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Cruise missile engines are relatively smaller in size. However, they might need the same level of expertise e.g. metallurgy, single-crystal blades, etc.

Do the design and manufacturing of cruise missile engines and aircraft jet engines require the same level of effort in terms of experience, infrastructure, and R&D investment?

Please, explain: Why.

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    $\begingroup$ It should be noted that many components, chemistries, and manufacturing techniques of aircraft jet engine components are highly-prized state secrets. Contrast with cruise missile engines which are generally expected to fall into enemy hands (albeit hopefully in pieces). To stress that point, if I'm not mistaken a US missile (air-to-air, not cruise) that failed to explode in a Chinese combat aircraft a few decades ago was the genesis of a few generations of Chinese air-to-air capability when the Chinese jet managed to land with the missile intact. $\endgroup$
    – dotancohen
    Mar 6 at 13:16
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    $\begingroup$ @dotancohen That was an AIM-9 launched by a RoC-AF F-86 and it plugged into a PLAAF J-5, intact. The Chinese tried to reverse engineer it but failed (of course, that was 1958) so it was transferred to Soviet Union. Then in 3 years we have a second source for AIM-9 for the other half of the world. $\endgroup$ Mar 6 at 15:39
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    $\begingroup$ Yeah, I thought it was a sidewinder but had no idea it was that early. Sidewinders in 1958? With that in mind the cannonless F-4s make a bit more sense. I thought that late 1950's missiles were more primitive. In any case, the point stands that missile components are expected to fall into enemy hands (today at least) whereas jet engine components can still be protected from such, to an extent. In fact I think an F-35 is being fished out of the ocean right now for that purpose. $\endgroup$
    – dotancohen
    Mar 6 at 16:04

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The service life of a cruise missile engine is a few hours at most, while airplane engines lasts 1000 hours at least. This alone makes things orders of magnitudes easier.

Also the engine accounts for less than 5% of the weight of a missile but more than 15% on a jet fighter, so less effort could be spent on weight reduction.

Additionally, for surface-launched cruise missiles, the weight budget of the missile is almost unlimited, meaning it could carry arbitrarily large amount fuel to compensate for bad fuel economy. The limiting factor for the range of the missile is more often navigation and homing, rather than fuel and fuel economy.

Finally, a cruise missile flies a at very predictable altitude and air speed and doesn't maneuver much, and always runs at full thrust. As a result, problems like compressor chocking and vibration only needs to be solved for those relatively simple working conditions.

As a matter of fact, other than some long range air-launched cruise missiles that are more constrained by weight and size (e.g. AGM-86), most cruise missiles below 300 km range are quite OK with turbojets even today.


As mentioned in the comments here, those comparisons are only between a cruise missile and military airplane. If you add civilian/commercial (passenger) airplanes to the scene, you'll have to deal with even more complications, such as:

  • State of the art fuel economy
  • Near-zero failure rate
  • Tens of thousands of hours of service life
  • Noise
  • Foreign object ingestion
  • Arresting engine fragments, had the engine been damaged while in operation

The list goes on and on. In short, an cruise missile engine is a minimum use case, i.e to generate sufficient thrust with acceptable amount of fuel for relatively short period of time, but airplanes engines, especially commercial ones, are concerned with much beyond that.

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    $\begingroup$ @MichaelHall Yes, updated. $\endgroup$ Mar 6 at 3:36
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    $\begingroup$ Isn't FOD just as important (if not even more) for military jet engines? $\endgroup$
    – Bergi
    Mar 6 at 13:31
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    $\begingroup$ @Bergi FOD is relevant for military jets, but for missile engines it's less likely to be an issue. When you're designing an engine for the many thousands of hours that a passenger jet will fly, chances are it'll meet a bird eventually. A particular design of missile accrues orders of magnitude fewer flying hours, with correspondingly lower chances of bird strike. Plus, cruise missiles are fired from launchers, rather than taking off from runways. Mostly the only time they have the engine running while near enough to ground for FOD risk is a split-second before impact, at which point it's moot. $\endgroup$ Mar 7 at 9:00
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    $\begingroup$ @anaximander I understand it's not an issue for missile engines. I just was surprised to see the point listed as a difference between military and civilian jet engines, not in the first part of the answer. And I wasn't even thinking of bird strikes but more of gravel on rough grass fields etc that military jets might need to operate from. $\endgroup$
    – Bergi
    Mar 7 at 10:21
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    $\begingroup$ @user3528438 of course civil aviation tends to get above most birds' flight ceilings, and stay there for much of the flight, while military aircraft can spend significant amounts of time down low where there are far more (big heavy) birds. I've often looked down on RAF jets in the Tornado days while walking in the mountains, but I've only seen civil aircraft from above at airfields (Concorde coming in to Filton was a good spot - they stopped the traffic and if you were lucky there was a really good view from the flyover just before the threshold) $\endgroup$
    – Chris H
    Mar 7 at 12:46
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An important difference between cruise missile engines and aircraft engines is their lifecycle.

A cruise missile has to perform only a single start and a single flight of at most a few hours. It is then disposed in an almighty bang. The majority of lifecycle costs are in the engineering and production (labour + materials).

A jet engine has to perform many starts, many flight hours. It goes through many temperature cycles. It will be maintained to extend its life. The total cost of the fuel it burns is a significant part of the lifecycle cost, and so are the maintenance costs. For jet engines, additional engineering costs to improve the fuel efficiency or reduce maintenance costs give a better return on investment than for missile engines.

Based on that,I expect the jet engine to require a higher level of effort in terms of experience, infrastructure, and R&D investment.

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    $\begingroup$ Are cruise missile engines ever started just to make sure they still work, to make sure lubricants and other materials have not deteriorated while in storage, etc? $\endgroup$
    – tml
    Mar 5 at 19:16
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    $\begingroup$ @tml the large Hound Dog cruise missiles once carried by the B-52 could have their engines started while on the pylon as needed "In 1960 SAC developed a method for using the missiles' jet engines to provide extra power for the B-52 carrier in flight or on takeoff. The missiles could then be refueled in flight from the bomber's fuel tanks. " web.archive.org/web/20071115211713/http://… $\endgroup$ Mar 6 at 1:44
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Not even the same order of magnitude.

The development costs for engines rank as follows:
Transport aviation >= Military jets >> Missiles

Some details on military engines are available in a report (admittedly an old one). It shows 100M to 1B to be spent on development per engine model. Smaller engines do cost less to develop. For major civil engines, the cost is up to 2B.

Most military engines are relatively low-tech, compared to airliners. Their reliability, efficiency and certification requirements are very relaxed. Fewer advanced technologies are used, simpler blade cooling, sometimes lower turbine inlet temperatures. A military engine will generally require more maintenance and repairs per flight hour compared to a civil engine of the same era.

However, special performance requirements, often unrealistic initial expectations, and large government budgets lead to some major high-cost military developments as well. Every once in a while, some of them do bear fruit.

For cruise missiles, the engine is almost an off-the-shelf item. The simple reason is that the greatest cost drivers in an aerial engine are reliability and fuel efficiency.

As far as reliability, missile engines are disposable items with low performance requirements. A reliability level of less than 99% per flight is considered sufficient for them. Some cruise missiles are expected to fail to launch or fail underway. More will fail for other reasons.

While efficiency means range for the military, turbofans already offer such an overwhelming advantage over rockets and ramjets that there's no need to work on a better turbofan to extend their range any further.

Simplicity is even more important in missiles, since their engines have to spend years in storage before actually being used.

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    $\begingroup$ I wouldn't say military jet engines are 'lower tech'. In fact, many share the same gas generator (F101/CFM56 or D-30) or are pretty much the same (JT8D/RM8). Specifically, turbine inlet temperature is almost invariably higher in military engines (even for the same/shared turbine), exactly because they can afford shorter lifespan / higher maintenance and risk. $\endgroup$
    – Zeus
    Mar 7 at 23:55

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