September 14, 2018, ©. Leeham News: In the last Corner we looked at the change of aerodynamic center when passing Mach 1 and the resulting trim change.
We also discussed the high pitch angle of a delta like the Concorde when landing, which brings visibility problems for the pilots. Now we look into the problem of skin heating at high Mach flight.
When airliners fly at normal cruise altitudes and speed, the temperature of the aircraft skin is around -25 to -30° Celsius.
When we increase the speed to Mach 2.0, the speed of the Concorde, the temperature of the outside skin varies between 90°C to 105°C, dependent on the position on the aircraft. At the tip of the nose, the skin temperature is 127°C.
Should the Concorde have flown at the originally projected speed of Mach 2.2, like the Boom Supersonic’s project, the highest temperature increases to 150°C.
The problem is not only the high temperatures but the temperature cycle of close to 200°C. When climbing to the altitude for passing the sound barrier, the aircraft will be chilled to around -30° to -40°C. Then the surface will heat to over 100°C when cruising at 50,000 to 60,000 feet.
At these varying temperatures, the long-term fatigue characteristics of materials become critical. The Concorde lowered the speed with Mach 0.2 to stay with Aluminum alloys in the aircraft skin.
Boom Supersonic is planning on using high-temperature Composites to master the temperature challenges.
While there has been research into high-temperature Composites, the proof of their suitability for commercial transport with a structural life of 30,000 hours, exposed to such temperatures, will be a long and hard process.
The problem is that testing for long-term hazards for aeronautical materials can only be accelerated to a degree. Long-term testing takes…..a long time.
The real hurdle will not be candidate materials. It will be getting FAA acceptance and certification of a Supersonic airframe flying at Mach 2.2 for hours, every day of the week, and lasting 30,000 hours.
Aerion Supersonic’s SST avoids the problem by flying at a modest Mach 1.4. At those speeds, the skin heating stays below the effect of the Sun on a parked aircraft. The skin will be heated to 20°C.
In the next Corner, we will start the discussion about the SST’s biggest challenge, the engines and nacelles.
One trick is to use Titanium Alloys on those few locations that are heated over Alu fatigue capability like on the wing l.e.
The Aircraft nose is often composite due to weather radar requirements and models found on supersonic bombers would work fine.
One advantage with the higher skin temperature is that the condensation of moist air on the inside skin is avoided.
ok Claes, what about CFRP as skin material and M2.2? Any idea?
I was hoping that you were going to give us the answer!Epoxy tooling resins have an easily high enough Glass transition temperature (this is the temperature at which point they go soft after post curing),but they often only guarantee them for a few dozen or a few hundred curing cycles. Yacht masts get up to a maximum 70/80°C (this is the worst case scenario for most dark things left out in the sun) ,which is below GTT,which is usually between 90 /150°C even for a standard resins.They normally seem to be alright, although I have come across heat stress cracking well below the GTT.Information is incredibly difficult to obtain about this subject.Yacht masts are much more likely to experience non compliance!
Well, there are a few NASA and NMAB reports about materials for SSTs but they are not conclusive. Funding has allowed some tests but as I write in the text the testing is far from looking at long-term effects which are needed for a certification of a civil SST.
As there have been no SST projects for 30+ years there has been no money for research. The military advances which have been made can direct to the promising materials which can be a start, but these are not qualified to the use and maintenance procedures of a civil airliner.
The problem is the projects which want to fly at M2+ and use new materials will have to conduct, pay for and take the time hit for the research and qualification themselves. There are no national civil SST research programs that would do it for them.
The carbon is not really the problem at these temperatures its the epoxy, they are developing more an more high temp versions, the first was BMI ( bismaleimide resins ) of A330 generation but here are even better versions used in engines and fuselage of the F-35, one problem is that they tend to be a bit poisonous and hard to repair.
Still for a SSBJ how many cycles do you need to design for as the engines most likely will have a disk life limit of 2000-3500cycles due to high speed cruise engine temperatures? Al-Li alloys might be good enough as Space-X use it for its Falcon rockets. For top performace like of the latest fighters you need CFRP or Boron fiber ceramic matrix but it cost and you have pretty low production volumes so my bet is that Al-Li allys will be good enough for most SSBJ and small high speed airliner applications. You can easy lose performace in wing/body design, engine inlet and exhaust design for perfomance and noise and you always have to design control surfaces to be able to handle an engine unstart at supersonic speeds. If Ed Heineman would be around I think he would design a small and light weight mainly Al-Li built SSBJ with carbon wings.
Materials good for 30,000 f.h?
At 4 hours per sector and 2 sectors per day, that would give an airframe life of 7500 f.c. and – assuming it flies 5 days out of every 7 – an elapsed time life of a little over 14 years. After that, it’s scrap.
In comparison, the structural limit of validity on an A320 is around 60,000 f.c, I recall (it may be more by now).
It would be good at the end of this series to bring together all the technical issues and assess them as economic issues, and assess their total effect on the business case for designing, buying and operating SSTs. I understand that BA and AF only got away with it because they bought the aircraft for £1 each. And that was back when £1 meant something!
I used 30k flight hours as these were the assumptions in the NASA reports I looked at. But agree, it will be higher. It’s a good idea to finish the series with an economic discussion, will do.
Spending endless hours at M2+ does at least anneal a metal airframe, at least to some extent. And it gets baked dry.
I’d heard (second hand admittedly) from the Concorde maintenance team in BA that airframe life wasn’t ever going to be an actual technical problem, they’d just have to constantly get the fatigue life reassessed. This happened once in its service life, the permitted hours being extended after dismantling one and finding no degradation. The only actual problem would be spending the money to do this, and whether the market would bear that.
I think the SR71 programme had a similar finding.
So could it be that a metal airframe might be more sensible than a composite one? Composite materials don’t improve with age.
AFAIK some radomes on supersonic military jets are either ceramic, or composite but with time limits on how long the jet could keep burners lit. The EE Lightning was like the former (fibre glass as far as I know), the joke being that it’d run out of fuel before the limit. And military jets these days aren’t being built to be as fast as their predecessors.
For the Concorde the various airframe checks were at:
A Check 210 hrs
B 420 hrs
C 1680 hrs
IL 6000 hrs
D 12000 hrs ( this could be 6 months and included upgrades)