[Kryptid]

Maybe you should draw a diagram of this engine of yours. I am having trouble visualizing how it is supposed to work.

[Professor Mega-Mind (OP)]

Otay , Imagine if you will...
A hi-pressure gas cylinder . Mount it upside-down , attach gas supply lines thru the sides , pump in 1 M. psi. , attach your harness to it , then take a sledgehammer and knock the valve assembly off and "get ready for a surpriise!" , yeeehaaw !! 

[Kryptid]

Otay , Imagine if you will...
A hi-pressure gas cylinder . Mount it upside-down , attach gas supply lines thru the sides , pump in 1 M. psi. , attach your harness to it , then take a sledgehammer and knock the valve assembly off and "get ready for a surpriise!" , yeeehaaw !! 

So where does the "continuing shock wave" come into that?

[Professor Mega-Mind (OP)]

 The hoses are rediculously long, so the 1 Mil.psi. is maintained , ergo ; an infinity of mega-shock-waves !   Hole size will determine  mass flow & ex.velocity .  Also , the P/V graphic relationship is a ginormous parabola .  You will travel in one too ! 
 Nyuck , nyuck , nyuck !     P.M.

[wolfekeeper]

You are thinking of combustion-powered projectiles , I am using a continuing shockwave .  Their velocity is definitely pressure-driven .  Combustion is so weak !

Carnot is not to be so easily circumvented.

What's the peak temperature at the shockwave, and what's the exhaust temperature?

[Professor Mega-Mind (OP)]

~300F° in pressure chamber , thermal-energy drops in direct proportion to increase in volume .  Can either have very short pipe dumping straight to low pressure , or very long pipe with massive electro-thermal induction , for increased efficiency .  This would be much easier for small flow rates , being somewhat like a jet engine that keeps heating the exhaust gases ever hotter , hopefully all the way to plasma .
Okeley-dokeley , must propulsion my vehicle .                P.M.

[wolfekeeper]

An engine that only reaches a peak of 300F is inevitably horribly inefficient. The laws of thermodynamics cannot be circumvented; you're trying to turn a hot fluid into a fast fluid, that's a heat engine. You seem to be unaware that high pressures do not raise the speed of sound in fluids- and fluids leave an orifice at only sonic speeds (supersonic speeds, if the orifice has an expansion nozzle as well).

Very high pressure engines enjoy plenty of thrust, but the exhaust velocity and efficiency is not in any way raised due to that. For that you need high temperatures.

[Professor Mega-Mind (OP)]

You are right , mahn .  I forgot the Post-chamber , where the liquid  is superheated , THEN exhausted ,possibly into another Post-chamber , etc. .  The end result is a superheated gas stream released at somewhat less than the initial 1 Mi.psi .
Hadn't thought of this branch in years .          P.M.

[wolfekeeper]

FYI the problem with most engines isn't the low exhaust velocity-  velocities can trivially be made arbitrarily large, even lightspeed (photon rocket). The problem is getting very high exhaust velocity at the same time you get very high thrust. That implies extremely, ridiculously high power. Normal rockets give you that because the nozzle doesn't directly touch most of the flow. But if you're trying to heat a flow electrically to raise the temperature then you've got to build electrical stuff with very high power/weight ratio. This is beyond the state of the art.

[Professor Mega-Mind (OP)]

Yes , the laminar flow issue is a fundamental limiter , it imposes size/efficiency parameters .  The rest is an engineering challenge , similar to early jet engine development .  Sounds like an opporknockity for someone , I am willing to coach .   
》
Note : Injecting that much heat by conduction is beyond the state of the art , but could be accomplished by microwave injection .
                     P.M.

[wolfekeeper]

It's not an opportunity unless you have a plan that can actually achieve it.

[Professor Mega-Mind (OP)]

Eeek !  Money required !  I feel like Nikolai !   "The world is a vampire !"
Seriously though , there have got to be organizations out there that could use an actual Impulse Drive .  Didn't you think the Shuttle sucked ?
   P.M.

[Professor Mega-Mind (OP)]

           Design Clarification
 The Carnot critique is valid .  In addition to being released under tremendous pressure , the LNG must be raised to very high temperature as it travels down the reheat tube ( afterburner ) .  The tube must be very long , so that as the liquid bursts from the pressure chamber , the thermal energy (microwave) injection causes it to phase-change to a supercritical fluid , then a superheated gas , & then a plasma .  All of the while , the initial burst velocity is maintained/increased by the un-believably extreme  (10s of thousands of times ) expansion of the highly compressed LNG into superheated plasma .  The "afterburner" does not have to withstand as much pressure as the pressure-chamber , but it does have to withstand very high temperatures . 
 The very-long reheat tube would likely require gas-turbine type active-cooling mechanisms .  High-pressure reinforcement would also be necessary .
 In the end , the unbelievable efficiency of this propulsion package would come from the astounding energy-density of nuclear fuel .  The nuclear power-plant would be the "engine" , and the high-pressure drive would be the "transmission " . 
Unbelievable isn't it ?  A practical Impulse Drive .  I welcome any quantitative analysis ...........P.M.

[Professor Mega-Mind (OP)]

Can a relativistic exhaust  (.99c) be generated by near-future ,
accelerator-type , propulsion mechanisms ?  Does such an "Impulse Engine" give us the stars?
P.M.

[Kryptid]

Accelerating an electron up to 99% the speed of light would give that electron a kinetic energy of about 3.11 MeV. Using plasma accelerator technology, we can already accelerate electrons to more than a GeV in a space of a few centimeters. So that particular aspect of such a thruster would be no problem: https://en.wikipedia.org/wiki/Plasma_acceleration

However, when I take a look at such lasers as BELLA (which was used in a plasma accelerator), the pulse used to accelerate the electrons had only 42.2 joules of energy and lasted for only 40 femtoseconds. If you had the entire BELLA laser apparatus and its powerplant in space, no doubt those electrons it accelerated with a mere 42.2 joules of energy would produce pitiful acceleration. I'm somewhat doubtful that current technology could scale that level of power up in a practical manner without also arbitrarily increasing the mass of the system. I would need more information to be sure: https://en.wikipedia.org/wiki/BELLA_(laser)

[Professor Mega-Mind (OP)]

I'm thinking massive amounts of plasma at above .99c . However ,  I am questing for near-future capabilities .  You are right about today's .  Performance parameters are of particular interest .  The public really wants believable avenues to the heavens , let's give'em what they want !
P.M.  .
Note : Such an engine would be called an accelerator-engine .

[Bored chemist]

The public really wants believable avenues to the heavens , let's give'em what they want !

Does that mean you are going to stop posting stuff that's not believable?

[Professor Mega-Mind (OP)]

I believe in reaction engines , and positive effort , not sniping .
P.M.

[Bored chemist]

I believe in reaction engines , and positive effort , not sniping .
D.

Last week you believed that the conservation of momentum didn't work.
If reality does not agree with your beliefs, it isn't because reality made a mistake.

Explaining that you need to learn stuff before you can make useful progress is common sense, not sniping.

[Professor Mega-Mind (OP)]

 Look , I saw a supposedly unbeatable battleship of a problem , proclaimed "Damn the torpedoes , full speed ahead !" . I then strapped on a steel helmet , launched myself  at it max-force , and made a dent .  Meanwhile , the nay-sayers did nothing to help .   Now I tackle a different problem .  I'd prefer that the negatives back off , and make room for other people to contribute  , in a positive environment .  Let’s not hear any more personal BS , then maybe the readers will feel more comfortable participating in this thread .  Got it ? 
P.