The Donor Airframe

Where to Start?

In order to build ‘The Grassroots SST’, we are not going to start from scratch, that would be foolhardy and very expensive indeed. We will do far better to start with an airframe, which is already as fast possible, and then modify it to get the extra performance we need.

This approach, akin to taking a stock family road car, and then re-engining and re-tuning it to create a race car, is much more likely to be successful than an attempt to design a complete SST from scratch, as some others in the business are attempting. (I’m looking at you, Aerion, Boom)


Hot Rodding

There are several potential benefits to the ‘hot rodding’ approach, including;

  • Aircraft certification. By taking an established airframe which is already certified, we may negate substantial development cost, and can certify our aircraft with an STC (supplemental type certificate) rather than certifying an entirely new type.

  • The aircraft can be ‘grandfathered in’ in terms of it’s ‘certification basis’, that is, the year in which it was certified, and what revisions of DO-160, DO-254 etc. it was qualified to. Those same regulations can arguably remain applicable for the life of the airframe. Later revisions of the rules and regs, known colloquially as the ‘FARs and JARs’, are getting harder to comply with year on year, so this is a potential timesaver.

  • Aircraft systems. There are opportunities for someĀ of the guts of the aircraft, including the fuel system, hydraulic system, bleed air system and electrical system to be re- retained and re-used in the revamped aircraft.

  • Heavy and expensive assemblies, machinings and forgings may be retained in some cases, primary candidates being the landing gear, the wing spar, the keel beam and engine pylons.

There are also some negatives, including;

  • The degree to which the original Donor Airframe can be modified must be limited, in order not to deviate too far form the original cert basis, possibly losing the link to the originally certified aircraft at great expense to the original equipment manufacturer!

  • The donor airframe itself, may be quite long in the tooth, and have a very high number of cycles, and as a result, a short useable fatigue life.

There are however some precedent examples, such as;

STC modification shops like Sierra and Nextant make their money by upgrading old airframes with new engines and new avionics, the aircraft performance is improved, new life is breathed into the airframe, and the end result is a machine which outperforms the factory variant. This sounds like what we want, does it not?


Let’s not go there…

So, which aircraft would best lend itself to the Grassroots SST effort?

Concorde? Get outta here – not going to happen. I hope our friends at SCG may achieve it one day, but it will only happen if they can pull off what Dr Robert Pleming did with XH558, but multiply the cost, prodject duration and difficulty by ten.

Let’s be Realistic

No, let’s pull our heads out of the clouds for a moment – all we need is, quite simply, the current fastest civilian aircraft in the world, and whatever that aircraft is, it shall be the basis for our Grassroots effort.

I give you, the Citation X:

Either the Mach 0.92 MMO Citation X, or the Mach 0.935 MMO Citation X+ (Read X as in roman numeral, ‘ten’) would be an ideal ‘design mule’ starting point from which we will create our SST. Above is an example, a flying example can be had for approx $2.4 Million, used. We can work with that.

The Gulfstream G650 is worth a mention, which has an MMO of Mach 0.93 – however, the G650 only managed Mach 0.99 in dive testing, whereas the Citation X achieved Mach 1.12, the G650s also cost 4 times the price of a ‘ten’, and is a bigger, thirstier aircraft.


The Need List

The basic design of the Citation X is already very well designed for the transonic regime – it has fully powered hydraulic flight controls, a trimmable horizontal stabiliser and a high and low speed rudder, it is highly area-ruled with smooth transitions in cross section from nose to tail, a highly swept wing, and is certified to fly at a cruise altitude of 50,000ft.

But it will not, and cannot, go supersonic in level flight. Here’s what we’ll change;

  • The engines. The current Rolls Royce AE3007C (read, Allison) engines are a pair of medium bypass turbofans, these are the same engines used on the Embraer ERJ145 regional jet – a 40 seater airliner, on our 8-seater business jet. Ergo, the Citation X is already overpowered for its size, but to achieve supersonic flight we need Turbojets rather than turbofans, with both variable geometry inlet ramps and reheat/afterburner capability. The powerplant options that fit the bill for this requirement are discussed on the ‘powerplant’ page, follow the link on the sidebar.

  • The wing. The existing supercritical wing on the Citation X features speed brakes, leading edge slats with bleed-air anti-ice and extendable fowler flaps. It’s not a great low-and-slow wing because it’s optimised for transonic cruise only, but that works to our benefit – to reduce the drag enough to allow sustained supersonic flight, we’re going to have to increase the wing loading by chopping the wingspan slightly, or replacing it altogether.

  • The flight controls. The existing trimmable horizontal stabiliser and elevator will need to be replaced with an all-flying one-piece stabilator, to mitigate against flutter and compression. The stabilators also need to become our roll control at high speed, as the ailerons will no longer be effective at Mach > 1. The rudder and vertical stabliser can be left relatively unmolested, although the bullet fairing which covers the join with the stabiliser will need to be redesigned to cope with the increased aerodynamic stress.

Generally conceived, here is a very rough three-view diagram, of how all of that would turn out;

Additional Donor Parts

Try as we might, a pair of all-flying stabilators are not going to fall from the sky, nor will a decent pair of PFCUs to drive them, it would be extremely helpful, for instance, if we stumbled upon a military surplus fighter jet from which we could steal said parts.

Lo and behold, here we have just such a potential donor aircraft for sale from which we can pinch a pair of stabilators, a pair of PFCUs, possibly even the wings too, let alone the engines, the nosecone, the pitot tube and a Machmeter which reads up to Mach 2… I could go on.

There’s no guarantee of course that any of the parts would be ideally suitable, let alone whether a similar aircraft may still be for sale when the time comes to go hunting for parts, but it is a tantalizing possibility, and would make a huge difference to the development cost of our SST. There’s no price on the website, but let’s assume this Tornado F2 costs $500,000 for arguments sake.


The Build

Let’s imagine this is all possible, let’s imagine we’ve bought a Citation X flightworthy donor aircraft, and we’ve rescued a Tornado F2 to be cannibalised for parts, setting us back approx $3 million.

Let’s remove, and sell, the expensive bits we don’t need… the AE3007C engines? Sold. The interior? Gutted. The tailplane and elevators? Gone.

Our airframe should now look something like this;

The above example, courtesy of Wikipedia commons, is actually a written-off Citation X previously belonging to the ‘Buckle’ clothing company, which was damaged by a Tornado (ironically), whereas we are going to use a Tornado to makeĀ ours better.

The closing image for this particular page of the site is below, this is my (very bad) photoshop (MS paint) rendering, of sort-of how the Citation X would look (Credit to Cessna corp for original image) wearing a pair of afterburning turbojets.

That’s more or less it, if we skim the surface. More updates will follow as the project progresses.