pusher plate. The vehicle can carry literally thousands of such charges, and with the amazing ISP and thrust generated (unrivaled by any other space technology we've developed, EVER) it's a lot more efficient than any of the alternatives- regardless of how much energy is lost to the surrounding area.

My point is that containing a large thermonuclear explosion is so difficult you're better off just shaping the charge (yes, this *IS* a shaped-charge fusion bomb, and it DOES work- this was one of the key technologies they successfully developed for Orion) and dropping it below a pusher-plate...

Regards,

Northstar

[KerikBalm]

With an ISP of 43,000 m/s? Hardly. Or did you miss the bit about nuclear shaped charges?

I might add that a 'conventional' NERVA has only an ISP of 8093 m/s at best.

Conventional NERVA only has an ISP of about 800 s, or an exhaust velocity of 8093...

ISP is not measured in m/s. The numerical value for ISP is basically exhaust velocity divided by Earth's gravity....

[Northstar1989]

Conventional NERVA only has an ISP of about 800 s, or an exhaust velocity of 8093...

ISP is not measured in m/s. The numerical value for ISP is basically exhaust velocity divided by Earth's gravity....

Actually, ISP *IS* measured in meters/second, so you're wrong.

There are two equally valid ways of measuring it- in units of time (seconds), which is the one used in KSP, and in units of velocity (m/s), which is basically equivalent to exhaust velocity, and sees some use in the real world...

SOURCE: http://en.wikipedia.org/wiki/Specific_impulse

As for Orion, ORION IS NOT NERVA . Orion is a type of nuclear-pulse rocketry, and achieves the ISP-equivalent of over 43,000 m/s (actual velocity of the explosions in higher- but spreads out in multiple directions...)

http://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29

Regards,

Northstar

Edited May 31, 2014 by Northstar1989

[meve12]

For a nuke? 43000 ISP -is- inefficent. the shape charge is more efficent than a pure inverse square, but it's still less than half the output of the bomb. (as the other half is required to launch the shape charge at the plate- every action has an equal and opposite, after all...)

It is, however, quite EFFECTIVE.

Shaped charges are actually 85% efficient, actually, compared to 10% for an unshaped charge. Project Rho here can explain it better:

http://www.projectrho.com/public_html/rocket/spacegunconvent.php#id--Nukes_In_Space--Nuclear_Shaped_Charges

But from what I read, gamma rays emitted from a nuclear initiation are contained and directed by a jacket of depleted uranium into a load of berylium oxide, which converts them into heat which is dumped into a plate of tungsten propellent, which becomes a focused jet of plasma in the desired direction. There's no humongous backblast like you seem to be assuming.

Edited May 31, 2014 by meve12

[Rakaydos]

Shaped charges are actually 85% efficient, actually, compared to 10% for an unshaped charge. Project Rho here can explain it better:

http://www.projectrho.com/public_html/rocket/spacegunconvent.php#id--Nukes_In_Space--Nuclear_Shaped_Charges

But from what I read, gamma rays emitted from a nuclear initiation are contained and directed by a jacket of depleted uranium into a load of berylium oxide, which converts them into heat which is dumped into a plate of tungsten propellent, which becomes a focused jet of plasma in the desired direction. There's no humongous backblast like you seem to be assuming.

A shape charge doesnt violate conseration of momentum. If one went off inside of a huge sphere, there would be zero net mmentum change, once all momentum was absorbed.

[nhnifong]

And yet another thread on KSP forums turns into a project orion discussion.