Posts about: "APU (Auxiliary Power Unit)" [Posts: 32 Pages: 2]

Considering the era in which the Concorde was produced I am curious as to why there was no APU fitted. It would seem to have been quite an inconvenience at times.


Was there ever any consideration given to fitting one and was the decision against the installation solely a weight issue ?

Hi Stilton, that is a question that we all used to ask ourselves; not having an APU was a major pain in the butt for the fleet, particularly at charter destinations, where air start trucks, GPU's and air conditioning trucks would all have to be pre-arranged.
One problem with 'Conc' was always one of weight, (for every extra pound you carried, another pound of fuel was required) so any APU installation would have to have been light, and worth the extra weight. But the main problem was one of 'where to put the darned thing. The only suitable space available for an APU was in the tailcone, aft of the tail wheel. Now a ready supply of fuel would have been available either from the aft trim tank, #11, or from one of the two trim galleries. (For stability reasons, tank 11 was invariably left empty during ground transits). The real crunch however, was how to arrange pneumatic services from an APU: Tank 11 was directly forward of the tailcone, so this would have meant either ducting the pneumatics THROUGH the fuel tank (not a particularly good idea ) or externally around the fuselage, which would have been 'draggy' to say the least.
You could still have had an APU powering hydraulics, and in essence electrics too (the emergency generator was powered from the Green System), but without pneumatics for engine starting and air conditioning, it would really have been a waste of weight. Still, it really is a shame that there was no APU.
Historically, there were 'sort of' aux power units fitted to development aircraft: The prototypes had two GTS's (Gas Turbine Starters), one in each nacelle pair, that could start the engines without an air start truck, but these never saw the light of day in later aircraft. The most unusual unit of all was the MEPU (Monogol Emergency Power Unit), located in the tail cone. This was manufactured by Sundstrand, and was fitted to all of the development aircraft. (A derivation of a unit fitted to the X-15!!). The idea was that if you had a four-engined flameout at Mach 2, this thing would fire up, power Green and Yellow hydraulics (plus the emergency generator, again from the Green system), and give you power and control down to a safe relight altitude. The MEPU was powered by Hydrazine rocket fuel (unbelievably unstable) and I seem to remember that the thing would run for about 8 minutes. There was no way that this monstrosity would ever be acceptable on a commercial aircraft, and so a conventional RAT was developed by Dowty for the production aircraft. (Also, the windmilling engines would give you full electrics down to Mach 1.1, and Hydraulics down to about Mach 0.7, so the thing had little practical use when supersonic anyway).
I hope this extended blurb helps answer your query Stilton.

TURIN ,
I've read "By the Rivers of Babylon" too..... and there is some fact behind the fiction.

History does not relate if and where the El Al Concordes would have had APUs.

However, history DOES relate, that the two Concordes ordered by Iran Air WOULD have had APUs (which would have made sense in the Middle East).
Now, Iran Air was the very last company to cancel its orders, and by the time they did, Concorde 214 (now G-BOAG) and 216 (now G-BOAF) were already well underway (they ended up flying initially as white tails).

As a result, both 214 and 216 still have the mounting holes and fittings for an APU. Don't ask me where, but experts know where to find them to this day....

ChristiaanJ
Both A/C 214 (OAG) and 216 (OAF) were Variant 192 A/C (British Unsold A/C). 216 was later converted to a 102 (British Airways) Variant, where 214 more or less stayed as a Variant 192. I'm not disputing what you say about possible APU mountings (I guess it would HAVE to be at the front section of the lower cargo hold somewhere) but I for one have certainly never seen any evidence of them. I'm still trying to imagine where the air inlet and exhausts would have to be arranged, not to mention pneumatic services ducting/hydraulics. Wouldn't it be interesting to find out?

It would be, yes!
I can try asking around, but can't remember my original source - it probably came up during a discussion of the MEPU on Delta Golf or on 01 several years ago. I very much doubt it would be mentioned explicitly in the SRM (Structural Repair Manual).
IIRC the MEPU lived in the tail (I'll look it up), so it's not impossible the APU was planned in the same location.

Hi Christiaan, yes THAT was the MEPU (Good photo of G-BBDG by the way). As far as fitting an APU in the tail cone, I still personally think that UNLESS you are prepared to pass a sizable pneumatic duct through a fuel tank , (Remember that tank 11 occupied the entire rear fuselage between the rear cargo aft wall and the front of the tail cone). then I don't think that this was really on. (It's quite possible of course that I'm missing something here, it comes with age ).
As far as the MEPU goes, all it really did was drive 2 hydraulic pumps; the Green System then powering the 40 KVA emergency generator; unless you are going to use the APU for engine starting and ground air conditioning, then I honestly don't think that there would be much point. It's interesting also to note that the MPU, being a rocket motor, needed no air intake, and as it was not driving any huge loads, the exhaust duct dould be quite narrow.

M2dude,

Nice set of photos of "The Thing" here :
MEPU at MAE at le Bourget .
This one is at the Air and Space Museum at Le Bourget, near Paris. My guess is that is was a spare, since the manufacturing date is 1973. 'SA flew in January '73 and 'SB in December '73.
IIRC, Delta Golf arrived at Brooklands with the MEPU still in place; I might have a photo.

As to the installation, we're obviously thinking along the same lines....

However, there were already several conduits through tank 11, such as hydraulics for the tail wheel, various electrics, and the 'backbone' fuel manifolds, that ended at the fuel jettison port in the tailcone.
A couple of fairly substantial air ducts would only have displaced a few hundred kgs of fuel at the most, out of the more than 10,000 kgs in tank 11.

And yes, of course, the whole point of the APU would be to have independent ground start and ground airco available, so clearly an APU would have been bigger and heavier than the MEPU (which was only just over 80 lbs), plus the problem of the air intake and bigger exhaust.
I'd have to get the drawings out to see how easy or difficult it would have been to fit one in the available space.

Since the tailcone was BAC, and both 214 and 216 were built at Filton, I wonder if anybody there still remembers?

ChristiaanJ
Thanks for the MEPU link, that really brings back memories (or was that nightmares ). I remember at Fairford, a small drop of Hydrazine leaked onto the hangar floor; the next thing you heard was a really loud crack, and a after the smoke cleared, there was a sizable hole in the floor.
I'd still really like to know what the 'thoughts' on this APU issue actually were. Although as you rightly point out tank 11 already had a fair amount of 'plumbing' running through it, we are talking here about a duct with sufficient size that can provide enough mass flow to turn over an Olympus engine to at least between 10 and 20% N2. You are looking at an least 10" diameter duct, not including the copious amounts of thermal insulation surrounding it, as well as an extremely sensitive overtemperature protection system. (This tank is going to be near empty, filled with fuel vapour). I'm not really convinced that this idea would even be considered by the CAA/DGAC/FAA etc. for safety reasons alone.
Still, it's food for thought though

There's an interesting piece here...

Concorde: story of a supersonic pioneer By Kenneth Owen, Science Museum (Great Britain)

Dr Hooker's argument that it makes more sense leaving the dead weight of an APU on the ground rather than carry it around at supersonic speed makes sense. (I presume this is the late Sir Stanley Hooker)

TURIN,
Thnks for the headup... I still have to get that book into my little library.

I've seen that quote before.
Dr. Hooker was right of course.

I think we'll never know what Iran Air's reasoning was......
Even with the vastly inferior infrastructucture in the Middle East at the time, it would seem to me it would have been much cheaper to station enough GPUs and ASUs at their various destinations (only two aircraft, after all), rather than go for an airline-specific APU fit.

Not really..
We are looking at two 4 inch diameter ducts, the same that are still used to this day on aircraft such as the B777.
One for the air start turbine, which was driven at about 45 psi nominal by the same kind of ASUs (Air Start Units) as are still in use today.
The other for the aircon... not to be confused with the huge near-ambient pressure hoses that were connected directly to the cabin, rather than to the aircon packs.

Absolutely fascinating, the fuel burns are way beyond what I expected.




One of my favourite sentences is that TOC=TOD !



It all makes sense though.




Not to beat a dead horse, but, on the choice of location for APU, the 727 had a problem with this but for different reasons. Because of the location of the engines that were all mounted at the rear, the Aircraft was quite tail heavy and adding more weight with an APU in the tail section was not desirable.



The solution found that I have not seen in any other Aircraft was to mount it in the wheel well transversely across the keel beam with the exhaust out and over the right wing. Quite unusual but it worked fine with the restriction that it could only be operated on the ground.



Its all academic now but, just out of curiosity could this have worked on the Concorde ?

Biggles78
I know these fuel flows seem crazy (If take-off fuel flows had been maintained the endurance of the aircraft would have been about 55 minutes!!). But as the majority of the flight was carried out at Mach 2 and above, with the relatively miniscule fuel flows, you can see how we were able to cross the Atlantic with relative ease. It was the subsonic bit that was the pain.
The powerplant was as you say truly amazing. We had an, as yet, unmatched engine/intake combination, with a variable primary and secondary nozzles. The variable intake allowed supersonic operation with maximum pressure recovery, minimum aerodynamic drag, as well as extreme operational stability. (Extreme temperature shears, that would have caused surge/unstarts in military installations) were dealt with as a total non event). It's astonishing to believe, but at Mach 2 cruise, the intake provided approximately 63% of the powerplant thrust. It was controlled by the world's first airborne digital control system. (The system computers were built by the Guided Weapons Division of what was then BAC). The combination of the variable intake, plus the LP and HP compressors gave an overall compression ratio of 80:1.
The engine itself, being supplied with air at an ideal pressure, could run at an almost conststant TET, thanks to the variable primary nozzle. This also allowed N1 and N2 (corrected for total temperature) to be controlled more or less independently and run as close as possible to their separate surge lines throughout the entire flight envelope.
The variable secondary nozzle (wide open above Mach 1.1) allowed the jet efflux to gently expand against a cushion of air that was passed over the rear ramp of the intake, through the engine bay and into the annulus of the nozzle itself. This prevented thrust being wasted by the jet efflux widely splaying as it met ambient air that was at a pressure of as little as 1.04 PSIA.
It was this integrated powerplant that made true supersonic cruise possible
The airframe life issue was sort of like 'how long is a piece of string?'. The airframes are lifed in supersonic cycles, (which had been extended before, with modifications) and studies were always underway as far as further life extensions were concerned. (Basically the airframe was as tough as a brick outhouse in structural terms). The only real area of concern was the crown area (the roof ). There was a design flaw here in that the structure had not been designed fail-safe (allegedly by designed a Korean designer at A\xe9rospatiale who, it was said, went a bit loopy). When the FAA evaluated the design (in order for the aircraft to be registered in the USA, for Braniff operations out of IAD) they wanted 'crown planking' to be fitted externally, which would have added over a tonne to the weight of the aircraft, as well as producing some not inconsiderable drag. Fortunately a compromise was reached and additional NDT inspections were carried out, as well as more limited structural modifications. There was a long term, cost effective solution being studied, which would have cured the problem altogether. (The changes would have been mandated, over new requirements for ageing aircraft)

Nick Thomas
Nick, the whole expansion issue was one of the biggest issues that had to be addressed. Wiring looms would 'snake' in some underfllor areas to take up expansion, but the biggest difficulty of all were the mulitudes of hydraulic lines. These required sliding expansion joints, with of course seals to prevent leakage. When a seal deteriorated YPU GOT A LEAK!! (Fluid at 4000 PSI tends torun for freedom very quickly ). As far as fittings go, ChristiaanJ is quite right, you tried to anchor at one end only. I seem to remember that the passenger seat rails travelled over a roller afair. Fuel lines wer less of a problem, because their relative lengths were less.
I also agree wholeheartedly with ChristiaansJ's explanation about the 'friction' thing, I never really liked those stories. As a matter of interest, 127 deg's, for Mach 2, that would be at ISA +5 (-51.5 deg's C). Any warmer than that and we could not achieve Mach 2, due to the Tmo limit of 127. I remember one year, for several weeks we had unusually high north Atlantic temperatures; these impacted both the flight time AND the fuel burn. The further away you were from Mach 2, the higher the fuel consumption. (The faster you flew, the less fuel you burnt. How's that for a paradox?).
At ISA (-56.5 deg's C) temperatures, the total temperature was at around 118 deg's C.

ChristiaanJ
I remember the 17.5 degree position on the nose; it always looked as if the aircraft was trying to eat ants to me . I can not recall personally anyone removing the 12.5 deg' stops for access, although this could of course have been done on your side of the 'puddle' I guess.
As far as the APU ducting issue goes (hee, hee, not often we disagree Christiaan ) we are just going to have to agree to disagee about this, although I accept that two 4" diameter pipes (PLUS THERMAL INSULATION) might have done it, BUT I still stand by the other points.

Stlton
Unfortunately not; the keel beam area was extremely thin and there was not anywhere near enough room. Interesting solution on the 727 though, I never knew that one.

I think we already do agree about the story that Iran Air was going to have an APU, that Rotring had already been put to Mylar to sort out the how and where, and that some traces are likely to be left in 214 and 216.

So I happily agree to disagree on the rest... between the two of us, each looking at our own clues, and with the help of anybody else who has more info, we might still find the answer!

One thought I had... with an APU in the forward baggage hold, you'd also have to take the air intake and exhaust through the pressure hull, and provide sound and thermal insulation for the entire APU itself.
From a design point of view, I'd have gone for the same location of the earlier pseudo-APU (the MEPU), and then solved the remaining problems from that starting point.

Yes we always started just the two inboard engines prior to push back and the outers when the push back was complete. This was for a number of reasons, but I do seem to remember it was not unheard of to break the tow bar shear pin on the initial push, so the less power the better

Remember that Concorde had no APU and no across the ship ducting for stating engines, therefore prior to push an air start unit was plugged into each pair of engines and the inboard engines would be started. This allowed, after push back, air from each inboard engine to be used to start it's outboard engine.

The other good reason for starting the inboards prior to push was that with no APU the cabin temp would rise quite quickly [specially in places like Bahrain in summer] and never mind the passengers
comfort, but some of M2dude and ChristiaanJ fancy electronic equipment was very temp sensitive , especially those intake control units down the rear galley. With Two engines running we could use their bleed air to at least try and hold the cabin air temp during the push back

When we first started LHR-IAD flights[ prior to thin lip///54% and other mods which improved our range] some thought was given to towing the aircraft to the taxi way near the end of the runway before starting engines so as to save fuel. I do not remember this actually ever being done though.

\

Notfred
Love the lightning story, hadn't heard that one before.
That would have been production series test aircraft G-BBDG, A/C 202 before a purpose built hangar (more shed really) was built to house her, with fin and U/C removed. This aircraft has now been beautifully restored at Brooklands museum.
You are quite correct about the pushback, not having an APU (THAT story again ) meant that a one engine in each nacelle pair had to be started on the gate, and the other in each nacelle started after push. Having a symetrical pair started enabled all 3 hydraulic systems, and hence most of the critical systems to be checked puring pushback.
Brake temperatures always had to be monitored; they really could get very hot. If a wheel was still too warm after T/O, then the gear would be left down just a little longer to aid cooling. (Each brake also had an electric cooling fan).
Idle thrust was always a problem in that it was too high; there was a 'lo idle' setting, but depending on the temperature of the day the difference was not that big. You could not just reduce idle some more because of a malady known as rotating stall. This can plague any engine, but the Olympus 593 was particularly susceptible. At very low idle speeds, pockets of air 'rotate' around the first few compressor stages and can completely alter the airflows through the engine. It is important that the engine is always accelerated quickly through this zone on start-up, because serious damage can occur if the engine runs for any period of time in the rotating stall region. If the engine DOES operate in this zone, then the combustion process can even occur in the last few stages of the HP compressor, causing extreme damage. This damage, although malignant, can result in blade failure and the subsequent damage to the combustion chamber and turbine areas. This can occur within a few flights of the event, so just cranking down the idle was never an option.

That's post #1 that started this thread.
The subject was dealt with in some depth, and in the course of the discussion is was described how the two preproduction and the first two production aircraft were equipped with an MEPU (monopropellant emergency power generator).

I've only just found this photo again... it's the MEPU of Delta Golf (202 - G-BBDG).
The photo was taken after 'DG was moved to the Brooklands museum, but before the tail cone was put back into place.





CJ

I don't think you'll be finding hydrazine on a pax-carrying aircraft anytime soon! And it wasn't an APU as such, but a source of power for non-normals potentially found in the flight test programme. M2D and ChristiaanJwill know far more.

The charter flights were different insofar as they often went to non-BA stations, so there was a bit more donkeywork to be done to get all the paperwork organised, but nothing a regular charter pilot won't be used to. Generally one would get a fuel plan/flight plan filed from Ops at LHR, but apart from LHR/JFK/BGI/IAD (and presumably MIA/BAH/SIN in earlier days) we produced our own loadsheet. I only once had to produce a fuel plan/route plan from scratch and that was at Sondrestrom (as it was) with a dodgy fax line. You'd have to file a flight plan occasionally.

We carried a ' PR ' on most of them - a line pilot or FE - to carry out a running PA and do general liaison. They were volunteered to do the loadsheet.

The atmosphere on board was very different - these were pleasure flights and so were the opposite of the JFK business run. Landlady may be able to elaborate on this.

The round-the-worlds were just a big charter in this respect. As you note we carried a 'flying spanner', since Concorde-qualified LAEs are hard to come by downroute. It looked like a great job on paper, but they were often at the airport for many hours before or after the sectors carrying out routine maintenance or dealing with snags.

I enjoyed the charters a lot - everyone was geared up for a good time and in general the flight had something different for us, too: Whether a lightweight departure on a 'round-the-bay', squeezing into a short runway (e.g. Bournemouth), visiting SFJ or Rovaniemi, or setting off around the world, predominantly to non-BA destinations. My favourites, though, were the RTBs out of Filton - EVERYONE was either connected to Conc development or manufacture, or was related to someone who was. Fantastic atmosphere. Shame the runway wasn't a bit longer........

If it wasn't for people like you, asking the questions, this thread would long since have petered out.
I know the story well... and I've seen some of the photos where all that was left of the aircraft was a huge black smear on the ground....

As mentioned earlier, hydrazine was "tamed" enough in the postwar years to make its use for EPUs in the military feasible (such as the F-16), where the weight gain is more important than the difficulties of handling the stuff.

Even so, just as well it was never used on the in-service Concordes... it's not the kind of product you want around on a civilian airport....
Just imagine a Concorde at the gate.. everybody being cleared away in a wide radius... a few people driving up with goggles, helmets and asbestos suits...
"What IS going on ??" "Oh, nothing, just topping up the APU"....

jodeliste ,
I have Stanley Hooker's book too. If you ever find that tape, put me on the distribution list for the DVD (all costs reimbursed, of course!).

CJ

May I just chip in with what a wonderfull read, and to think this thread started with a simple?? question about the non existant APU 3 months ago!!

ps Landlady where are you?? we need another recollection please from the cabin to break up the technical stuff we may not fully understand but still find totally enthralling in a typical understated British way

ChristianJ and M2dude et al many thanks