Nuclear pulse propulsion with low Z propellant?

[Decimator]

I've been doing some reading about Orion-drive spacecraft , but I don't understand the requirement of a high Z propellant like tungsten. Assuming that the pulse unit were detonated further away from the craft to prevent it from drilling a hole in the pusher plate due to its narrower angle, why couldn't the pulse units use a low Z propellant to gain a higher exhaust velocity? I assume there's a reason, but it isn't easily found information.

 

[Astronuc]

I'm not sure where one found the requirement for tungsten.

The ORION concept is one of detonating a nuclear (or thermonuclear device) behind the spacecraft , and capturing the blast wave from the detonation. One does not need tungsten, except possibly that is used for the 'pusher plate' because it has a high melting point. One could of course use graphite.

Some ORION concepts use an ablative surface that would be ablated or evaporated/sublimate(?) from the surface. But then it's a matter of get a more or less uniform coating.

For high specific impulse, one would want the lowest atomic/molecular mass possible, but then liquid hydrogen is not amenable to being coated on a hot surface.

 

[Decimator]

My question relates to Orion drives which do not require an ablative surface. I got the high z propellant requirement from http://www.projectrho.com/rocket/enginelist.php#orion", which I understand to be a fairly reputable site.

Specifically, I'm talking about the design of the nuclear shaped charge pulse units themselves:

Note the use of tungsten as propellant. This is the part I do not understand.

 

[Bob S]

I have seen (over 50 years ago) either a tungsten or tantalum disk that had ≈1 mm of surface ablated by x-rays from a small (few kt) blast. I believe the recoil force (impulse) was measured. I think the reason for high Z is to maximize the momentum transfer (thrust) per unit x-ray energy. Recall p²=2ME

See equation for ablation pressure in

http://nuclearweaponarchive.org/Nwfaq/Nfaq4-4.html#Nfaq4.4.3.3

Bob S

 

[Astronuc]

My question relates to Orion drives which do not require an ablative surface. I got the high z propellant requirement from http://www.projectrho.com/rocket/enginelist.php#orion", which I understand to be a fairly reputable site.

Specifically, I'm talking about the design of the nuclear shaped charge pulse units themselves:

Note the use of tungsten as propellant. This is the part I do not understand.

It seems to be someone's wild fantasy.

 

[Bob S]

It seems to be someone's wild fantasy.

I firmly believe that this type of propulsion is unworkable, but if it could work, the only way is if there is a large thick tungsten shield between the nuclear device (a few kt or less) and the rocket body. The tungsten shield protects the rocket and its contents from the radiation. The x-rays from the detonation will heat up and ablate the tungsten surface. Recoiling material ablating from the tungsten surface will transfer a momentum impulse to the rocket. To get the maximum momentum impulse p·Δt from the available x-ray ablating energy, p² = 2ME, showing that M should be maximized, where M represents the gram-molecular-weight of tungsten.

The only propulsion method that is less likely to work than ORION is the antimatter drive. See

http://en.wikipedia.org/wiki/Antimatter_rocket

Bob S

 

[Decimator]

Why do you think it won't work? Why can't a wave of plasma from a pulse unit impart momentum on the pusher plate?

 

[Bob S]

Why do you think it won't work? Why can't a wave of plasma from a pulse unit impart momentum on the pusher plate?

Momentum can only be transferred to the rocket if there is an equal and opposite momentum (thrust) in the direction away from the rocket. High mass ions are a lot more efficient than x rays in producing thrust.

Here is a homework problem for you. Suppose there is a tungsten shield (pusher plate) between the nuclear device and the rocket. The tungsten shield intercepts 25% of the total x-rays emitted isotropically from the explosion. Assume that one Avagadro's number of 10-keV x-rays (E_xray = hν) hit the tungsten shield. What is the impulse (Newton-seconds)? Assuming the x-rays are perfectly reflected away from the rocket (like a mirror), what is the additional impulse? Now, instead of the x-rays being reflected, suppose each 10-keV x-ray causes one tungsten atom (184 grams total) to be ablated from the pusher plate surface (very simplistic model), and the tungsten atom ablates with an energy E_W=E_xray=½MV², where M and V are the rest mass and velocity of the ablated tungsten atoms. What is now the total momentum transfer to the tungsten pusher plate (Newton-seconds)? Would the momentum transfer be more or less if the pusher plate were made of copper?

Bob S

 

[HowlerMonkey]

What is the source matter that expands greatly when superheated that get's expelled for propulsion?

Or

If a nuke blasts in space and there is no matter for it to heat, does it make a sound?