Technical Talk

[Engine 711]

Gnat XM691 was the first of 14 pre-production prototypes of what became the Gnat T.1, so it was far from a standard example of the type.

We could conclude that Engines 709 & 711 were effectively Flight Test engines - no more. Airworthy, but only just - lols.

[Renegadenemo]

We could conclude that Engines 709 & 711 were effectively Flight Test engines - no more. Airworthy, but only just - lols.

As I understand it, 711 flew but if the other one did it's either not recorded or i've not heard of it doing so. The fuel system dates back to 1959 and was a very early version. It was a real challenge to get it back to running order then mate it to a later 101 core engine such that it would all work - one of our most complex and proud restoration/conservation efforts backed by Rolls-Royce Plc even though they sought to distance themselves from it all post-Shoreham.

Sorry, R-R, we're still grateful for all your help and will continue to tell people that you made this possible.

[mtskull]

We had a failure with the hyd when the accumulator let go but we can run without that.

Are you sure? The purpose of a hydraulic accumulator, in addition to storing pressure, is to smooth out the pulses from the pump and protect the system from any "hammer blow" effect.

[Engine 711]

As I understand it, 711 flew but if the other one did it's either not recorded or i've not heard of it doing so. The fuel system dates back to 1959 and was a very early version. It was a real challenge to get it back to running order then mate it to a later 101 core engine such that it would all work - one of our most complex and proud restoration/conservation efforts backed by Rolls-Royce Plc even though they sought to distance themselves from it all post-Shoreham.

Sorry, R-R, we're still grateful for all your help and will continue to tell people that you made this possible.

The Shoreham Hunter crash....? https://en.wikipedia.org/wiki/2015_Shor ... show_crash

Not surprised - corporate liability & image and all that. RR (IMO) has always been at best a reluctant supporter of any speed record. The support for K7's Fuel System was from AEC - when they were part of RR, but still more... free wheeling. Less Treacle....

[conistoncollie]

Gnat XM691 was the first of 14 pre-production prototypes of what became the Gnat T.1, so it was far from a standard example of the type.

We could conclude that Engines 709 & 711 were effectively Flight Test engines - no more. Airworthy, but only just - lols.

A late friend of mine worked at BS in the sixties. He had this to say about XM691
It would be silly to say that I knew XM691 well, but it feels like it. It was used by Bristol Siddeley not only for flight test, but also a chase plane, for observation and for photography. The in-flight photograph of the Valiant, used briefly as Pegasus test bed, was probably taken from it.
In 1964 or thereabouts, in the course of my apprenticeship, I spent two weeks in the Flight Shed, and XM691 was there. It was much used and much liked.
Pegasus of course had its origins in the Orpheus, later powered the Harrier.

[Engine 711]

A late friend of mine worked at BS in the sixties. He had this to say about XM691
It would be silly to say that I knew XM691 well, but it feels like it. It was used by Bristol Siddeley not only for flight test, but also a chase plane, for observation and for photography. The in-flight photograph of the Valiant, used briefly as Pegasus test bed, was probably taken from it.
In 1964 or thereabouts, in the course of my apprenticeship, I spent two weeks in the Flight Shed, and XM691 was there. It was much used and much liked.
Pegasus of course had its origins in the Orpheus, later powered the Harrier.

Interesting insight - so XM691 was a Flight Test aircraft...?

According to Stanley Hooker's book, the Pegasus began as an amalgam of an Olympus LP (as the fan) and an Orpheus (as the core or HP).

[Mike Bull]

Interesting insight - so XM691 was a Flight Test aircraft...?

As I've already said, it was the first of 14 pre-production aircraft, and was pretty well hammered testing-wise. It also appeared a lot at shows such as Farnborough- here it is (from 3:50) in 1959-

[Richie]

Mtskull- to answer your question, yes it works and works fine without the accumulator, however in the event of a flame out, Ted / Stew must ensure they deploy the brake ram before the engine spools down and stops spinning.

Post Bute I will pull the thing back out of the bilges and thoroughly investigate !

At the moment it is the weak link in the system, which thankfully after much running seems ok at present (however I have said it now haven’t I)

[conistoncollie]

According to Stanley Hooker's book, the Pegasus began as an amalgam of an Olympus LP (as the fan) and an Orpheus (as the core or HP).

Engine 711 - I am enjoying reading your technical questions and feedback. Indeed it did. Prior to the Orpheus they used two compressors of the Orion which made it too long. The Orpheus itself used the low pressure spool of the Orion. The full story is beautifully and intimately described in my late colleague's book 'Pegasus The Heart of the Harrier'. Written with the clear insight that comes from being responsible for the engine's commercial fortunes for 16 years.

[Engine 711]

Engine 711 - I am enjoying reading your technical questions and feedback. Indeed it did. Prior to the Orpheus they used two compressors of the Orion which made it too long. The Orpheus itself used the low pressure spool of the Orion. The full story is beautifully and intimately described in my late colleague's book 'Pegasus The Heart of the Harrier'. Written with the clear insight that comes from being responsible for the engine's commercial fortunes for 16 years.

@conistoncollie - Thank you. I am ex-RR - mainly Marine - so Olympus, Tyne & Spey - for various Frigates & Destroyers, UK & overseas. Only did about 30 knots, but.... they were over 5000 tonnes.....

Can definitely recommend the book 'Not Much of an Engineer' - by (Sir) Stanley Hooker - which covers a lot of the Bristol engines, including Oly, Pegasus, Orpheus and more - Oly for TSR2, Oly 593 for Concorde, etc. Am sure the Orion got a mention, too.

https://www.amazon.co.uk/Not-Much-Engin ... 1853102857

Hmm... cheaper than the Pegasus book..... lols.

https://www.google.co.uk/shopping/produ ... CNsQ6SQIZg