Boom Aerospace

[Star One]

Have they really found an engine supplier?
Guess we'll see tomorrow.

https://onemileatatime.com/news/boom-supersonic-engine/

Maybe Santa’s elves!!!

[butters]

Boom has partnered with Kratos Defense & Security Solutions, which acquired a small turbojet supplier called in Technical Directions Inc. in 2020. They make engines for low-cost cruise missiles and drones, with up to 200 pounds of thrust. It's quite a jump in scale from that to an 80-passenger airliner, but they say that have engineers that have worked on supersonic military engines, so maybe they have what it takes.

[Robotbeat]

Good to hear. And while it definitely is higher risk for the project to partner with such a tiny engine maker… it’s better than nothing and is potentially an opportunity to introduce more competition to the big engine manufacturers.

The risk is higher for the project, but so is the potential gain for the companies if successful.

[Star One]

Boom has partnered with Kratos Defense & Security Solutions, which acquired a small turbojet supplier called in Technical Directions Inc. in 2020. They make engines for low-cost cruise missiles and drones, with up to 200 pounds of thrust. It's quite a jump in scale from that to an 80-passenger airliner, but they say that have engineers that have worked on supersonic military engines, so maybe they have what it takes.

This seems to be stretching credibility to expect a small scale engine maker to suddenly scale up to a large clean sheet supersonic engine just like that.

[Asteroza]

Kratos is involved with USAF unmanned fighter loyal wingman drone work, the XQ-58, but that's a high subsonic drone using a low-lifetime turbine as an attritable drone.

They seem to be confident in low cost design and production of turbines for attritable drones (reusable, but cheap enough to be expendable), based on their various military turbine contracts. I wonder if the math works out for a low lifetime turbine being replaced more often on a commercial vehicle?

[Robotbeat]

Or they could, you know, build it with sufficient margin for a longer lifetime.

[john smith 19]

Or they could, you know, build it with sufficient margin for a longer lifetime.

Actually there is a long history of smal expendable jets being upgraded to being reusable. A classic example was the upgrade from the Adder to the Viper turbojet driving the Jindyvik target drone. These flew for decades with the UK and Austrailian armed forces.

The joker with margin is that usually implies an increase in engine weight.

IIRC what actually happened with the viper is they upgraded the materials to materials with better long term properties.  So same size part, slightly heavier perhaps, but much longer operating life, but more expensive partly due to higher raw material cost, partly harder to machine. Of course that was with 1950's cutting tools.

But that is a pretty small engine. either they are going to need a lot of them at that size, or the move to a substantially bigger clean-sheet design. I'm putting the "same size" option out there because it might be they are maxed out on machine capacity, in the same way that the F9 is at (or close to) the limit that SX's FSW machines can build a stage.

 It's unlikely (but not impossible) that Kratos have already maxed out the size of engine they can make, but there will be some limit, beyond which they a)Subcontract that part of the work b)Raise a loan for upgrades on the strength of their Boom contract c)Ask Boom for direct help.

The other question then becomes how well they move (or establish a division to move)  from small size/high(ish) volume/low price engines  to large(ish)/low(ish) volume/much higher price engines. Successful companies have a mindset that focuses them in certain directions. Re-focussing that mindset is a bit more than just saying "OK, we used to build this stuff, now we're going to build this different stuff"

That in turn depends on how happy operators would be to move from their current maintenance regime to something more  frequent.  The question is will the unique speed advantage justify that?

 The CIA and USAF did with the SR71. For commercial operators IDK. I think it'll depend on if they sell Boom as "Just like a normal biz jet, but with a higher top speed," or "Boom is the future of biz jet travel. It's a whole new way of flying and it needs a whole new approach by your support staff, but the results will be awsome."

Time will tell

[Asteroza]

Or they could, you know, build it with sufficient margin for a longer lifetime.

I don't begrudge this rational response, but if the Overture is going to be a hangar queen anyways and likely using an engine lease pool, does reducing the upfront engine development cost by going with a low lifetime design and accepting more engine swaps a counterintuitive solution to making the project startable/viable? It's similar to the classic solid vs liquid rocket dev costs vs ops costs comparison.

Though commercial certification requirements may automatically push the engine design to higher margins/lifetime by default, as an attritable turbine design base may not be deemed safe enough. Where is the original cost savings coming from though? Sandbagged derated components with lots of hidden margin but a mass penalty? Low spec materials (with implied safety issues)? Simplified manufacturing by reducing machining through increased parts counts (less monolithic parts with lots of CNC machining)? Simpler parts at the cost of less performance (centrifugal compressor for instance)?

[Barley]

Is the life of an engine mostly a function of it's design or how you choose to operate it?  I.e. if you derate a drone engine, will it last much longer?  While this doesn't help with the suitability of a particular engine it may inform us of the capabilities of the manufacturer.

I'm thinking of things like War Emergency Power which if used shorted the time between overhauls and the life of engines by orders of magnitude.  A 2000hp engine for a fighter and a 1000hp engine for a transport could be exactly the same engine.  There are a lot of systems that have steep performance v. life span curves where different users make different tradeoffs.

[Star One]

Sceptical appraisal of the proposed engine choice.

https://www.ainonline.com/aviation-news/air-transport/2022-12-13/boom-supersonic-outlines-its-own-engine-design

[Robotbeat]

I mean, the odds of the company succeeding to first flight are maybe 25% (and odds of keeping schedule are 1%, optimistically). But better to try and fail than to do nothing.

Or, as Teddy Roosevelt put it:
 

It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming; but who does actually strive to do the deeds; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the end the triumph of high achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who neither know victory nor defeat.

So I applaud Boom, and their engine partner.

[butters]

Kratos is involved with USAF unmanned fighter loyal wingman drone work, the XQ-58, but that's a high subsonic drone using a low-lifetime turbine as an attritable drone.

They seem to be confident in low cost design and production of turbines for attritable drones (reusable, but cheap enough to be expendable), based on their various military turbine contracts. I wonder if the math works out for a low lifetime turbine being replaced more often on a commercial vehicle?

One unique aspect of supercruise engines is that the compressors operate in high-temperature working fluid for hours on end, not just the turbines. Concorde's Olympus engines, for example, had a 10,000-hour life for both the compressor and turbine blades, whereas the rest of the engine was certified for 25,000 hours. Kratos will certainly need to use nickel-based superalloys and/or ceramic matrix composites in the turbomachinery. Titanium won't cut it for the later stages of the compressor. It's a whole different ballgame than subsonic.

[Asteroza]

The joker in all this is is the involvement of GE Additive, bringing 3D printing to the table. For example, 3D printing a near-net blisk for the engine could reduce machining costs and lost material costs from machining. 3D printed monolithic aerospace parts are a thing now.

There's also some wacky engine design choices that could help with materials choices as well, such as the proposed D-16 turbocompound, which utilizes a reciprocating engine core in a diamond 16 piston arrangement to drive the high pressure compressor coupled with a conventional single spool low pressure turbine/compressor. Seeing a deltic take on the ICE core of such an engine would be fun.

https://www.flightglobal.com/systems-and-interiors/hybrid-geared-fan-and-piston-concept-could-slash-fuel-burn/127860.article

https://web.archive.org/web/20180826030208/http://www.ultimate.aero/page/media_items/composite-cycle-engine-28cce2928.php

https://www.researchgate.net/publication/327601208_A_Composite_Cycle_Engine_Concept_for_Year_2050

[Robotbeat]

The joker in all this is is the involvement of GE Additive, bringing 3D printing to the table. For example, 3D printing a near-net blisk for the engine could reduce machining costs and lost material costs from machining. 3D printed monolithic aerospace parts are a thing now.

There's also some wacky engine design choices that could help with materials choices as well, such as the proposed D-16 turbocompound, which utilizes a reciprocating engine core in a diamond 16 piston arrangement to drive the high pressure compressor coupled with a conventional single spool low pressure turbine/compressor. Seeing a deltic take on the ICE core of such an engine would be fun.

https://www.flightglobal.com/systems-and-interiors/hybrid-geared-fan-and-piston-concept-could-slash-fuel-burn/127860.article

https://web.archive.org/web/20180826030208/http://www.ultimate.aero/page/media_items/composite-cycle-engine-28cce2928.php

https://www.researchgate.net/publication/327601208_A_Composite_Cycle_Engine_Concept_for_Year_2050

3D printing is partially what enabled the proliferation of all the new pumpfed rocket engines (and therefore smallsat launchers) of late.

Rocketlab, Relativity, Astra, SpaceX, ABL, etc all use either 3D printed engines or major components of their engines are 3D printed.

So I agree that this could end up being part of what enables a novel jet engine to be developed quickly.

(Still don't expect them to meet their schedule, of course.)

[Asteroza]

The joker in all this is is the involvement of GE Additive, bringing 3D printing to the table. For example, 3D printing a near-net blisk for the engine could reduce machining costs and lost material costs from machining. 3D printed monolithic aerospace parts are a thing now.

There's also some wacky engine design choices that could help with materials choices as well, such as the proposed D-16 turbocompound, which utilizes a reciprocating engine core in a diamond 16 piston arrangement to drive the high pressure compressor coupled with a conventional single spool low pressure turbine/compressor. Seeing a deltic take on the ICE core of such an engine would be fun.

https://www.flightglobal.com/systems-and-interiors/hybrid-geared-fan-and-piston-concept-could-slash-fuel-burn/127860.article

https://web.archive.org/web/20180826030208/http://www.ultimate.aero/page/media_items/composite-cycle-engine-28cce2928.php

https://www.researchgate.net/publication/327601208_A_Composite_Cycle_Engine_Concept_for_Year_2050

3D printing is partially what enabled the proliferation of all the new pumpfed rocket engines (and therefore smallsat launchers) of late.

Rocketlab, Relativity, Astra, SpaceX, ABL, etc all use either 3D printed engines or major components of their engines are 3D printed.

So I agree that this could end up being part of what enables a novel jet engine to be developed quickly.

(Still don't expect them to meet their schedule, of course.)

That actually brings up an interesting point, that for rockets at least, the previous rule of thumb was turbopump rocket engines made sense at 5000+ lbs thrust, but below that the machining difficulty for the drive turbine drove up costs, thus pushing small rockets to pressure fed or alternative pump architectures (Whitehead and his work on small reciprocating pumps as covered in Kare's Mockingbird (bricklifter) SSTO work). The recent update to that rule of thumb was electric (centrifugal? axial?) compressor motor driven rocket engines are best for less than 10,000 lbs  thrust engines.

Are there similar rules of thumb for jet engine scaling, driving the use of centrifugal versus axial setups? I get the impression a lot of small engines use a centrifugal compressor for the final stage of their compressor, or as the high pressure compressor if twin spooled.

[john smith 19]

One unique aspect of supercruise engines is that the compressors operate in high-temperature working fluid for hours on end, not just the turbines. Concorde's Olympus engines, for example, had a 10,000-hour life for both the compressor and turbine blades, whereas the rest of the engine was certified for 25,000 hours. Kratos will certainly need to use nickel-based superalloys and/or ceramic matrix composites in the turbomachinery. Titanium won't cut it for the later stages of the compressor. It's a whole different ballgame than subsonic.

Good point. Without a pre-cooler a lot more of the engine runs a lot hotter. The othe issue is that AIUI earlier generation turbojets relied a lot less on air cooling through the blades. Modern ones use it a lot but where does the cold air come from when all the air is pretty hot to start with?

OTOH coolerer might be good enough. The engine on Concorde for example was first fired up in 1950 and the Concorde version dates from 1964.The engine on the SR71 has a similar timeframe.

 IOW a modern engine could use an alloy developed in the late 80s/early 90s based on at least 25 years more alloy development work and still have a 30 year track record behind it to avoid certain mishaps from deploying materials with unknown responses to certain environmetal conditions.

Then we have the ability do directional solidification of the alloy and it's ultimate version of single-crystal blades, so better is definitely possible. No (material) development programme needed. Couple that with higher maximum operating temperature and silicon nitride bearings (now available up to 1 3/4") and air cooling might still be viable.

That said avoiding air cooling is quite attractive. Those holes are tricky to machine or build into the blades.

[john smith 19]

The joker in all this is is the involvement of GE Additive, bringing 3D printing to the table. For example, 3D printing a near-net blisk for the engine could reduce machining costs and lost material costs from machining. 3D printed monolithic aerospace parts are a thing now.

There's also some wacky engine design choices that could help with materials choices as well, such as the proposed D-16 turbocompound, which utilizes a reciprocating engine core in a diamond 16 piston arrangement to drive the high pressure compressor coupled with a conventional single spool low pressure turbine/compressor. Seeing a deltic take on the ICE core of such an engine would be fun.

https://www.flightglobal.com/systems-and-interiors/hybrid-geared-fan-and-piston-concept-could-slash-fuel-burn/127860.article

https://web.archive.org/web/20180826030208/http://www.ultimate.aero/page/media_items/composite-cycle-engine-28cce2928.php

https://www.researchgate.net/publication/327601208_A_Composite_Cycle_Engine_Concept_for_Year_2050

AIUI one of the reasons for early adoption of the jet engine was the massive reduction in parts conpared with piston engines to drive compressors.  But for a subsonic engine this might work better.

I see they are targeting a 35 000lb thrust engine as a "medium bypass" turbofan. That would be a big pure turbojet (in the XB70 class) but well within the limit of turbofan sizes (currently around 90 000lb) and they don't want to use an after burner, which the 17th Concorde was also planning to phase out.

OTOH that's 80x 175x bigger than the stuff Kratos seems to have experience of.

A key issue with both the plane and the engine are the "uncertainty bars" around drag and performance.  If they have improved enough then you can design a vehicle with no more than a certain level of drag confident that it will be below that and that level will not be a huge excess over nominal levels. Likewise you can design an engine with at least X 000lb of thrust confident you will get that on the test bed.

Time will tell how far this goes.  But the fact they lowered performance speaks to them being a flexible and pragmatic management team.  That maybe what you need to get something like this over the finish line.

[Asteroza]

So being a two spool with fan engine design, the Symphony at first glance appears to slap on a fan in front of a regular tubojet core. Doesn't seem to be one of the fancier variable bypass 3 stream type engines that are increasingly popular.

Compare this with Hermeus, which just announced they are going to use a Pratt and Whitney F100 turbine as the core of their engine (which if I remember correctly is a precooled turboramjet TBCC). CotS, and possibly available used.

https://www.hermeus.com/press-release-f100-engine

[john smith 19]

So being a two spool with fan engine design, the Symphony at first glance appears to slap on a fan in front of a regular tubojet core. Doesn't seem to be one of the fancier variable bypass 3 stream type engines that are increasingly popular.

I think you'll find most turbofans of any size are two spool. You're able to harvest more energy from the combustion process.

Compare this with Hermeus, which just announced they are going to use a Pratt and Whitney F100 turbine as the core of their engine (which if I remember correctly is a precooled turboramjet TBCC). CotS, and possibly available used.

https://www.hermeus.com/press-release-f100-engine

Which is also a 2 spool design.

In theory Hermeus could use any engine because of their claimed use of a pre-cooler in front of the engine. At least that was the pitch for Chimera I, but now they will be using  this as the basis of "Chimera II" so it's not clear if that still applies or if they hit problems scaling up.
 Let's keep in mind Hermeus has USAF funding to be the next Air Force One

Ironically the freedom to use any engine a pre-cooler gives you is exactly the freedom Boom needs to broaden it engine choices.

Inlet and outlet nozzles were a key part of the Concorde engine system and each engine had 13 controllers/computer/regulators to run its associated inlet. They were designed by the French half of the consortium. AFAIK only 1 was what people today would recognize as a proper computer (processing with a using a stored program that can be changed)  Given the Z80 was powerful enough to run the FADECs of civilian turbofans decades ago (military engines were running the equivalent of an original Macintosh in term of procesor and clock speed) I think that problem can be managed considerably better.

One interesting question would be how constant are the inlet conditions really during cruise? They are assumed contant if you stay at the same height and speed. If the engine is flying through patches of changing air then it's fuel efficiency may be compromised. The question of course would be (if it's happening at all) can you sense this happening and adjust the engine during the passage of air through the engine? Could be a totally non-problem. Could be significant.

Given the extreme difficulty of sourcing an engine for this market segment ( market size for a bespoke design) It's kind of amazing that Boom have found anyone willing to take on the job at the price they are willing to pay. 

[Star One]

So being a two spool with fan engine design, the Symphony at first glance appears to slap on a fan in front of a regular tubojet core. Doesn't seem to be one of the fancier variable bypass 3 stream type engines that are increasingly popular.

Compare this with Hermeus, which just announced they are going to use a Pratt and Whitney F100 turbine as the core of their engine (which if I remember correctly is a precooled turboramjet TBCC). CotS, and possibly available used.

https://www.hermeus.com/press-release-f100-engine

That’s because there has been in the past I believe work on using the F100 by DARPA in such a way.