« Last post by lamontagne on Today at 05:43 pm »
As far as I can see you have rediscovered solid fuel boosters.We don't need a metal liner! We are not going to penetrate the armor of the tank, all we need is a high-speed jet directed in the direction we need. In a conventional rocket engine, this jet is formed and accelerated by the Laval nozzle, in our case, this jet is formed and accelerated by the charge itself due to the conical recess.To form a jet, a shaped charge is accelerating a tamper, not the charge itself*. That tamper is not part of the combustion process, so is pure dead reaction mass that reduces your effective MJ/kg (e.g. if you have 1kg of explosive accelerating a 1kg tamper into a EFP, then your energy density if halved).TNT has an energy density of 4.184 MJ / kg. Explosives apparently exist with up to 2.38 times greater energy density than TNT (https://en.wikipedia.org/wiki/TNT_equivalent), but those explosives have not been synthesized in any quantity, probably for good reasons, so I'll use 1.90, the highest value for an explosive that seems like someone actually seriously considered using. With perfect efficiency the best possible exhaust velocity is sqrt(2 * 1.90 * 4.184 MJ / kg) = 3,987 m/s. That's not bad, but that assumes 100% efficiency, i.e. that the products all leave the rocket in the desired direction at the same speed. I don't know how efficient shaped charges can be but my hunch is you'd get less than half of this, probably much less, which makes this idea not competitive with traditional rocket propellants. Two things are hurting this idea: the energy density of explosives isn't as good as for bipropellants, and the efficiency of shaped charges is probably much worse than a nozzle's efficiency.Shaped charges typically use explosives with an energy density of 9 MJ/kg, which is comparable to methalox. But the most important thing is the gigantic pressure of hundreds of thousands of bars during the formation of a cumulative jet, which allows it to be accelerated to a speed of 10 km/s or more. For comparison, in the combustion chamber of the Raptor engine, the pressure is "only" 300 bar, and the jet velocity is 3.5 km / s, and this is an incredible achievement - it is not possible to significantly increase this value in a classic rocket engine.
Something similar has been suggested using nuclear bombs: https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) .
A nozzle in a rocket is needed for only one task - the formation of a directed high-speed jet. In a shaped charge, this jet is formed by the charge itself, so no nozzle is needed.
Then you have the problem of harvesting that energy for propulsion. A single shaped charge floating in space may be able to fling a het of hot copper to a few km/s. but there's no vehicle that accelerates in the opposite direction in that scenario. You'd need to contain the explosion somehow and capture the gasses that are produces as a result of the explosion in order to use them to push a vehicle forwards. The problem there is that whilst a shaped charge is good at accelerating a tamper in one direction, it's no so good at accelerating the combustion products in one direction: instead, they expand mostly radially: you do not get the equivalent of a lump of gas travelling at the opposite speed to the tamper jet.
You can compare this with an ion engine where the Laval nozzle is also missing due to the fact that the flow of ionized gas is already accelerated and directed in the right direction.
Even if the gas flow velocity is uneven (which is not a fact in the absence of a liner), then the average velocity value (and hence the specific impulse value) will still be higher than that of any chemical rocket engine.