Category: Propulsion

A Wacky Tilt-Rotor Concept From DARPA

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It’s called the Folding Advanced Stopped Tilt Rotor (FASTR), and the basic idea is that you have some sort of variable cycle engine which powers tilt rotors on takeoff, and after you get to speed, the rotors are stopped, folded back, and the engines convert to turbojets.

I’m inclined to believe that this won’t amount to anything, as the weight penalties of a stop start folding rotor and variable cycle engine would likely make any advantages irrelevant, but it’s DARPA’s job to try out weird stuff like this, and see what happens.

Veto Threat on F-22 and F136 Authorizations

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President Obama has threatened a veto on both programs.

I’m of a mixed mind on the programs.

While it is clear that the F-22 is overpriced, and does not address foreseeable threats, the F136 alternate engine for the JSF seems like a good idea.

The P&W F100/GE F110 engine competition saved a lot of money, and produced more reliable engines.

What’s more, the F136 seems to have more growth potential than the F135, and this is needed, particularly for the STOVL F-35B.

Additionally, the funds to continue F136 development are about the cost of procuring just 2 more F-22s.

GE-Rolls Gets Smart on F136 JSF Alternate Engine Bid

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With the Pentagon gunning for the GE-Rolls Royce alternate engine, because they want their pig, the F-35 JSF, to fly as soon as possible, and the Pratt & Whitney’s F135 is already paid for, the alternative engine is behind the 8-ball, even if all indications are that it will provide superior performance, and that the competition between the two engines is likely to provide significant cost savings over time.

The F100/F110 competition between P&W and GE saved about 20%, but the basic F-110 was already flying as the F-101 on the B-1 bomber before it challenged the F-100.

The problem is that the services, particularly the Air Force and the Marines, want the JSF right now even more than they want the JSF get it right, and the money spent developing the F136 engine could be used to accelerate keep the schedule from slipping quite so much.

I think that it’s penny wise and pound foolish, but things being what they are, the Pentagon, and Lockheed are right about the fact that once the aircraft enters squadron service, it will be much harder to cancel.

Well, the F-136 team just had a cunning plan.

In fact, it’s so cunning, if you put a tail on it, you could call it a weasel: They are looking at making a firm, fixed-price contract proposal (paid subscription required) for the engine, which would make it a “known known”, as opposed to a “known unknown” (or maybe an “unknown unknown”, I gotta stop trying Rumsfeld-Speak, my head hurts).

In any case, the F136 does have friends on the House Armed Services Committee, who just added money for it back into the budget.

Lithium Hydride Scramjet Shows Promise

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Lithium Hydride is a fairly dense and easy to handle way to store hydrogen, which is one of the reasons that it’s used, with Deuterium substituting for Hydrogen, as the fusion fuel supply for thermonuclear warheads.

Well, Claudio Bruno, of the University of Rome, has begun testing the material for use in scramjet engines.

Gaseous, or for that matter cryogenic, hydrogen is bulky and difficult to store, but it also is the material that is best suited to burn quickly enough to work well under supersonic combustion.

One of the techniques used is to use a catalyst, and the heat generated at speed, to crack hydrocarbon fuels, and use the hydrogen generated, or to use a preburner, basically a ramjet inside the scramjet, to preburn the fuel and create a hot reducing atmosphere for further combustion.

LiH would decompose naturally at operating temperatures, and would appear to be a simpler solution.

Company Claims Throttlable Solid Fuel Rocket

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A company called Digital Solid State Propulsion claims to be able to throttle a solid state rocket electronically. (See vid below).

This has a number of potential applications, such as variable blast bombs and thrusters for satellites.

To my mind, one of the applications would be for an air to air missile, where the motor would be on continuously for short/medium range flight, but for longer range, it would coast much of the way to the target.

Neat tech.

Thielert Reenters Aero Diesel Engine, Diamond Pursues Their Installed Base

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Thielert (top picture), and Diamond (bottom picture) are now both aggressively competing for market share for aircraft diesel engines. (paid subscription required)

Both engines are based on the same Mercedes diesel engine block, so the real differences are design philosophy and corporate history.

In terms of design philosophy, Diamond’s engine, the AE300, is somewhat heavier, and it is implied, more robust.

In terms of corporate history, Thielert went into bankruptcy, largely driven by the warranty costs on their engine, production ramp-up issues, and financial irregularities, and the trustee basically tried to shake down the installed base by demanding ruinously expensive service and parts.

They have since returned to a going concern status, though their warranty terms are now far less generous, and their engine division has been rebranded “Centurion”.

Diamond aircraft, as Thielert’s best customer, was left in a serious lurch by this, and so developed its own engine, which has now been certified by the EASA, and they are using it on their new build aircraft.

The two main draws of Aero-Diesels are better fuel economy, and the ability to run on Avjet fuel, as opposed to the increasingly expensive and hard to find, particularly in austere locations, Avgas.

I think that for Theilert to compete, it needs to get a handle on reliability, as is evidenced by the line from the article, “By year-end, the company hopes to have doubled the service life of the two vital components to 600 hr,” (emphasis mine) which means that they were operating at a 300 hour life (!!) (IIRC, it’s the reduction gear and clutch).

By comparison, the TBO (time between overhaul) on the 0-360 is 2000 hours.

Both diesels are pushed harder to get their power than current commercially available engines, with he competing Avgas is something like the Lycoming 0-360 (whose origins are more than 50 years old) generating 180 horsepower out of 5.91 liters (361 cubic in) at 2700 rpm, while the Theilert gets 135 hp at (it’s been certified for 155 hp) out of 1.9 liters at 3700 RPM, and the AE300 gets 166 hp out of 1.9 liters at 3800 rpm .

Note also that the prop speed on the diesels is 2300 RPM, while on the direct drive Lycoming it remains 2700 rpm, so propulsive efficiency per hp should be a bit better for the diesels, and their fuel consumption is on the order of ½ that of the gasoline engine.

Boeing’s Looks to Uprated Engine for Super Hornet

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As a results of GE’s work on the Integrated High Performance Turbine Engine Technology (IHTET) research program, GE has managed to increase the thrust of the F-414 engine by 20%, which would take it from its current 22,000 lbs to about 26,400 lbs of thrust, and Boeing is looking to using this engine as an inducement to sell more of it’s F/A-18E/F Super Hornet and EF-18G Growler on the export market (paid subscription required). (See also here).

Basically, there is a new core, which the US Navy wants to increase durability, and reduces fuel consumption by about 3%, and a new fan, which can be used to increase thrust, and which the navy does not want.

Boeing is offering the 2nd option to other customers, which is a sort of acknowledgement that the “market” finds the F/A-18 E/F (and G) to be a little bit poky compared to the competition.

It might also make SAAB’s Gripen NG, my fighter obsession du jour, more attractive, as the increased durability engine would save costs and give it somewhat longer legs, and the 20% increase in thrust, which might require a new inlet, because mass flow increases, would offer a very significant increase in performance.

More Scramjet News

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While DARPA’s Blackswift hypersonic , the dual mode turbojet/scramjet Falcon combined-cycle engine technology (FACET)soldiers on, (paid subscription required) and has actually successfully completed tests up to Mach 4 in a wind tunnel.

The technology used a turbojet to get to high supersonic speeds, at which point the scramjet kicks in, and the turbojet is cocooned, and it is supposed to take a vehicle to around Mach 6.

Additionally, the X-51A scramjet test vehicle has been moved to the Boeing plant at Huntington Beach, CA for structural tests.

It’s not as sophisticated as the FACET, it’s a pure scramjet, and boosted to speed by a rocket booster after being dropped by a B-52, and it’s rather crude, using existing equipment wherever possible, using a rocket booster from ATACMS, an existing fuel control system, and geriatric igniters from the TF-33 engine, along with using a rather heavy tungsten nose cone and ablative coatings elsewhere to deal with the heat.

Hypersonic Developments

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Alliant Techsystems (ATK) isworking on getting its Thermally Throated Ramjet (TTRJ) onto the X-51B hypersonic demonstrator (paid subscription required) that is due to succeed the X-51A demonstrator (2nd pic down) that will be conducting flights later this year.

The flight weight engine is now in test (see inlet diffuser and combustor combination, top pic), and as opposed to using the rather exotic, and difficult to ignite JP-7 that was used for the SR-71, it uses the more common JP-10 missile fuel, with the fuel cooling the combustor walls and being vaporized prior to ignition.

It’s interesting in that the boundary between subsonic and supersonic flow inside the engine is controlled thermally, rather than through inlet geometry, with the heat of forward (subsonic) combustion taking the stream to supersonic speed.

In the meantime, the X-51A scramjet demonstrator appears ti be continuing on both budget and schedule, (paid subscription required) largely because the development team has limited requirement creep and “bleeding edge” technology wherever possible, with a first flight in October of this year.

There are a number of potential technical breakthroughs, a hydrocarbon fuel cooled scramjet, unstable aerodynamics, and long powered flight time (300s as vs NASA’s 10s for its X-43-A), but structurally, it’s simple, with a tungsten nose cone, aluminum structure, and an old fashioned ablative coating for much of its thermal protection.

Additionally, the 4 planned flight tests are designed to be identical, in order to provide redundancy.

While it’s not an issue for the relatively slow (!) Mach 5-6 X-51A, future hypersonic vehicles, which can be expected to exceed Mach 10, will have issues with communication, as the speeds create an ionized sheath around the vehicle that will cause a communications blackout, which makes getting telemetry out of a vehicle problematic.

The potential solutions are interesting, ranging from using lasers or magnets to poke holes in the plasma, dropping capsules with the telemetry as the flight progresses, using the plasma to generate a signal, magnetic fields, etc.

Ceramic Composites Starting to Find Way Into Engines

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Production verification ceramic matrix composite vane

Well, we now have another reason to support a 2nd engine for the JSF, it appears that the GE/Rolls-Royce F-136 alternate engine will be the first production engine to use ceramic composites. (paid subscription required)

Right now, they are looking at using it on stationary parts of the engine, “vanes, shrouds, combustor liners,” though there is investigations of using it on rotating parts too.

There is both a significant weight savings (Nickel is heavy), and a reduction and/or elimination of the need for cooling air.

Diamond Aero-Diesel Gets Euro Certification

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I have been following the story of Thielert’s insolvency, and Diamond Aircraft has been hit particularly hard by the loss of their engines, which were a major part of their business strategy.

Diamond has now certified Austro AE 300 turbo-diesel, which it has been feverishly working on since Thielerts troubles, which means that it can start shipping aircraft with the highly efficient, and perhaps more importantly, jet fuel compatible, engines shortly.

They still need type certificates, but that should be relatively straightforward, as their engine uses the same Mercedes block as the Thielert, and so fit and mounting should be very similar.

Hypersonic Engine Tests Continue Despite Blackswift Demise

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Facet Common Inlet

It looks like DARPA is continuing to develop turbine/ramjet hypersonic propulsion systems, (paid subscription required) called the Turbine-based combined-cycle (TBCC), despite the cancellation of the Blackswift demonstrator last month.

Williams International XTE88 (Histed)

The concept remains the same, a turbine to get the vehicle off the ground and through Mach 1, with some level of cocooning beyond that as a Ram/Scram jet takes over.

DARPA is continuing to test the Falcon combined-cycle engine technology (Facet) developed for Blackswift, along with the high-speed turbojet engine development (Histed).

Basically, they are looking at integrating previously pieces, inlet, combustor and nozzle primarily, into a complete engine system.

Following this, they intend to combine Facet and Histed.

Obviously it is a non trivial issue to test the engines throughout their range without a flight test.

Because the USAF Thinks That There No Other Needs But Them

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So the boys in blue, despite officially eschewing a supersonic aircraft for their Next Generation Bomber, and canceling the Blackswift hypersonic demonstrator, are still looking for something that flies faster than Mach 1 and carries bombs. (paid subscription required)

If you’re confused, that’s what happens when you look at Air Force procurement programs recently. As Bill Sweetman notes, when talking about USAF procurement, “If your track record is Ishtar and Howard the Duck, and you tell me that you’ve got something that beats Gone With The Wind and Star Wars, you are going to have to prove it with more than a PowerPoint, or ‘trust me, but it’s secret.’“

They are looking at a tailless supersonic design, which would achieve the requisite broadband stealth, only, no one has gotten a tailless design to fly at supersonic speeds.

Basically, they are looking at advanced applications of fluidics for both the serpentine air inlets and the control systems.

Considering the drains of Iraq, Afghanistan, and now the US economy, and the complete lack of utility on the highest tech desires of the USAF, they have to smoking something pretty powerful.

Pulse Detonation Engines Move Ahead in Hypersonic Race to Mach 4

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One of the problem with the hypersonic scram jet is that it does not work well at speeds lower than about Mach 4 or so, and a turbojet pretty much runs out of steam at about Mach 3, and even then it gets big and heavy and complex, as evidenced by the size and weight of the bypass turbofans in the SR-71 (bottom pic), and it looks like DARPA is has put the pulse detonation engines (PDE) at the head of a list to bridge this gap in its Vulcan program. (paid subscription required) (top picture)

Similar in concept, though much different in execution to the pulse jet that powered the V-1 “Buzzbomb”, it is increasingly being looked at as a way to bridge the gap between turbojet and scramjet.

Additionally, it is being examined as a possible replacement for both combustors in turbine engines, and for afterburners behind turbine engines.

One thing that I’m not clear on here though is why you need the turbine engine in the first place with a PDE, as they do operate at 0 airspeed, as evidenced by the (very noisy) flight of a Long-EZ powered by a such an engine about 11 months ago.

Thielert Aero-Diesel Manufacturer Back In Black

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Honestly, I am pleasantly surprised by the news that Thielert Aircraft Engines’ insolvency administrator has announced a (probably small) profit, and is looking for a buyer.

You can find a rundown of my previous stuff here, but here is the nickel tour:

  • Thielert makes a revolutionary aircraft turbo diesel based on a Mercedes block, which is fuel efficient and runs on AvJet fuel, which is easier to find, and cheaper, than AvGas.
  • Thielert gets a number of deals to supply engines in the aftermarket, to OEMs (particularly Diamond), and for UAVs.
  • Thielert gets into financial problems.
    • This appears to be 1 part fraud, and 1 part that the engine, particularly the reduction gearing to the prop needs more maintenance than anticipated, and Thielert is getting hammered by guarantee costs.
  • The founder is kicked out, and the company is placed under an insolvency administrator (in the US, we would call this Chapter 11).
  • The administrator cancels guarantee support and jacks up the price (gouges really) on spares, forcing most of the operators to ground their aircraft.
  • Diamond tells Thielert to go Cheney themselves, and starts to develop an aero-diesel based on the same engine block.
    • Diamond also takes a major hit in their business too, as they had used the Thielert engine extensively, and certification of a new engine is a non-trivial process.

What has happened over the past few months apparently is that the insolvency administrator, Bruno Kübler, has come to understand the market, reduced spares prices, and started to supply a limited amount of support under its old guarantees.

Additionally, they are looking to sell the engine in the future without the extensive guarantee, so as to avoid this problem in the future….It’s a change in business model, with the majority of future profits being in engine support, rather than the initial sale.

It’s still a promising technology, though I am still dubious of Thielert’s continued existence.

A Thrust-to-Weight Ratio of 6.6:1

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Bill Sweetman teases out some information which sheds some light on the relatively low thrust to weight ratio of the F-35 JSF, the fact that the Pratt & Whitney F135 engine has a thrust to weight ratio of only 6.6:1.

By comparison, the F-100 engine (F-15, F-16), whose contract was awarded in 1970, has a thrust/weight ratio of 7.8:1, and the more modern F414 and EJ2000 (Superbug and Typhoon) exceed 9:1 thrust to weight.

Additionally, he notes that the vertical lift system used on the F-35B weighs 10,472 pounds with a thrust of 40,550 pounds, giving a thrust to weight ratio in that mode of 3.87:1, as compared to 5.6:1 for the Rolls Royce Pegasus used on the Harrier.

There is a saying that you can’t make a race horse out of a pig, but you can get an awfully fast pig, the F-4 Phantom comes to mind, but this is not a fast pig.

Pics of the F135 promotional brochure are from here.

Japanese Company Developing Regional Jet Engine

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Japanese aero engine manufactuer IHI is working on a a low fuel consumption turbofan (paid subscription required) to compete with US and European engines on future regional jets.

One of the interesting features of the engine, IHI calls it the Eco turbofan, mockup shown is how the fan wraps around its hub, so as to cover more of the frontal area of the engine, which is intended to improve efficiency.

In its initial form, it should put out around 9,000 lbs of thrust with a 7:1 bypass ratio.

Son of HOTOL

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When I was in college, I recall, reading about a British program called HOTOL (Horizontal Take-Off and Landing, click picture to be taken to a page on it) which was supposed to use a “liquid air” engine in order to more efficiently reach orbit.

A short description of the cycle was that it was fueled with LH2 and for much of the way up, it would use the cold liquid hydrogen to extract liquid oxygen out of the air.

It got canceled, or more accurately, when the program ended it was not taken to the next phase.

I kind of figured that technical issues, my guess at the time (which appears to be wrong) was getting the heat exchangers small enough and light enough.

The real problem appeared to be that the engine was heavy, and in an aft mounting configuration, maintaining the center of lift far enough aft compromised performance.

In any case, a company called Reaction Engines are having another go at the concept with the Skylon, (paid subscription required) , which uses the Sabre (Synergic Air-Breathing Engine).*

As is clear from the picture, they dealt with the CG issues by placing the heavy engines at the center of lift on the wing tips.

It’s supposed to function as an air-breather until it reaches 30km altitude and “around Mach 5”, and then function as a normal liquid fueled rocket the rest of the way up to orbit.

They are looking at testing a 9% scale precooler (left) in January on a test stand, using a RR Viper turbojet behind the cooler (left below)

There are two problems with Liquid Air Cycle Engines (LACE), hydrogen embrittlement of the heat exchanger, and condensation of water and CO2 on the heat exchanger, which will plug it up.

It solves these problems by not cooling the air to full liquid state, but instead having a high-pressure turbo compressor behind the air to liquefy it, and by using helium as an intermediate medium for the heat exchanger, along with an unspecified proprietary frost control technique. (bottom pic)

It’s a neat concept, but I’m dubious of any project that is not fully funded, or almost fully funded by government sources, and at this point, they are getting just a trickle from the British National Space Center and the the European Space Agency (ESA).

*Let me note here that the Brits come up with cool names.

Rolls Plans ‘Puller’ Open Rotor Work While GE Readies for Noise Tests

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It looks like Rolls Plans ‘is looking hard at a “puller configuration” for it’s advanced propfan. (paid subscription required)

They believe that the issues involved in mounting the fan in the immediate vicinity of hot exhaust from the core are challenging, so they are looking at a more conventional “turboprop like” layout.

Of course, it’s all tradeoffs, as the “puller” configuration has issues of the interaction between the prop wash and the pylons and inlets too, which need to be carefully managed to handle noise, which is generally considered to be the Achilles heel of propfans.

New “Piston” Engine Enters the “Mogas” Sweepstakes

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The Mistral Engines G-300 (paid subscription required) is a multi fuel Wankel rotary engine.

Though the term “multi-fuel” means that it can run on both aviation (100LL) and automotive (87 Octane) gasoline, it’s spark ignited, though their web site states that they are working on Jet fuel capable engines.

Obviously there are fewer parts than a conventional piston engine, there are no conventional valves in a Wankel, but it still has a reduction gear, which was the major maintenance headache in the Thielert engine.