Orion drive and related physics

[lobe]

Ever since I have looked into the vacuum I have noticed something with the Orion drive: what does it push against? In normal explosions/reactions chemicals form a gas to expand against a suface to propel forward. With a nuclear weapon in deep space they only have particles and radiation off the bomb, or am I missing something?

[K^2]

9 minutes ago, lobe said:

Ever since I have looked into the vacuum I have noticed something with the Orion drive: what does it push against? In normal explosions/reactions chemicals form a gas to expand against a suface to propel forward. With a nuclear weapon in deep space they only have particles and radiation off the bomb, or am I missing something?

There is quite a bit of matter in the bomb. All of it turns into a superheated plasma during the explosion. For the Orion, they supplement it with additional "propellant" material. About half of all that gets ejected into the space. But the ship has a damper plate on the back that absorbs the rest, giving ship impulse.

[Temstar]

Each orion bomb has a slab of tungsten above the channel filler that gets turned into a cone of plasma, it's this plasma that hits the pusher plate. The other bomb debris and gamma ray play no part in the propulsion.

[K^2]

Sure they do. Anything that hits the damper will impart momentum to the ship. The only reason to have that "propellant" buffer there is to increase total mass and reduce energy of particles striking the damper.

You can just detonate plain old nukes. You'll get nearly the same amount of impulse out of it. But you'll be damaging the damper as well, and possibly, irradiating the ship quite a bit more.

[lobe]

So the propellant is part of the bomb or propellant charges the drive drops? What is happening is the nuclear weapon launchs the propellant material at the plate the imparts the momentum from the propellant to the plate to impart acceleration?

[Temstar]

The propellant and the bomb are integrated into a single physical package and are ejected out of the ship together.

Edited February 17, 2016 by Temstar

[SomeGuy123]

This creates a major problem.  The hotter the nukes you use (higher yield) and the faster you set them off, the more hot plasma impinges on your pusher plate.  At a certain point, the excess heat from this (if we're talking about a sustained burn) limits you from accelerating any harder.

This radiation limiting point could be a fairly low acceleration.  I've done some rough estimates for directed fusion drives - where you have a lot more control over the reaction and a lot less wasted gas - and once you start talking millions of ISP, your thrust becomes miniscule unless you can reject unrealistic amounts of waste heat.

Orions do have the advantage of using fusion devices we have versus theoretical ones that might not ever work, and it works amazingly well in the atmosphere because the air is free propellant mass, creating a gigantic shockwave to carry your ship like a cork in a bottle.

 

[lobe]

Assuming this, it means that the Orion drie doesn't rely on radiation pressure?

[Temstar]

18 minutes ago, SomeGuy123 said:

This creates a major problem.  The hotter the nukes you use (higher yield) and the faster you set them off, the more hot plasma impinges on your pusher plate.  At a certain point, the excess heat from this (if we're talking about a sustained burn) limits you from accelerating any harder.

This is not a huge problem, because the only type of Orion that need that kind of Isp are interstellar ships, and those have plenty of time for thrusting anyway. Dyson's "energy limited" Orion takes this into consideration and it has 0.00003g of acceleration. This ship by the way is 20km in diameter and has a 5 million ton copper pusher plate.

The alternative is to use open cycle cooling where you accept that each bomb will cause some material to ablate away and you design the ship so the pusher plate or whatever ablation system it use can just survive all the bombs. The ship will be useless when it reaches its destination (as in, it can't be refueled with more bombs) but can achieve 1g acceleration for 10 days and so reach 3.3% c.

Edited February 17, 2016 by Temstar

[DerekL1963]

24 minutes ago, SomeGuy123 said:

Orions do have the advantage of using fusion devices we have versus theoretical ones that might not ever work

Keep in mind that Orion is itself just barely this side of theoretical, pretty much no significant part of the propulsion system has been tested beyond the crudest of scale models.

[kerbiloid]

32 minutes ago, lobe said:

Assuming this, it means that the Orion drie doesn't rely on radiation pressure?

Orion drive relies on X-ray pressure which accelerates vapourized tungsten.

[lobe]

This does mean what ever impacts the plate is matter rather than energy from what I expected, which makes more sense than x-rays, gamma-rays, ultraviolet or infrared waves producing any kind of pressure.

[kerbiloid]

Energy to sectional area ratio matters, and a nuke has the greatest value of it - that's the only reason why Orion uses nukes.

[lobe]

47 minutes ago, Temstar said:

This ship by the way is 20km in diameter and has a 5 million ton copper pusher plate.

This is the most amazing fact I have learnt today. Does this happen to have Graham's number and Poincare recurrence time embedded in the paper?

[shynung]

1 hour ago, Temstar said:

The propellant and the bomb are integrated into a single physical package and are ejected out of the ship together.

Imgur mirror:

[SomeGuy123]

53 minutes ago, DerekL1963 said:

Keep in mind that Orion is itself just barely this side of theoretical, pretty much no significant part of the propulsion system has been tested beyond the crudest of scale models.

I guess?  I feel like the evidence we do have is pretty overwhelming that Orion would work, it just might not work as well as optimistic estimates now.

(a) Nukes do detonate

(b) if you build a thick enough pusher plate with good enough shocks, and you set the nuke off far enough away, it will give you some push without blowing up your ship.

I don't see any possibility that Orion doesn't work.  I'd say it's about as theoretical as SpaceX Falcon Heavy.  It just might suck and have performance that is a fraction of what it is calculated to be.

[DerekL1963]

22 minutes ago, SomeGuy123 said:

I don't see any possibility that Orion doesn't work.  I'd say it's about as theoretical as SpaceX Falcon Heavy.

I didn't say it won't work - I said it's theoretical.  Words mean things, and "won't work" and "theoretical" aren't synonyms   Nobody has ever actually detonated one of the propulsion units in space and actually determined how much push it will give.  (Though it makes one king h--l of a shaped charge and if the rumors are true it's been tested on the ground.) and is the reason all but the gross details of the propulsion units are still classified.)  Nobody has ever exposed a pusher plate to the resulting plasma.  Nobody has ever built the shock absorption system except at the crudest scale.   Etc... etc...  Precisely nothing about the performance of the drive system is known with certainty.

Meanwhile, the F9H is an expansion of an existing and more-or-less well proven vehicle.

If you call them both equally theoretical, well...  that just makes no sense.

[SomeGuy123]

4 hours ago, DerekL1963 said:

I didn't say it won't work - I said it's theoretical.  Words mean things, and "won't work" and "theoretical" aren't synonyms.   

Fair enough.  I guess I was trying to differentiate truly theoretical concepts like black hole engines, or antimatter engines, from something that we know will work.  Nuclear shaped charges might be super secret but it's hugely unlikely they just "wouldn't work"(it's just a nuke with some propellant material at the side facing the pusher plate when it goes off).  Hot plasma might ablate more plate than you might pencil in on an estimate, but this was always part of Orion - it isn't all that high an ISP for the versions proposed for the near future.

Black hole engines - nobody knows if a black hole can be made synthetically.  Nobody knows if you could feed it.  Nobody knows if you could move it.

Antimatter engines - nobody has actually made antimatter from a method that doesn't waste an absurd portion of the input energy.  Nobody has found a way to keep it in it's tank so you can have a starship that is 50% or more antimatter fuel at launch.  

Edited February 17, 2016 by SomeGuy123

[Steel]

12 minutes ago, SomeGuy123 said:

Fair enough.  I guess I was trying to differentiate truly theoretical concepts like black hole engines, or antimatter engines, from something that we know will work.

Personally, I'd probably go as far as classing those as hypothetical rather than theoretical, as most designs I've seen make some fairly major assumptions that only time will tell us are OK or not to assume.

[Bill Phil]

7 hours ago, lobe said:

Ever since I have looked into the vacuum I have noticed something with the Orion drive: what does it push against? In normal explosions/reactions chemicals form a gas to expand against a suface to propel forward. With a nuclear weapon in deep space they only have particles and radiation off the bomb, or am I missing something?

There's a shaped charge with a cone in front of it, which is the propellant.

5 hours ago, DerekL1963 said:

I didn't say it won't work - I said it's theoretical.  Words mean things, and "won't work" and "theoretical" aren't synonyms   Nobody has ever actually detonated one of the propulsion units in space and actually determined how much push it will give.  (Though it makes one king h--l of a shaped charge and if the rumors are true it's been tested on the ground.) and is the reason all but the gross details of the propulsion units are still classified.)  Nobody has ever exposed a pusher plate to the resulting plasma.  Nobody has ever built the shock absorption system except at the crudest scale.   Etc... etc...  Precisely nothing about the performance of the drive system is known with certainty.

Meanwhile, the F9H is an expansion of an existing and more-or-less well proven vehicle.

If you call them both equally theoretical, well...  that just makes no sense.

Nuclear physicists tend to understand nuclear reactions.... I'm just saying. The bomb would impart a certain amount of energy to the shaped charge and propellant, which then hits the plate, imparting its kinetic energy and then dispersing.

They know it'll work, and it's beyond theoretical. It relies on tests and calxulations of already proven physics. It works well on paper, but not much practice has occurred.

[Bill Phil]

7 hours ago, K^2 said:

Sure they do. Anything that hits the damper will impart momentum to the ship. The only reason to have that "propellant" buffer there is to increase total mass and reduce energy of particles striking the damper.

You can just detonate plain old nukes. You'll get nearly the same amount of impulse out of it. But you'll be damaging the damper as well, and possibly, irradiating the ship quite a bit more.

A plain old nuke would actually be very inefficient. The shaped charge redirects a large portion of the nuclear energy.

[fredinno]

3 hours ago, Bill Phil said:

There's a shaped charge with a cone in front of it, which is the propellant.

Nuclear physicists tend to understand nuclear reactions.... I'm just saying. The bomb would impart a certain amount of energy to the shaped charge and propellant, which then hits the plate, imparting its kinetic energy and then dispersing.

They know it'll work, and it's beyond theoretical. It relies on tests and calxulations of already proven physics. It works well on paper, but not much practice has occurred.

That's actually still considered theoretical.

[Bill Phil]

12 minutes ago, fredinno said:

That's actually still considered theoretical.

Even with tests?

Theoretical means that it's described by a theory. This is fact. It'll work. We know explosions produce energy and it's possible to focus that energy. They used chemical explosives to test the pulsing, they blew up a sphere coated in graphite and only ax small portion ablated away ( it was near to a nuke).

Edited February 17, 2016 by Bill Phil

[Steel]

13 minutes ago, Bill Phil said:

Theoretical means that it's described by a theory.

Exactly, and by most definitions a theory is a explanation based on well known laws or measurements. 

' As used in science, a theory is an explanation or model based on observation, experimentation, and reasoning, especially one that has been tested and confirmed as a general principle helping to explain and predict natural phenomena ' From www.fsteiger.com/theory.html :

Generally, anything that is not physically built and measured is theoretical, even if all calculation and expert opinion say it will work.

Edited February 17, 2016 by Steel

[magnemoe]

6 minutes ago, Steel said:

Exactly, and by most definitions a theory is a explanation based on well known laws or measurements. 

' As used in science, a theory is an explanation or model based on observation, experimentation, and reasoning, especially one that has been tested and confirmed as a general principle helping to explain and predict natural phenomena ' From www.fsteiger.com/theory.html :

Generally, anything that is not physically built and measured is theoretical, even if all calculation and expert opinion say it will work.

We have good knowlede in how nuclear explosions work. That said an ground launched Orion would have an high chance of failing because of lack of practical experience.
Shock absorber fail who then deform the trust plate is one fail node, 
Now if we talk about an large orion we are on thinner ground, I think the Orion starship specifications looks too good, yes its uses large nukes where 95% of the energy is fusion but you would still need an reaction mass I think, stats listed is better than an fusion engine, the fusion engine should work better as it don't need the bomb around the fusion material.