For brevity’s sake, I used “always” in the title when that should be “always/almost always”. I don’t remember running into cruise missiles that used ramjets, turboprops and internal combustion engines. If there are, please mention them.
I understand why cruise missiles would often use turbofans and turbojets but I’m at a loss as to why they seem to always use those.
I get that in applications where space is at a premium, internal combustion engines and turboprops won’t do. So, no using them in vertical launch systems or carrying them inside airframes.
In applications where stealth is at a premium, high flying ramjets and exterior blade-spinning turboprops and ICEs wouldn’t do.
Yet, ramjet, turboprop and ICE offer significant advantages.
If one wants maximum speed with significant range, airlaunch + ramjet or booster + ramjet offer that. I’m not sure here but I would think that a ramjet engine would be cheaper than a turbofan or turbojet given its greater simplicity.
If one wants maximum range, ICE and turboprop offer that. Especially since cruise missiles often fly close to the ground where ICE and turboprop are typically better suited than turbofans and turbojets.
Wouldn’t sticking an ICE or turboprop on a small diameter bomb ( GBU-39 Small Diameter Bomb - Wikipedia ) give us a missile with hundreds of km of range for pretty cheap given the SDB low wing loading and 40 000$ price?
Ok, so jets are faster than turboprops. Ramjets are not available in inexpensive, easily available forms, are they? All commercial aircraft use some form of turbine engine. Turbine engines provide far more power/weight than ICEs, and you don’t care about fuel efficiency in terms of cost, you care about missile range.
So, working this out :
ICE : Too slow
Turboprop : too slow
Turbofan (with bypass) : high range, right below the speed of sound, you can develop a cheap jet engine using the same technology in commercial aircraft. A major aerospace company will have a horde of engineers who have designed jet engines before who you can put onto the cruise missile project.
Ramjet : see #3. Who’s going to design and build it? How reliable is it going to be? How much is the unit cost going to be?
I think that’s what it is. A “mature” technology - something you’ve built a lot of versions of and used on a large scale over decades - is going to perform a lot better than a prototype version of a technology. I suspect that actual ramjets prototypes available at the time the engine selection was made were not better, or not much better, than high bypass turbofans available at that time. Sure, on paper, not having blade drag means more performance, but you have to actually do years of development and testing and building prototypes and testing and then building more advanced prototypes and so on to get the theoretical performance in the real world. Even then, your engine won’t have the reliability of the competing conventional technology.
Some fast cruise missiles do use ramjets, but for the many that travel slower than sound, and at low altitude to boot, a turbofan is simply more efficient in addition to its wider availability. A ramjet would far rather poke along at Mach 2 or more.
Ramjets are inefficient below about Mach 1.25. They need something to get up to Mach 1, and then accelerate rapidly from there to the much higher speeds where they are efficient. Also a ramjet doesn’t have a way to power a generator on it’s own so an additional engine or batteries would be needed to power the controls and guidance system.
For subsonic speeds turbofans are much more efficient, and you need efficiency to get range and payload capacity. Turbofans and turbojets are essentially the same thing, turbofans just have a very high percentage of thrust produced by the fan. A turboprop is a form of turbofan but it requires gearing to power the prop at a lower speed than the turbine. Turboprops typically have a narrow range of operation efficiency and may not suit the requirements of a cruise missile. Piston engines might actually be efficient, but they will weigh more than equivalent turbines, and their top speed and altitude would be limited, however they might cost less than turbines which is important for an engine that only gets used once.
All of these possibilities must have been considered and turbojet configurations came out ahead.
A pulse jet cruise missile was proposed by an individual experimenter in New Zealand. Pulse jets were used in the Nazi V1 ‘Buzz bombs’, essentially a dumb version of a cruise missile. A pulse jet is simple and lightweight like a ramjet but operates at it’s peak efficiency at subsonic speeds. The theory is the weight saved in the engine allows it to carry more fuel to make up for not matching the efficiency of other engine types.
The full range of propulsion devices might somewhat depend how we defined ‘cruise missile’. If including any missile which is mainly supported aerodynamically on its fins/wings in relatively prolonged level flight then there are plenty of relatively shorter ranged ones powered by rockets, the French Exocet antiship missile being a very numerous example, but also true of some Soviet/Russian antiship missiles. Likewise as linked by another poster, some supersonic Soviet/Russian anti-ship missiles are ramjet powered. Long range land attack cruise missiles in the West have been pretty much strictly powered by turbojet/turbofan, but that’s not the full definition, and some of the faster missiles designed to attack ships also nowadays have land attack modes or versions.
The Russians were also developing a propfan (unshrouded fan turbofan, ie quasi turboprop) version of the Kh-101/102 air launched land attack cruise missile but it never reached production.
Ninety-nine bucks for a pulsejet that can wake the dead. Not stealthy. But why would a larger one be difficult and expensive to make? There is nothing to them, except a reed valve, a combustion chamber, and a tuned tailpipe.
Slower, yes. But too slow for so many applications that it’s not worth using even if it would result in great payload/range? I’m wondering why cruise missiles need to go at at least Mach 0.8. A lot of cruise missiles are intended for stationary targets. anyway
Do cruise missiles travel most of their journey at high altitude and only stay close to the ground in dangerous areas?
In theory, well into the stratosphere, so over 100,000 feet easily. But in reality I don’t know how high a ramjet has ever flown. The SR70 Blackbirds used turbojet augmented ramjets and flew over 85,000 feet. At supersonic speeds those engines were just ramjets.
The Soviets were developing a rocket-augmented ramjet ICBM. The ramjet was the middle stage of a 3 stage rocket. At some point when you are going fast enough and high enough you may as well use a missile following a trajectory.
It’s going to get fuzzier too. The success of drones show that remote fly-by-wire is feasible and practical. I would expect a new generation of remote control long range bombers, dropping smart bombs, and firing smart missiles, and launching cruise type missiles.
The X-47 was a research project intending to do that. The military was unsatisfied with it because it wasn’t stealthy enough. They’ll probably have to scale it up to be sufficiently stealthy.
I can’t find the page but I remember stumbling upon a UAV that was launched from a plane and then used missiles. So, the manned plane launches the launcher. I don’t know how many steps that process will end up having.