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Stealth aircraft have low visibility on radar.

I assume that they strive to achieve low visibility optically as well, e.g. spend some effort to not stand out too much visually from the surroundings.

A condensation trail (during daytime against a bright sky) could definitely make an invisible aircraft visible.

So what do stealth aircraft do against condensation trails?

Do they have technology aboard to avoid or minimize them? Or it just a matter of choosing the right conditions (clouds, humidity, altitude, temperature) to fly?

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wikipedia only mentions choosing right altitude –  ratchet freak Aug 29 at 11:30
    
@ratchetfreak and the WikiPedia article about stealth aircraft does not mention contrails at all. –  florisla Aug 29 at 15:11

3 Answers 3

up vote 27 down vote accepted

Do stealth airplanes need to limit contrail generation?

Not just stealth aircraft, most military aircraft are required to avoid contrails. Contrails form due to moisture in the aircraft's exhaust. A tried and tested method by NASA is NOT to fly in regions of air that support contrail formation.

Ophir's Pilot Alert System which is used by the B-2 stealth bomber uses LIDAR (light detection and ranging) to differentiate contrails from clouds and tells the pilot to change his altitude as/when necessary.

That being said, it is essential to remember that though contrails are visible to the naked human eye they are NOT visible to Radars. Ergo, until the stealth aircraft is really close (assuming it is on a SEAD/DEAD mission) one cannot see the contrails and by the time they do, it is too late. Moreover, stealth aircraft generally operate during night when contrails are less visible.

Stealth aircraft have low visibility on radar.

Not necessarily. VHF radar should be able to detect them at long range. Moreover, RCS of aircraft is a complex phenomenon, depending on many stuff (frequency, aspect angle, polarization of signal etc). You will never find a "static" figure for this.

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not to forget their infrared signature. A high flying B-2 can be spotted from 400 km distance with a good IR sensor due to the heating of the leading edge (just a few degree, but very hard to avoid). And that is long before the airplane arrives, not when it is almost overhead. –  Peter Kämpf Aug 29 at 14:27
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"one cannot see the contrails and by the time they do, it is too late" .. not true for all cases. For example -> the jet(s) are flying to a far off target (reactor? ) and need to avoid detection for quite some time. In the meantime, if some farmer on the ground spots the jets early and gives warning, the target may have time to prepare resistance. –  a20 Aug 29 at 15:09
    
Leave alone a farmer without the help of an acquisition radar it is impossible to differentiate between jets that are flying at altitudes of 20,000 ft plus.Fighter aircraft(including stealth) are NOT supposed to fly very long distances.They are based close to the theater, so that they can react quickly.Countries like the U.S, China, Russia who have a constellation of satellites at their disposal can very easily track a Stealth aircraft from the moment it takes off, contrail or no contrail. –  D_S Aug 30 at 9:48
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@D_S: Who said anything about fighters, though? The B-2 stealth bomber is reported to have a range of 6,000 nm, and its Wikipedia article describes a number of missions where it flew nonstop from the continental US (with in-flight refueling) to attack targets in Africa, Central Asia, and the Middle East. –  Nate Eldredge Aug 31 at 22:35

Stealth aircraft include options to reduce visual signatures, so yes.
This is achieved as a side effect of reducing the infra red signature, which is done by mixing the engine exhaust with environmental air before it is ejected, cooling the air a lot and thus making contrail generation far less likely.

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actually some option include heating the air so the trail that forms evaporates again more quickly –  ratchet freak Aug 29 at 12:55
    
I also find it counter-intuitive that making air colder would avoid condensation. –  florisla Aug 29 at 15:07
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The cooling of the air is to lower the IR signature of the engines. This makes it harder for IR radars and heat seeking missiles to track the aircraft. I am unaware of air cooling having any effect on contrail generation which is generated by pressure changes in a high humidity environment. –  tmptplayer Aug 30 at 0:40
    
"cooling the exhaust" does not mean down to condensation temperatures - it means lower than the 1500+ degrees of normal jet exhaust. –  paul Aug 31 at 9:17
    
@paul true, but lowering the temperature will help, even if not 100% effective. They sometimes also inject chemicals into the exhaust. Also not 100% effective but it helps. –  jwenting Sep 1 at 6:31

To directly answer your question, stealth aircraft have no special equipment to limit their contrails. The military does though incorporate atmospheric data into their flight planning to minimize the creation of contrails especially during daytime operations. This is true not just for stealth aircraft, but any aircraft which is concerned about being detected visually (such as reconnaissance aircraft, etc).

NASA has a website that projects the formation of contrails. Essentially it looks at the relative humidity with respect to ice (RHI) in the atmosphere, and if it is greater than 70% the region is flagged. Areas with greater than 100% RHI are very likely to support the formation of contrails.

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