There is a Czech engine called the PBS TJ-100 that I’ve seen used on some jet powered gliders. It’s about 290 lbs of thrust and retails for $75k. There is also an uprated version, the PBS TJ-150 that’s about 340 lbs. I think that would be enough for an antiship missile with a smaller warhead like they’re building.
Is there any reason not to use much cheaper piston engines? If you're going for saturation of and defenses you don't need speed that much, you only need the ability to hit the target if not shot down.
That's the basic philosophy of what people are now calling "fixed wing suicide drones." It's just a cruise missile with a propeller and a piston engine instead of a jet.
Here are the tradeoffs as I see them:
1. The piston/propeller missile is slower, so it's easier to shoot down. If you have a certain detection range, you have more time between detection and impact. The traditional AEGIS type air defense on a high end ship has three layers -- long range missiles, short range missiles, and guns. Guns are the cheapest with the most ammo, but they're short range, so with a fast missile you only have a matter of seconds to shoot it down. If there are a lot coming at you simultaneously, you need more guns to get them all. If they're moving slower, this is a lot easier.
2. Lift is proportional to the square of the airspeed, so the slower you go, the bigger the wing you need. Compare for example the LRASM (the latest U.S. anti-ship missile), to a Cessna 172. They each weigh about the same (2500 lbs), and have very roughly comparable payload and range. Since the Cessna goes about 120 knots and the LRASM goes roughly 600 knots, that's a 5x difference in speed, so the Cessna needs roughly 25x bigger wings to stay in the air. You see this with the span, the LRASM has a span of 8 feet and the Cessna is 39 feet. Obviously the chord on the Cessna is a lot bigger too. The bigger wing will give you a bigger radar cross section, so you're detected sooner. It also makes the missile cost more due to all of the structure, and it makes it hard to put it in a launch tube if the wings are supposed to fold up.
I'm sure there are a lot of creative solutions to this with flexible wings, etc, but there are good reasons to prefer a faster missile. Whether that ends up being the war winning solution though, time will tell.
Things are a lot different than 20-30 years ago in terms of tradeoffs though. Back then your guidance system with a radar, computer, radio/satellite transceiver, and inertial navigation would weigh over 100 lbs, and cost over $100k in today's dollars, so making smaller missiles made less sense. Now, you can do guidance with GPS, strapdown inertial, and optical, and there are low power and low cost radio transcievers. Instead of a radar, you can do the terminal guidance optically with a multi-teraflop GPU that costs $50.
This sounds like a major liability, GPS is an extremely brittle and vulnerable system--you'd only need one or a small handful of nuclear warheads detonated in space to knock it out completely.
lmao at equating "extremely brittle and vulnerable" with "susceptible to multiple nuclear missiles shot to geostationary orbit". thanks for the chuckle.
If you (a superpower) deploy a bunch of GPS dependent equipment, and I (another nuclear power) am fighting you--and I'm less dependent on GPS--why would I not destroy GPS immediately, given that it's not very hard?
EDIT: Destroying GPS completely with nukes lobbed into space isn't the only way to mess it up either, it's susceptible to more terrestrial electronic countermeasures as well. It's generally not a good idea to depend on GPS alone, you need other backups as well. Pilots know this It's why they fly with a paper chart and a compass as well as GPS.
EDIT: Also, if my opening move is nuking space, I don't just deny you GPS, I also deny you all your other space-based advantages. I'd guess that would be one of the very first things any nuclear adversary of the US would do, because the damage is so asymmetric.
But the piston engine design really have to be slower?
For example, some WWII fighter aircraft could be quite fast, easily comparable to the claimed top speed of Tomahawk missile that can be found on Wikipedia and since you'd have less drag I don't think you'd need a huge piston engine.
I'm guessing you'd want something like 500 kW out of 200 kg of engine, but that's similar to the claimed performance of the apparently not-quite production ready Zoche Aero-Diesel.
Looking this up, Zoche Aero-Diesel were something like 1-3 horsepower per kilogram. You can buy a jet engines for model planes today that will generate 150 HP of exhaust gas power while weighing under 4 kg, giving you something like 30-40 horsepower per kilogram. The difference is just tremendous.
Piston engines are not even that much cheaper, and the only reason they are is that we produce literally billions of them every year, so it pays to invest in tooling. High performance piston engines are more complex, have more parts, and more complex assembly than simple jet engines designs that handily outperform them.
Yes, but those engines have tremendous fuel consumption.
90 ml per minute for an engine weighing 2.4 kg. Over an hour that's 5.4 litres, 2x the weight of the engine.
But with a heat exchanger to heat the compressor air with the exhaust the situation will be changed, so I still you're right to some degree but I am not sure about cost. Competing turbine engines are much more expensive than piston engines and you can actually build pistons engines at reasonable cost.
Turbotech in France produce a small fuel efficient jet engine, but it's like 90 kW weighs 90 kg and probably costs US$50,000+.
I think there's a reason a lot drones have gone with Wankel engines.
> Over an hour that's 5.4 litres, 2x the weight of the engine.
This perspective is completely wrong, because it actually penalizes smaller, lighter engines, all else being completely equal. Imagine you take a 100 kg diesel engine that produces 200 HP, and uses 10 kg of fuel per hour, and through insane feat of engineering, you minimize it to only be 5 kg, while producing same power and using as much fuel. This makes the engine obviously better, but by your logic, it now uses 2x its weight in fuel, versus just 0.10x as before. This logic is obviously wrong.
You need to instead ask: how much fuel does each engine need to produce equivalent amount of energy? On this metric, jet engines are not significantly worse than piston engines.
> Competing turbine engines are much more expensive than piston engines and you can actually build pistons engines at reasonable cost
Yes, if you compare a slightly modified off-the-shelf automotive piston engine produced in millions, to a small batch jet engine, then yes, the former is cheaper. However, if you need your drone to be both very fast and fly very far (and these two are very important in military context), then off the shelf automotive piston engines are really not an option, and instead you need to invest in something very custom, which would destroy the per-unit cost advantage of piston over jet, and that’s ignoring benefits like smaller form factor allowing for much bigger payload (extra 100 kg available for warhead does make a difference).
Yes, and that is in fact not my perspective, I only mentioned the ratio in order to illustrate that the fuel weight is substantial.
The right condition is of course J/kg for the fuel + engine + propeller/shroud/etc. package, and also taking into account cooling drag and frontal area.
>Yes, if you compare a slightly modified off-the-shelf automotive piston engine produced in millions, to a small batch jet engine, then yes, the former is cheaper. However, if you need your drone to be both very fast and fly very far (and these two are very important in military context), then off the shelf automotive piston engines are really not an option, and instead you need to invest in something very custom, which would destroy the per-unit cost advantage of piston over jet, and that’s ignoring benefits like smaller form factor allowing for much bigger payload (extra 100 kg available for warhead does make a difference).
The problem is that jet turbines of the required efficiency and weight are actually expensive. They have high temperatures internally, and continuously, and have all these blades and bearings. I think it might be feasible to make them much more cheaply than they can be manufactured today, but a Wankel engine can be mass produced with present technology and is not incredibly bad.
We are talking about American-built products on an American-built website. I think people would happily use metric units when talking about foreign products on foreign websites, but for some inexplicable reason, these are much less often the center of the conversation.
Ramjet has a high minimum flight speed. (Although this is an issue across most missiles--ground launch versions of missiles are the air launch version with an extra rocket on back that boosts it to a suitable flight speed.)
Really the cheapest and most reliable thing would be an electric launcher that threw JDAM-like gliding bombs. Probably could get pretty good range with a strong enough launch system.
Shaheds can fly at 4000m, good luck with your Maxim. They also try to catch them far from target areas by spreading roving AA squads all over the border, somewhat impossible at sea.
Something similar could also dive from 4000m gaining quite some speed.
