# How come cruise missiles always use turbofans and turbojets?
**URL:** https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973
**Category:** Factual Questions
**Created:** [July 1, 2015, 6:28pm UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973 "2015-07-01T18:28:44Z")
**Posts on this page:** 18
**Page:** 1
### Author:  [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)
#### Post date: [July 1, 2015, 6:28pm UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/1 "2015-07-01T18:28:44Z")
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](https://en.wikipedia.org/wiki/Small_Diameter_Bomb) ) give us a missile with hundreds of km of range for pretty cheap given the SDB low wing loading and 40 000$ price?
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### Author:  [@Habeed](https://boards.straightdope.com/u/Habeed)
#### Post date: [July 1, 2015, 7:08pm UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/2 "2015-07-01T19:08:07Z")
> [@MichaelEmouse](#):
>
> 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](https://en.wikipedia.org/wiki/Small_Diameter_Bomb) ) 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 :
1. ICE : Too slow
2. Turboprop : too slow
3. 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.
4. 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?
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### Author:  [@Habeed](https://boards.straightdope.com/u/Habeed)
#### Post date: [July 1, 2015, 7:15pm UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/3 "2015-07-01T19:15:35Z")
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.
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### Author:  [@Your\_Great\_Darsh\_Face](https://boards.straightdope.com/u/Your_Great_Darsh_Face)
#### Post date: [July 1, 2015, 8:38pm UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/4 "2015-07-01T20:38:53Z")
Some fast cruise missiles [do](https://en.wikipedia.org/wiki/Cruise_missile#Supersonic) 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.
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### Author:  [@TriPolar](https://boards.straightdope.com/u/TriPolar)
#### Post date: [July 1, 2015, 9:31pm UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/5 "2015-07-01T21:31:52Z")
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.
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### Author:  [@Quartz](https://boards.straightdope.com/u/Quartz)
#### Post date: [July 1, 2015, 10:11pm UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/6 "2015-07-01T22:11:16Z")
Edit - didn’t spot that **TriPolar** had beaten me to mentioning the V1.
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### Author:  [@Corry\_El](https://boards.straightdope.com/u/Corry_El)
#### Post date: [July 2, 2015, 1:17am UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/7 "2015-07-02T01:17:42Z")
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.
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### Author:  [@dropzone](https://boards.straightdope.com/u/dropzone)
#### Post date: [July 2, 2015, 1:58am UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/8 "2015-07-02T01:58:18Z")
> [@Habeed](#):
>
> Ramjets are not available in inexpensive, easily available forms, are they?
[Ninety-nine bucks for a pulsejet that can wake the dead.](http://www.hobbyking.com/hobbyking/store/ __34247__ Hobbyking_Pulse_Jet_Gasoline_Engine_34_Red_Head_34_with_Ignition_System.html) 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.
But yeah, they are really freakin’ loud!
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### Author:  [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)
#### Post date: [July 2, 2015, 2:15pm UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/9 "2015-07-02T14:15:59Z")
> [@Habeed](#):
>
> 1. ICE : Too slow
> 2. Turboprop : too slow
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?
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### Author:  [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)
#### Post date: [July 3, 2015, 2:06am UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/10 "2015-07-03T02:06:09Z")
> [@TriPolar](#):
>
> They need something to get up to Mach 1, and then accelerate rapidly from there to the much higher speeds where they are efficient.
> .
Yes, that’s why I mentioned that the ramjet would either be air-launched or have a rocket stage first.
Any idea about the approximate altitude limit for a ramjet engine?
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### Author:  [@TriPolar](https://boards.straightdope.com/u/TriPolar)
#### Post date: [July 3, 2015, 2:38am UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/11 "2015-07-03T02:38:15Z")
> [@MichaelEmouse](#):
>
> Any idea about the approximate altitude limit for a ramjet engine?
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.
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### Author:  [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)
#### Post date: [July 3, 2015, 3:24am UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/12 "2015-07-03T03:24:23Z")
> [@MichaelEmouse](#):
>
> 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?
IAI makes at least a couple of prop UAVs that also have warheads, which would make it conceptually a cruise missile.
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### Author:  [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)
#### Post date: [July 3, 2015, 4:31am UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/13 "2015-07-03T04:31:15Z")
> [@Throatwarbler\_Mangrove](#):
>
> IAI makes at least a couple of prop UAVs that also have warheads, which would make it conceptually a cruise missile.
I looked through several of them but found nothing. Would you happen to know their designations?
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### Author:  [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)
#### Post date: [July 3, 2015, 8:13am UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/14 "2015-07-03T08:13:07Z")
> **[IAI Harpy](https://en.wikipedia.org/wiki/IAI_Harpy)**
>
> The IAI Harpy is a loitering munition missile produced by Israel Aerospace Industries. The Harpy is designed to attack radar systems and is optimised for the suppression of enemy air defences (SEAD) role. It carries a high explosive warhead. The Harpy has been sold to several foreign nations, including South Korea, Turkey, India, and China.
> According to Air Forces Monthly magazine, In the late 1980s, Kentron sold the designs for its ARD-10 loitering drone to IAI. IAI then used those designs to d...
This one is powered by a rotary engine.
> **[IAI Harop](https://en.wikipedia.org/wiki/IAI_Harop)**
>
> The IAI Harop is a loitering munition developed by the MBT division of Israel Aerospace Industries. Loitering munitions are designed to loiter above the battlefield and attack targets by crashing into them and exploding.
> The IAI Harop has a loiter (flying) time of 6 hours and a range of 1,000 km both ways. It is a larger version of the IAI Harpy and is launched from ground or sea-based canisters, but can be adapted for air launch. The Harop uses a man-in-the-loop mode, being controlled by a remo...
This one looks like a turboprop.
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### Author:  [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)
#### Post date: [July 3, 2015, 9:23am UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/15 "2015-07-03T09:23:13Z")
Thanks. I see that both are used for SEAD.
This ([http://www.dror-aero.com/ac/iclean/iclean\_fr.pdf](http://www.dror-aero.com/ac/iclean/iclean_fr.pdf) ) on page 16 does say that the Harop is a push prop
It certainly looks like it: [http://www.satnews.com/cgi-bin/display\_image.cgi?2064323018](http://www.satnews.com/cgi-bin/display_image.cgi?2064323018)
With the SDB being launchable using a rocket booster, air turborocket ([Air turborocket - Wikipedia](https://en.wikipedia.org/wiki/Air_turborocket) ) and the Harop, the lines between planes, missiles and glide bombs might get fuzzy.
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### Author:  [@TriPolar](https://boards.straightdope.com/u/TriPolar)
#### Post date: [July 3, 2015, 1:48pm UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/16 "2015-07-03T13:48:48Z")
> [@MichaelEmouse](#):
>
> With the SDB being launchable using a rocket booster, air turborocket ([Air turborocket - Wikipedia](https://en.wikipedia.org/wiki/Air_turborocket) ) and the Harop, the lines between planes, missiles and glide bombs might get fuzzy.
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.
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### Author:  [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)
#### Post date: [July 3, 2015, 2:44pm UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/17 "2015-07-03T14:44:00Z")
> [@TriPolar](#):
>
> 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.
> **[Northrop Grumman X-47B](https://en.wikipedia.org/wiki/Northrop_Grumman_X-47B)**
>
> The Northrop Grumman X-47B is a demonstration unmanned combat aerial vehicle (UCAV) designed for aircraft carrier-based operations. Developed by the American defense technology company Northrop Grumman, the X-47 project began as part of DARPA's J-UCAS program, and subsequently became part of the United States Navy's Unmanned Combat Air System Demonstration (UCAS-D) program. The X-47B is a tailless jet-powered blended-wing-body aircraft capable of semi-autonomous operation and aerial refueling.
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.
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### Author:  [@TriPolar](https://boards.straightdope.com/u/TriPolar)
#### Post date: [July 3, 2015, 2:47pm UTC](https://boards.straightdope.com/t/how-come-cruise-missiles-always-use-turbofans-and-turbojets/723973/18 "2015-07-03T14:47:21Z")
> [@MichaelEmouse](#):
>
> 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.
> [Northrop Grumman X-47B - Wikipedia](https://en.wikipedia.org/wiki/Northrop_Grumman_X-47B)
> 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.
So if a manned plane launching an unmanned plane launching missiles is trying to take off on a conveyor belt…