Making of the “huffer” – Garrett-Airesearch GTC-85 Air Transportable APU – scratchbuilt, 1:48

Hi All! Happy new modelin' year to everyone first as this will be my first article this year. Ops! I didn't do as much modeling recently as I wanted but there are seome progress. I made the mistake last year that I wached Ken Burns and Lynn Novick's ten-part, 18-hour documentary series, THE VIETNAM WAR. I really enjoyed the experience and - guess what - after 27 years of absence I begin to make jets again (OK there had been a He-178 and a Pulqui but these don't count as "modern jets" right?). Well I thouth that prop. and golden age racers will be enough for me and no, I don't buy or collect any other era's aircraft anymore but such is life. I hooked again. I choosed the magnificent Hasegawa/Eduard A-4 as my "first" jet and at the same time I begin to collect articles and goodies for the project. I saw this strange little fellow on some pictures - on Vietnam war ones also - and I thought that I want one for my Skyhawk. For my surprise there is no one any on the market (except in 1:72 from F4Model - a great stuff judged from the pictures and the M1A1 starter cart from Special Hobby resin as a future release in 1:48 but that is stricly an USAF thingy not NAVY) so it was time to some more research and the usual "hard way".

The first I learned that the thing is a "huffer" The urban dictionary says that the huffer is "Someone who is addicted to inhalants. ... Spray paint is commonly used." - well that's probably me playing with my airbrush and some Alclad or Gunze Mr 🙂 . In aviation, a huffer is an external engine that creates large volumes of pressurized air used to start large gas turbine jet engines.

Here are some more info (if You are familiar with what is a huffer or APU does just jump it over): In general, the main challenge with starting a turbine engine is ensuring there is sufficient airflow before introducing fuel. If the turbine blades are not pushing enough air through the engine, introducing fuel and beginning combustion will cause the engine to overheat and will damage it. This is called a "hot start". The Auxiliary Power Unit is a smaller turbine engine that generates high-pressure air. The APU will spin up and some pressurized air will be 'stolen' from its compressor to supply "bleed air" for engine starting, the air conditioning packs, etc. The APU, being smaller, is usually can be started with a battery. Most jet engines use this method. Military aircraft often hasn’t got onboard APU to save weight (ie. A-3D, F-4D, A-4 until „M”, F-8 or NAVY A-7 etc). The Huffer is an external APU device powered by a gas turbine compressor and is the primary air start unit for Naval Aviation. The unit is housed in both a shore based and carrier configuration, in this case in an auxiliary fuel tank shaped pod that can be transported on an aircraft’s external storage point making it versatile in any environment. Because of its low to high flow start capability, it has the ability to support every aircraft in the Navy and Marine Corps inventory. The “Gas Turbine Compressor model 85” or the “Gas Turbine Compressor & Power model 85” must be the World’s most common and successful gas turbine compressor for aircraft starting. Many thousands have been built and used both in ground air-start carts and as APUs in aircraft. The model 85 consists of a two-stage compressor and single stage radial turbine and a single can combustor. Several different models of 85 series exist. A unit known as the GTC85 consists of a compressor unit with only a small accessory gearbox and so function only as a compressor unit. A small gearbox carries the engine accessories and a small generator for battery re-charging. The GTC85 series unit was also mounted in the USAF starting cart known as the MA1A and not to be confused with the gas turbine powered M1 Abrams tank of similar designation. This kind of air ransportable huffers - the subject of my model - were widely used int he 50’s 60’ and during the Vietnam war by the NAVY and the MARINES both onboard ships and on forward air bases. Blue Angels' Skyhawks always took their huffers with them on their tours. There are still many in use even today but You won't see them on NAVY aircraft carriers anymore as more modern solutions replaced them.

I collected lot of pictures and as many drawings, acticles diagrams etc. as I can and made some scale drawings - You can see some of them among the pics. After that I made a sketch how I want to make the model. I wanted to replicate some of the interior so I planned to make a "shell" like body first with vacuumforming or so. Fortunately I found some auxiliary tanks and fuselage parts in my scrap box close enough the shape I wanted to create as a starting point. I reshaped these parts as per the drawings and I could begin the carving and sanding and cutting and sanding and... oh did I say that sanding already? The various parts made from plastic sheet thin aluminium (aluminum? as You want...) sheets (for cover doors for example), wires and two spare wheels also from my scrapbox. You can see the main steps on the pictures - all just good old modeling stuff and elbow grease. When I finished and tried all the parts I made silicone molds to resin cast them. I obtained a vacuum camber last year and this stuff makes a real difference in molding and casting. My next project is to make a pressure camber. The most difficult part was the air hoose but somehow I could manage to make it - finally I could even bend and form the resin copy into shape in hot water quite easily. Just a hint.

The build was quite straightforward after this - I made two "huffers" at the same time because I can't decide which one looks better the white or the yellow one - now I like the yellow more. I even designed a decal sheet for the project but finally (as my printer striked than) I used some spare decals this time and simply paint the stripes - these indicate where You doesn't suppose to stand when the turbine is working (the moving parts are between the stripes so when the 42000rpm spinning turbine accidentally fell apart that's where something may fly out... well...). I hope You like it - and the article wasn't too long - sorry for my typos it's quite late here. All C&C are welcome as usual. Next time maybe about my attempt to make a small carrier deck display base... Have a great modeling time!

75 additional images. Click to enlarge.