Radar Alcohol - Why?

[xxJohnxx]

Another thing they use the alcohol for is de-icing the wind shield.

Seems like they designed the plane around not having to use any air from the turbine. The only thing relying on the engine bleed air is the SPS/BLC system. Maybe given the relatively weak and simplistic engine they only had enough engine bleed air available for that.

 

De-icing and cooling is done with alcohol, brakes, brake-chute and other things are done by compressed air stored in bottles.

[Witchking]

Good to know. I remember thinking about the alcohol because even in the Su-27 documentary, you can hear the pilot say the little cylinder in the front landing gear bay door is full of alcohol. Pretty awesome and simple IMO...very nice to use more commonly available materials than the fancy alternatives often seen in western aircraft.

[CookPassBabtridge]

So hang on, was this purely an evaporative system then, as opposed to a close loop refrigeration type cycle? I guess that would make sense, otherwise there is no reason why you would run out! Was there a vent for it?

[CookPassBabtridge]

There will be a vent on both closed or open system. On the closed system, it's an over pressure safety device. On the open system it's necessary in order to produce the amount of evaporation needed for heat transfer. Alcohol is way more efficient in transferring heat than water, because it evaporates faster and have a lower boiling point. The con of such a system is gradual loss of alcohol as you use it. But very simple, efficient,cheap, trouble free and if having to refill it after each flight doesn't create undue logistical problems, then sure, a good idea.

In a nut shell, alcohol is routed (pumped) around the radar array with tubes. The alcohol being cooler than the radar, the heat will transfer to the liquid. The liquid will keep going til it gets to an expansion chamber, if it were. Evaporation will occur and heat will be dissipated as it is vented out in the atmosphere by it's own pressure create as a byproduct. Very simple system, a fraction of the parts (and cost) needed compare to closed circuit. You can also see an advantage in weight and space saving here.

 

You could always tell that Russian designers were very practical and ingenious and had to work within constraining limitations. The ruling parties were forever demanding better design but with a fraction of the money that would be available to western countries. They wanted maintenance to be idiot proof so as to shorten training time and enlarge the pool of potential candidates for technical schools. They wanted super reliable airplanes to minimize downtime between flights. On top of that, their aircrafts had to be able to operate at temperatures ranging between -60 to plus 60 Celsius. And lets not forget crappy unprepared landing strips as a requirement in case it becomes necessary to deploy some jets in remote areas. All in all, i think these guys did (and are doing) pretty good.

 

Thank you, what a great answer :)

[Flamin_Squirrel]

In a nut shell, alcohol is routed (pumped) around the radar array with tubes. The alcohol being cooler than the radar, the heat will transfer to the liquid. The liquid will keep going til it gets to an expansion chamber, if it were. Evaporation will occur and heat will be dissipated as it is vented out in the atmosphere by it's own pressure create as a byproduct. Very simple system, a fraction of the parts (and cost) needed compare to closed circuit. You can also see an advantage in weight and space saving here.

 

I'd bet money that the expansion happens just prior to the radar, not after it. You'd lose the benefit of the latent heat transfer otherwise.

[effte]

But you do need the evaporation to occur where the radar is dumping its heat. To allow it to evaporate after passing through the system would be to waste it.

 

In the simplest form, you just spray or drip the liquid on the cooling baffles of the system.

[Flamin_Squirrel]

It makes no difference if the system is open or closed. The process of vaporising a liquid into a gas (at a constant pressure, e.g. while expanding in a heat exchanger) requires energy, in this case provided by a hot radar. If this expansion occurs after the heat exchanger, the cooling benefit of the evaporation is wasted.

 

If the system is open, the hot gas gets vented overboard; if it's closed the hot gas is compressed and condensed (cooled by another heat exchanger) back into a liquid.

[fable2omg]

You win. You can be assured from now on i will not try to give a bit of what i've learned and did back to the forum to help.

Aw come on don't get yourself all worked up because some kid think they know it all, people like me who don't remember everything from school and don't know what to look for on wikipedia really appreciate your simple explanations. Thanks.

[effte]

Dan, old chum, is that you?! I had no idea! Don't worry, I have you covered. Here you go.

 

 

The explanation with an expansion chamber at the end of a total-loss system isn't a simplification or a broad description. It is incorrect, will not work (at least not efficiently) and will confuse those trying to understand the subject. That's why it needed to be corrected. Help those trying to understand the subject by just saying "oooops, that's right, sorry about that" rather than this, which only adds confusion.

 

A kettle is a different cup of tea. :D That was a good example, and is actually rather similar in principle to some systems which are in use. See below.

 

http://www.enginehistory.org/Reno/Reno2011/Reno2011Pub.shtml

 

Boilers for Cooling - In opposition to going fast the airplane confronts drag. One look at the Gold class racers and it is apparent that everything possible is, or has been done, to minimize aerodynamic drag. The wings and fuselage have been waxed, polished and seams taped, all to make a slick airplane. Another, and not so obvious source of drag is the air taken onboard for cooling, this creates drag because its speed must be slowed down to function in onboard services such as carburetor air, oil coolers, cylinder cooling and radiators. To minimize the onboard losses due to cooling and ventilation air most Gold teams go to elaborate lengths to fit and seal their cowlings so that every pound of air performs its intended function, then is ducted through a properly controlled exit. Even so, there are real benefits in minimizing the amount of cooling air. One way this can be done is to seal off the oil cooler inlets and reconfigure the oil coolers for evaporative cooling. Below is a diagram showing a system AEHS member Pete Law designed for this. It was flown on the one-time world speed record holder (piston driven propeller), Conquest I, when it broke the 1939 German record with a speed of 483.041 mph. Different physical arrangements have been utilized over the years, but the concept is that the oil cooler functions as a boiler, with ADI fluid (Anti-Detonation-Injection fluid, a 50:50 mixture of methyl alcohol and water) serving as the boiling medium. This mixture has the advantage that it boils at about 158oF (75oC) (at Reno’s 5,000 foot elevation), which results in the oil returning to the engine at near its optimum temperature of about 185oF (85oC). Since all of the equipment is internal to the airframe and no air is required, the result is a very clean airframe and low drag installation. The one downside is that range is severely restricted, that is, the aircraft must have sufficient quantities of ADI fluid to sustain it for the duration of the flight.

 

If I was to guess as to the actual design of the MiG's radar cooling, I'd expect nozzles spraying alcohol mixture directly onto the cooling baffles of the radar.

 

BTW, I single-handedly designed and flew both the space shuttle and the Spruce Goose, if claiming credentials on the internet is the way to establish credibility in technical matters. ;) Personally, I think I'll stick with going on the content of people's posts rather than their stated background.

[dpatt711]

My understanding is the Mig-21 is an interceptor. It gets to it's target and back in less than 30 minutes. 90% of the time is coordinated via ground control. Very small portion of the flight uses the radar. The radar was only designed to be used for fire control.

[Teapot]

Its Soviet tech. It doesn't have to have a reason!

 

Soviet tech ALWAYS has a very good reason .. it's always practical and always effective. Can't always say the same about western gear ...

[NeilWillis]

Or perhaps it's just that the MiG-21 is grizzly old campaigner, and it needs a drink in the morning to get it going!

[Random]

I wonder what would weigh more. Enough alcohol to cool the radar for the whole flight or the pumps and rads to go closed loop...

 

I also wonder why air cooling wasn't used. Air density too low for good heat transfer at altitude?

[Flamin_Squirrel]

I also wonder why air cooling wasn't used. Air density too low for good heat transfer at altitude?

 

Yes. Not only is air (even at sea level) 1,000 times less dense than water (alcohol won't be exactly the same, but you get the idea), it's also less than 4 times as efficient at transporting heat per unit of mass.

 

Therefore, if you needed to pump 10 litres/sec of water through the radar to keep it cool, you'd need to pump 40,000 litres/sec of air through.

 

In alcohol's case, as it evaporates, it's even more efficient still.

[DocSigma]

That's not a search radar, so it's supposed to be operated only for target acquisition. 20 min is more than enough for the job.

 

:pilotfly: