I had this stupid idea about hypersonic flight

[juanml82]

I was reading about supersonic and hypersonic flight the other day (mostly perusing wikipedia, I'm not an aircraft engineer nor an expert of any kind in the field) and was reading how one of the challenges is that different engines work best (or work at all) at different speeds. So, for instance, you can't design a plane around ramjets because it wouldn't take off the runway and you can't expect a plane with turbojets to cruise at match 4.

But then I got thinking: Wait, there is the Kerbal way! You just combine engines!

Which, since none is even planning in real life, probably means the Kerbal way doesn't really work and you can't "just" combine engines. But hear me out:

Imagine a small business jet. You fit a central turbofan (or turbojet, if they engine has to propel the plane higher than match 1) at the end of it much like engines in fighter jets are positioned. This engine will power the aircraft during take off, landing and subsonic flight where supersonic flight is forbidden. It should be a common, off-the-shelf commercial engine, which is easily available, has a reasonable price point and is common enough so maintenance and spares are just regular business. Nothing fancy, in other words.

Then, on the wings, you put a pair of ramjets powerful enough to propel the plane above match 3. Now, ramjets have very few moving parts (according to wikipedia, only the turbopump), and they are built in large scales for missiles, even if those are likely to be smaller than ramjets powerful enough to work in a business jet. So they should be relatively cheap to maintain and manufacture.

So, the plane takes off and heads for the ocean on the central turbojet, activates the ramjets, closes the turbojet intake to prevent damage and goes as far and as fast as it can!

Even more, at least as far as the engines are concerned, there isn't that much R&D involved. The turbojet is an existing commercial design (uh, wait, turbojets are for military jets, right? Maybe there are purchase regulations?) and the ramjets are upscaled version of existing designs (just use tweakscale? ). The extra weight of unused engines cuts the aircraft useful payload, but it might remain economically efficient because of the much lower investment in R&D and the possible maintenance costs of more complex and unique designs like the SR-71 engines (which I doubt the USA would allow to be sold for commercial use, plus the production lines are closed) as well as the lack of economies of scales in designing an expensive engine that only works for one type of aircraft.

 

Of course, that still leaves all the issues of commercial supersonic/hypersonic flight: regular aviation materials melt at speeds above match 3 (use active cooling with liquid nitrogen or something?), you can't have a civilian aircraft leaking fuel on the airport like the SR-71 did, and it will be able to carry very few people in far less comfort than rich executives are used to - specially if they have to use pressure suits like the pilots of the SR-71, and the limited routes where supersonic flight is allowed.

But, since in any case none who knows of the subject is contemplating it, which are the drawbacks of trying to apply this Kerbal way to real engineering?

[Streetwind]

Not that stupid of an idea... it's actually one that real-life aerospace engineers have had. And refined upon it. See, nowadays, you don't even have to mount different engines on the same vehicle. You literally have "two-in-one" engines. KSP actually has two of those, and both are inspired by real life counterparts.

 

First, you have the Whiplash turboramjet. The word 'turboramjet' should already give you an indication as to what it is. The engineers took a ramjet, and mounted a turbine right in front of it, so that the exhaust of the turbine goes straight into the ram combustion chamber. The turbine drives an intake fan, much like in a turbofan engine; you could consider this a super-high-bypass turbofan, with almost all the air being routed around the turbine to the combustion chamber, and only as little as possible being consumed by the turbine itself to keep itself running.

So on the runway, where the ramjet is unable to do anything, you spin up the turbine and force large amounts of air into the ram combustion chamber, which lets the ramjet ignite. It doesn't work all that well, you don't have anywhere near peak thrust, but it's enough to let the plane accelerate and take off. The faster the plane is moving, the more air gets into the intake. You start routing air around the turbine to avoid it becoming a bottleneck, feeding it straight into the combustion chamber. Eventually, at mach 2-3, the turbine does almost nothing and the ramjet largely powers itself.

Realworld inspiration: Pratt & Whitney J58, the engines of the SR-71 Blackbird. While officially designated as a "turbojet with afterburner", it featured the above-mentioned air routing, and the afterburner had a dedicated combustion chamber, so for all intents and purposes it worked as a turboramjet. The Blackbird was easily able to take off under its own power, while also being able to exceed mach 3 with those very same engines. (Even the exhaust graphics of the Whiplash are inspired by how these engines looked under full thrust.)

 

The issue is that ramjets cap out eventually, and if you want to go hypersonic, you need a scramjet ("Supersonic Combusting Ramjet") instead. And now you suddenly have three engines you need to stuff into one. That doesn't work, especially since the geometry of the combustion chambers and intakes differ noticably between ramjets and scramjets. You need a whole new type of engine.

In KSP, we have the R.A.P.I.E.R. engine. It is largely defined by its ability to switch between open cycle (air-breathing) and closed cycle (rocket) operation, but the realworld inspiration - the S.A.B.R.E. engine from Reaction Engines Ltd - can do much more than that. In contrast to the J58, this one has never flown before; in fact, it has never been built before. But the science behind its operation principles have been verified. It too involves an internal turbine forcing air into dedicated combustion chambers, but it's not a ramjet, it's not a scramjet, and it works without any complex air-routing shenanigans.

Instead, it has a very special air intake with absurd, almost magical-sounding abilities to gather and cool down supersonic air. It's already working on the test stand though, now it just needs the rest of the engine to be built. By choosing the right kind of turbine and the right kind of combustion chambers, this thing supposedly scales smoothly all the way from standstill on the runway to hypersonic velocities - at least mach 5 has been claimed, probably more than mach 6. And then it can also switch to closed cycle mode on top of all of that. You can imagine now why this engine is kind of a big deal, and why it had trouble securing development funding initially (nobody would believe this was physically possible).

But if it works out, it will mark the birth of an entirely new class of airbreathing engine, the likes of which the world has never seen before. We will find out in the next roughly 10 years, which is the timeframe Reaction Engines has named for building a prototype, testing it, and bringing it to production.

Edited November 6, 2016 by Streetwind

[pincushionman]

Weight and complexity. It may seem like an engine that's not running is simply empty weight, but the fuel systems in particular become more than twice as complex, and structure now has to be good for wildly different load paths deending on which engines are on or off.

You also can't usually close off intakes like we can in KSP, and if you could, it would also impose a significant weight and complexity penalty.

Now, that's not to say multiple engine types haven't been used before. In the early days of jet power, turbojets had little static (ground/low-speed) power and responsiveness, and were unable to get naval aircraft off the shorter carrier decks of the time reliably (and, frankly were simply unreliable), so a concept called "composite propulsion" was investigated, in which aircraft had both turbojets and piston-powered propellers. This worked, but it was heavy and complex, and the real answer was of course better jets (more specifically the low-bypass turbofan) and the steam catapult.

But for the application you describe, you'd probably be better off pursuing a combined-cycle concept like the J58 or SABRE @Streetwind described. At least that way you're getting two engines for the price of three, which is a lot easier to swallow than the price of six you'd eat for having separate systems.

And you can just forget about hypersonic planes right now. We can't even get the hypersonic part to work well by itself yet, let alone in a combined application.

[KerikBalm]

The SR-71 essentially does this, it has a central turbine that functions as a turbojet at low speed, but at high speed the air bypasses the turbines and it functions simply as a ramjet. Hence the term "Turbo-Ramjet"

https://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58#Partial_ramjet

 

Also note that they used a small turbojet in this design

https://en.wikipedia.org/wiki/Nord_1500_Griffon

Sooo... its not a stupid idea... and you may see some similar designs in KSP where panthers are used to supplement thrust for Rapiers when trying to pass mach 1 (and I've modded some engines that don't work at below mach 0.5, so they must be combined with another engine to start as well)

[juanml82]

Yeah, I had been perusing through both the SR-71 and the Sabre articles I've found after quick google searches. The problems with trying to use them for civilian aviation (and I guess very fast business jets for the very rich are the only market for civilian turboramjets) are, I guess:

The capital costs of R&D, which can't be spread across hundreds or thousands of units because it's a niche product.

The J58 was already built, but that doesn't mean civilian manufacturers would be allowed to purchase them ("Hey! let's sell military technology to rich sheiks who may, or may not, be supporting terrorist groups on the side!" or "the Iranian and North Korean governments are interested in buying a couple of them. For civilian purposes, they swear" would raise quite a few eyebrows), although then again, the same applies to ramjets, but since those are simpler to build, a prospective manufacturer could probably purchase them from countries with fewer security concerns (or more love for money). And even if the J58 can be manufactured again and purchased for private civilian operation, wouldn't it be rather expensive, both to purchase (restart production lines, relatively few orders, only one manufacturer in the world to choose from) and, I guess, to maintain?

And the other issue with both engines is fuel. Neither uses regular aviation fuel, so any prospective purchaser/renter is not only limited by flying over the ocean, it would also be limited by airports who have those fuels at hand. And since it's a niche product, few airports could justify the expense of keeping liquid hydrogen or JP7 depots.

 

I guess those are also reasons why these types of aircraft don't exist, though.

Edited November 6, 2016 by juanml82

[Tullius]

8 hours ago, juanml82 said:

But then I got thinking: Wait, there is the Kerbal way! You just combine engines!

Lockheed Martin got you covered: https://en.wikipedia.org/wiki/Lockheed_Martin_SR-72

[FleshJeb]

Just a sidenote, this idea applies to subsonic flight as well: https://en.wikipedia.org/wiki/Convair_B-36_Peacemaker#Addition_of_jet_propulsion

"six turnin' and four burnin' "

[magnemoe]

6 minutes ago, FleshJeb said:

Just a sidenote, this idea applies to subsonic flight as well: https://en.wikipedia.org/wiki/Convair_B-36_Peacemaker#Addition_of_jet_propulsion

"six turnin' and four burnin' "

Was thinking of this too then someone talked about piston+ jet.
Has been plenty of ideas lately of hybrid planes who has extra electrical propellers for landing and takeoff as STOL or VTOL. During cruise the propellers are folded back or covered.