On the Broader Question of the Appropriateness of Airbreathing Propulsion for Launch Vehicle Applications

Thanks to John Hare and his interesting post on “turbo rockets” and Jonathan Goff creator of Selenian Boondocks, a great discussion commenced focused largely on the Air Turbo Rocket engine cycle, but also relating to turbine-based combined cycle (TBCC) engines in general.  This discussion represents the best of what the internet can offer:  open, insightful discussion on a topic of interest to the participants from all over the country, and world, and with people providing all kinds of interesting and thought-provoking insights, knowledge, and experience.

This discussion caused me to reflect further on a fundamental issue associated with launch vehicles, and that has to do the appropriateness of airbreathing propulsion to space launch applications.  By space launch applications I mean such things as suborbital vehicles, like Spaceshipone and two, XCOR’s Lynx, RASCAL, etc., and orbital vehicles, such as Two Stage to Orbit (TSTO) or even Single Stage to Orbit (SSTO) concepts, plus many, many others too numerous to mention.

I think it’s fair to say that airbreathing engines for these types of applications are out of favor.  Given the demand to develop space access concepts as rapidly and as inexpensively as possible, airbreathing engines are seen not as an enabling technology, but quite the opposite: as an impractical propulsion option whose low thrust-to-weight, complicated packaging, and constrained launch trajectory requirements are not compensated by their superior specific impulse performance, which only applies for a portion of the launch trajectory anyway, and certainly not out into space.  As an industry, we’ve also learned the hard lesson that the development of propulsion systems takes a great deal of time and effort, maybe as much as the vehicle itself, and that’s just for rocket engines.  Airbreathers are more complicated than rockets, so by extrapolation, these systems are going to be even harder to bring online.

It wasn’t always this way.  There was a time when airbreathing engines were not only seen as an enabling technology, but as the answer.  The National Aerospace Plane (NASP) program, started in the early 80’s and terminated in 1993, envisioned a vehicle powered by scramjet engines that could go all the way to orbit.  When I worked at Aerojet in the mid-1980’s, our systems engineering office was bustling with all sorts of airbreather-powered launch vehicle concepts, and the ATR was even a featured concept for the “orient-express”, a hypersonic, suborbital spaceplane that could span the pacific rim in an hour instead of tens of hours.

In the 1960’s, a great deal of work on scramjets and combined cycle engines to power high-speed test aircraft, like the X-15, was underway.  It’s been said that, if you asked aerospace engineers from the 1950’s how we would get men up into space, they would have replied that we would continue to fly aircraft higher and faster, until we reached orbital velocities.  Now, I can’t confirm that quotation, but manned, air/spacecraft that could fly up and out of the atmosphere were actively being conceived, build, and flown.  And airbreathing engines that could power these vehicles were under active development as well, and were considered an important, if not indispensable, component of those vehicle concepts.

So what went wrong?  Why has airbreathing propulsion fallen out of favor?  Well, the truth of the matter is that it really hasn’t, or more specifically, not everyone in the aerospace community has given up on airbreathing propulsion for space launch applications.  Nevertheless, within the “new space” community, airbreathing propulsion is pretty unpopular.  Some of this has to do with the fact that, as we’ve gained more experience and understanding with airbreathing propulsion, problems and technical challenges have arisen making their advantages less attractive.  Some would claim that their advantages were oversold, but I don’t subscribe to this notion.  Technical people tend to be naturally critical and are always looking for technical weaknesses, it’s part of our training and experience.   I think the reason that airbreathing propulsion has become unpopular is simply that we are in a big hurry to develop space launch systems now, we don’t want to wait five or ten or twenty years to develop some new airbreathing engine that may or may not help us build and fly a spaceship.  We want something that we know will work, is affordable, and that is available now.  Rockets are the answer now.  We can design and build them, and know that they will work for our launch vehicle concepts.

But there is a hidden cost to this push for what will work now.   It’s the same process that led us to where we are with our current Government/Industrial space industry.  We were so insistent on getting into space, on beating the Russians, that we went for the fastest, easiest, most assured approach for getting us there, which was to scale up our ballistic missiles.  The old complaint about the space industry being nothing more than “man-rated artillery” is now the very approach we’re choosing once again.  We’re choosing expediency over evolved development.  We’re choosing the “good-enough” of now over that of the promise of tomorrow.

But maybe this is the right choice for now.  Maybe to get to the place where we can accommodate “evolved development”, to the place where we in fact demand significant improvements to propulsion performance, we must build the rocket-powered systems of today, and let that develop an industry, an economy, a space infrastructure which can be self-sustaining, and will need and demand improvements to propulsion technology.

This is how free-market economies and civilizations grow and expand.  And it is this intrinsic flexibility of free-market economies which accommodate and nurture evolving methods and technologies, and feed back on their successes.

Historically, the development of launch vehicle systems has not experienced free-market forces.  Until very recently, all of our manned space launch capability has emerged from government programs, from a top-down, national priority to maintain manned space flight as a component of our country’s international prestige.  The priorities of these national programs were decided by a very few, specially-chosen government elites, who by their very intelligence and skill, could presumably choose the best technical approach.  But this is changing, and we are seeing the emergence of entrepreneurial efforts to realize manned space access.  This is why the “new space” movement is so exciting, and so important.

In the final analysis, the argument about whether or not airbreathers have a place in launch vehicle systems becomes secondary to how we will approach launch vehicle development.  Anyone who doubts whether free-market forces can do a better job that government elites in deciding what is the correct approach for something as relatively straightforward as launch vehicle development, need look no further than the current debacle of our home-mortgage industry, or our nationalized car companies.  Perhaps no better example exists than to look at our current national launch vehicle concept, a concept chosen by a elite cadre of our nation’s finest aerospace technologists, and compare the success of that program with that of launch vehicles being developed by private companies.

I would claim that if we allow it, nay, if we demand it, we can let free-market forces decide what the right approach is, and whether airbreathing propulsion has a role in launch vehicle development.  We can let all-comers try their hand.  Let a plethora of concepts take to the field, and let free-market forces separate the winners from the losers.  Cheer your champions!  Raspberry your competition!  But whatever you do, support the process, be an enabler of the free enterprise and entrepreneurism, and do what you can to make the field open to whoever has the fortitude to try.

So, does airbreathing propulsion have a role in launch vehicle development?  I’m betting that it does.  You don’t have to agree with me, you just have to let me get to the field.

Comments

[john hare]

The big question which faces air-breathers as well as the rest of newspace is, "How do we get on the field to play?" I think the existing market is even more inappropriate for ATR based vehicles than 'conventional' rocket vehicles.

New markets must be opened that the existing base cannot serve. Better for ATR is if it is a market segment that 'conventional' newspace cannot serve either. IMO, that market segment is barnstorming suborbital services from densely populated areas. An ATR has the ability to fly at relatively low noise and fuel consumption compared to a rocket. This implies the ability to self ferry, and operate from airport runways near cities, which is where the money resides.

An ATR vehicle could probably find usable runways closer to NY city than a pure rocket system for instance. A short flight at minimum noise to a low traffic area over water before lighting the afterburners for a suborbital mission. By flying parallel to the coast, a runway could be within 50 miles of the reentry zone. Land and refuel for return or on to the next customer out of Boston or Savanna.

Convenience to the customer is absolutely a selling point.

[plasma wind]

Well said,John. You make a number of excellent points. "The innovator's Solution" identifies just the approach you are advocating: under or non-served market segments.
The ATR may indeed be able to offer turbojet-like flight operations in terms of sub-orbital vehicles.
Regarding the noise-level of ATRs, on this point I will have to admit that I am unsure. I would imagine that an ATR would sound much like a turbojet on afterburner, through much of its power range.
But I find the idea of ATRs for sub-orbital vehicles to be a good match.

[jsuros]

Okay, here's my craziest post yet. The mention of barnstorming above made me wonder if ATR engines might make possible a two seater VTOL craft built along the lines of the Human (http://en.wikipedia.org/wiki/Star-Fury) or Minbari (http://en.wikipedia.org/wiki/Nial_class_heavy_fighter) fighter designs in the show Babylon 5. I don't know if it would make sense to put the engines at the ends of the wings or not, but the shapes seem aerodynamically stable for going straight up and straight back down.

I imagine you could sell quarter hour flights that go straight up, give you a few tens of seconds of "vomit comet" style zero gee at the top of the trajectory, then fall back to earth and use the ATRs for a powered landing. I'm not sure you could take off and land in a corn field like the barnstormers did, but it wouldn't take much of an airfield (or helipad?) to support something like this. Could it be made small enough or come apart to fit on a trailer?

For an extra treat, let the passenger spin the craft around its center of mass and fire a firework or two at the top of the trajectory. I'd pay a couple hundred bucks for a ride like that! Maybe more, just to be able to say I'd done it.

Ideally, the lightest craft possible would have 1 passenger and be flown remotely from the ground. We are building up a pool of military drone pilots who fly harder missions every day. Hire some of those guys to be the barnstormer pilots.

I can see competition pushing the performance of this sort of entertainment craft farther and farther until you got to the point where it was useful for bigger things.

[john hare]

I'm not really a fan of airbreathing engines for VTVL, however the NG-LLC 180 second flight could be flown with a mass ratio in the region of 1.3. (it doesn't qualify) For both your idea here and the barnstorming one, rockets and their propellants for the upper range get surprisingly light. If you are trying to reach 100 km from a mach 2 verticle climb at 30,000 feet, rocket mass fraction is around 1.3.

Simplifying, a VTVL takes off at 3 G (2 effective) on ATRs hits mach 1 at ~8,000 feet in 16 seconds. Mach 2 at 32,000 feet in 32 seconds. The ATRs keep thrusting during the rocket burn from this point to more than compensate for drag. Landing plays to the ABEs strengths also at low altitudes and airspeeds.

Total mass ratio for the above is about 1.5. With the ATRs only providing thrust to mach 2 as primary and mach 3 or so as assist, inlets and engine mass are easier than a mach 5-6 primary propulsion system. A system T/W of 20 seems reasonable.

Mass break down
ATRs 15%
propellants 34%
rocket engines 3%
Tanks 3%
structure 20%?
payload 25%?

At first cut, 1,000 pounds of pilot and passengers in a 4,000 pound GLOW vehicle. Three passengers in a trailerable vehicle. There are many other issues, such as two engine systems in a single airframe. The above is optomistic to say the least.

I think a more viable early market is a propulsion system that allows companies to wring out their airframes simulating rocket behavior. To about mach 0.5, the inlet can be almost eliminated sacrificing some performance for installation simplicity. The resulting installed engine should reach T/W in the 25 range. While still twice as heavy as the rocket engine, it would allow 3-5 times the powered flight time per flight. This would give margins in the low speed end during the very early test phase.

Later on, pilot training would be well served by installing ATRs in a similar vehicle to the operational article. Reference Paul Breed using a RC helecopter to develop VTVL flight software and personal practice flying with it.

Lynx, Pixel, and Mastens manned units could all use something of this nature.

[John Bossard]

The level of creativity here has caused my head to explode. Please allow me some time to collect the pieces, reassemble them, and try to regain sentience. I will respond when I recover.

[john hare]

The noise level seems managable from where I sit. The vehicle is going to need thrust in excess of the vehicle mass during the boost phase. A decent airframe will allow a thrust to weight of 0.25 at take off and climb out. With little or no afterburning, the ATR should compare to a medium bypass turbofan at this phase of 15-25% power. The commercial turbofan has a core pressure of about 30 atm, which is similar to the gg pressures in the ATR.

[john hare]

To illustrate my point on airframe check out and pilot training, consider the shuttle Enterprise. It was used for a few drop tests and then retired.

If mature ATRs had been available in the 70s, then 10,000 pounds of ATR engine could have provided over 200,000 pounds of thrust in place of the 21,000 pounds of SSME. By eliminating a very few systems not required for flight test in addition to the 11,000 pounds saved on main engines, the vehicle could have been flown 2-4 minutes at full power. Considering the XCOR experience with the EZRocket and Velocity, This would have been good for several minutes of actual flight per fueling.

Flights could have been performed early enough in the program to possibly affect some of the design decisions. Without the need to mate it to a carrier aircraft before each test, many more flighs could have happened. Test flights could have been in the hundreds instead of handfull(s?). Low speed handling would be much better understood if this had happened.

For training purposes through the decades, the Enterprise could have put many new pilots in hardware to back up the simulators. Mission pilots could have flown proficiency flights before every real mission for an added degree of confidence. No shuttle pilot has enough real hardware landings to be type rated in any other aircraft, and the shuttles have never been landed by a pilot that would have been considered current.

[jsuros]

I had another idea for the barnstormer engine. Could you place a second gas generator inside the engine coaxial with the turbomachinery but just downstream of the turbine? You could use this gas generator to feed fuel into the combustion chamber in ATR mode and burn fuel and oxidizer in it in a rocket thrust mode. This should get around the turbine heating problems in other combined mode engine ideas.

I got the idea when I saw that turbines designed to maximize crankshaft power rather than thrust, as in an ATR gas generator, expand the axis of the compressor just before expelling the gasses. I assume this has something to do with getting more work out of the flow. The Saturn & Centaur lines from Solar Turbines are good examples of this.

So, why not fill that void just south of the turbine with a rocket combustion chamber?

[Randy Campbell]

I'm noticing we "blew-his-mind" and it's only been a couple of days :o)

So, John H. who won the pool I know it wasn't me, I expected his sanity to last a whole week at least...

Engineers... they just don't build em like they used to ;o)

Randy

[Randy Campbell]

John H:
Noise levels are going to be a very real concern given the current and projected noise level allowences over inhabited areas.

But I think you all have managed to "slip" past the most probable "killer-app" for the ATR:
"Economical" long and short range Point-to-Point service.

Randy

[john hare]

At low thrust levels, I expect the ATR to have an acceptable noise level as the afterburner is not in use.

I don't have any understanding of the point-to-point. If someone will pay $500.00 a pound or more to jump to 100+ km and sight/float/see, I don't see the market matching that price for short hops. Long jumps are a serious challenge that may be in the second generation at least.

I've done a couple of skull sessions with John B. His mind blown is just an integration of the pieces and accuracy check. Now we need to watch him because his revenge is probably going to be one of those subtle things from left field.

[John Bossard]

Ok, I'm reassembled.
John Hare:
In principle, an ATR should make thrust without afterburning, but this would not be considered a typical operating condition for an ATR. Normally, the ATR’s combustor stays lit.
The difference in noise-level between lit and un-lit combustor I would suspect to be quite large. So maybe “quiet” operation with an ATR is possible is possible with an un-lit combustor, if it can make sufficient thrust.

I, too, am not super enthusiastic about ATRs for VTVL vehicle concepts, but that doesn’t mean that they might not be a good fit there. I simply haven’t looked into that application deeply enough. But, if any airbreathing engine could perform well it that application, it would be the ATR: high T/W, deep and fast throttling, reduced-sensitivity to inlet distortion.

Regarding, NG-LLC, I thought one had to use rockets. It’s for the moon, right?

Regarding, ATRs on the enterprise for a landing trainer: not a good fit, IMHO. As a matter of practice, trying to put airbreathers on vehicle initially designed for rocket propulsion is just not a good idea. You can sometimes get away with putting rockets on an airbreather-based vehicle, but again tricky. The proper approach is to design the vehicle use ATRs from the beginning, rather than as a retro-fit.

We’ll have to get together for some more skull sessions, I’ve missed those days.

Jsuros:
Yes, you can put a gas generator downstream of the turbine, use it to make rocket propulsion. This concept is known as the Air Turbo Rocket/Rocket (ATRR), and there are a number of variants. Challenges include integration, compromises to the combustor in ATR mode, and compromises to the rocket’s nozzle performance. I think this concept has merit, but it will take careful design and a good understanding of design performance compromises to get a good design. Search the AIAA publication data base, and I think there are some papers there that discuss these concepts.

Randy:
The application of ATRs to suborbital vehicles I think holds a great deal of promise. ATRs have a relatively high T/W, a relatively low TSFC. Another nice thing about ATRs is that they are (relative to turbojets) easy to turn off and on; very much like rockets. That means that, for an exo-atmospheric suborbital trajectory, you can shut the ATR off as you leave the sensible atmosphere, then turn it back on again when you reach suitable flight conditions once your endo-atmospheric. So this seems to potentially good application.
A relevant paper on this subject is: “Introduction to the Problem of Rocket-Powered Aircraft Performance”, NACA Technical Note (TN) No. 1401, December, 1947. This paper explores the use of rockets for endo and exo-atmospheric flight. If one looks at this paper through the lens of an ATR, the results seem quite interesting.

Tourist flights to 100 km, and point-to-point transportation seem like good applications to me. But critical to this development, whatever propulsion option is selected, it must be integrated into the overall vehicle design. Providing people and/or cargo transport is obviously a different market than that of space tourism, and I don't have a feel for whether or not you could turn a profit, but that's the kind of high-traffic businesst that drives innovation; exactly the kind of free-enterprise approach I talked about in the above article.

Just my 2 cents.

[jsuros]

FYI - “Introduction to the Problem of Rocket-Powered Aircraft Performance” is available for download at the NASA Technical Report Server (ntrs.nasa.gov).

[jsuros]

John,

$500 a pound is WAY out of the range of a first generation barnstorming ATR vehicle. That's virgin galactic money with an insanely platinum cost structure and a state government buying you an airport.

Think about giving the passenger a chance to join the tiny fraction of one percent of the human race who have gone mach 1+ and not worrying about the limits of the engine. Think of getting high enough to see the sky go dark in broad daylight and not approaching the technical definition of the edge of space. Think about poking along with a vertical ascent for a minute or more before going supersonic to minimize the stress on your glorified sailplane of a craft. Think about a VTOL flight as cheaper to do because you offer zero cruise in any direction but up and down. Think about launch infrastructure that consists of one each Air Products delivery trucks holding LOX and LH and equipment that fits in the bed of your pickup truck tow vehicle to interface the tankers and your vehicle. Think of $5 a pound ticket prices and operators who lease their vehicles.

Think of barnstorming, in other words.

Sorry. End of rant. That price assumption pissed me off is all.

[john hare]

Jsuros,

I accept your feelings on the price matter. As a businessman, if somebody is willing to pay $500lb, I might cut to $200lb, but not $5lb. You may be right on the second or third generation, but I think I am on the first. There will be an investment to recoup.

[John Bossard]

IMHO, talking about $/lbm is way premature. There is so much information that has to be created and understood before any meaningful discussion on prices and cost can be undertaken.

We spend a lot of time speculating about and trying to identify potential “markets” that suborbital vehicles could satisfy. But one customer that gets neglected, probably because they generally don’t have the funds, is yourself. What need or want to you have that could be fulfilled by the suborbital launch market?

New markets are created, and they are created by the wants and needs of individuals, sometimes without knowing the exact nature of what is wanted and needed. One of fundamental ways that a new market is created, is that you become your own first customer. If the free market cannot or will not supply what you need, then you have to create it for yourself. Sometimes others follow, and sometimes you remain your only customer.

I believe the same will hold true for certain markets in the “new space” industry.

So, my thoughts about creating new markets for suborbital vehicle applications are to consider a two-fold process: First, identify what are your needs and/or wants. Do you want a 2 minute joy ride to some altitude? Do you want to see the blackness of space, the curvature of the earth? Do you want the experience of operating a supersonic aerospace craft?
Secondly, you have to scale your wants and/or needs to the funds you have available. This is an important step, and one of the great freedoms and powers that being your own customer grants you. You get to find the intersection between resources and needs/wants.

Here’s a classic example. When guys get together with cars, they want to race them. In the south, moonshiners liked to race their tricked-out cars. Guys would get together, on a dirt track carved out of somebody’s bottomland, and race their cars, just for fun and bragging rights. Over the years, as more and more guys wanted to participate, or even just to watch, this enterprise became known as NASCAR, one of the great sporting venues in the US. It started not because some marketing guru put together a great business plan, but because guys wanted to race cars. They started small, and the activity simply grew organically.

Here’s an example from my own experience. I wanted to experiment and develop the ATR engine. Obviously not a big market, I couldn’t find any ATR to buy, let alone one I could afford. To date, I am the only customer for the ATR engine that I built. It certainly would have been possible to put together a proposal, submit it to the government, and maybe win some funding to build such an engine. I’ve done it before, I know how to do it. But I decided I wanted to use my own resources to create an ATR. So I scaled down the size of my ATR concept until it fit my available resources. I was no longer trying to build a 2500 lbf thrust ATR, I was building a 80 lbf thrust ATR, and now a 20 lbf thrust ATR.

“Build it and they will come?” Hard to build a business on that assumption. But by being your own customer, you have a place to start. And don’t discount the fact that what you want/need may likely be something similar to others wants/needs.

[jsuros]

Working for the cheapest multinational on the planet gives one a sense of how costs can be minimized. I have an idea for lowering the cost of an ATR engine I wanted to run by you.

Unlike a turbojet, all of the air that passes through the turbomachinery of an ATR is either from outside the engine or from the gass generator. Given this, have you ever considered using ablative composite materials for the body and thrust chamber of an ATR engine? Microcosm (www.smad.com/scorpius/lowcost) has been promoting such materials for years in LOX/HC rocket engines. They cite low fabrication cost and part count as advantages of this technology.

Could an ATR design benefit from these materials and construction techniques?

[Randy Campbell]

John B:
Interesting approach and outlook.I like it :o)

Now lets examine that philosophy; how about we (ok you :o) push up instead of down? How rough to make a single 800lb thrust ATR? Or maybe two 400lb thrust engines? Why? Why not? What's "in-it" for the group that test flies the first "technological demonstrator" ATR?

The example exists:
http://www.xcor.com/products/vehicles/ez-rocket.html

What's needed to make that idea happen really? As a 'barnstorming' aircraft it would be ideal also you realize folks, heck the chance to sell engines is even there! XCOR for a while had plans for sale on their website for sub-scale copies of the X-1 and Me-163 powered by thier engines...
Comments?

Randy

[John Bossard]

Jsuros:
Ablative materials for ATRs could well make good sense for expendible engines, such as you might use on a tactical missile.
For aerospace vehicle propulsion, i.e. applications where you need to use the engine again and again, ablative materials would not be a good choice. I'm very familiar with Microcosm and the good work they do.

Randy:
Building an ATR of any particular thrust class is a relatively straightforward proposition. But I think you’ve identified the key issue: what’s in it for them? And is their motivation sufficiently strong so that they can come up with the funds?

XCOR is indeed a good example, however I gave an example of how an exchange would go with them in comment #17 of John Hare’s post on Turbo Rockets in Selenian Boondocks. IMHO, the X-1 and Me-163 vehicles would not make good vehicles in which to try and install an airbreathing engine like the ATR, nor would their new vehicle concept, the Lynx. These are rocket powered vehicles, and as I’ve mentioned before, it is very difficult to try and put an airbreather on a rocket vehicle, although vice versa can be done (aka the EZ-rocket). I think the best way to use the ATR is to design and build and purpose-built vehicle. Short of that (which would be quite a large effort), it might be possible to find an existing homebuilt aircraft of more limited performance that could accommodate an ATR. A good candidate, in my opinion, would be the BD-5J. The BD-5J uses about a 350 lbf turbojet engine. A comparable ATR would be even smaller (and thirster), but the vehicle would need to carry an oxidizer (LO2), which adds quite a bit more risk. There are some other candidates as well.

FWIW, in 1999 I gave a presentation at EAA Airventure (Oshkosh) entitled: Homebuilt Aerospace Planes: A new Arena for the Homebuilder. The premise of this presentation is that the intersection of aircraft homebuilders and experimental rocket/engine builders may make it possible to build your own homebuilt aerospace plane. In that presentation, I suggest that for under $1M, one might be able to put together a modest suborbital craft. In that example, I assumed the rocket motor might cost on the order of $350k.
I still believe that the “homebuilt aerospace plane” approach might be viable, but it will take quite a bit of money (from a homebuilder’s perspective), and quite a few years.

If anyone is interested in my slides from this presentation, let me know and I’ll make them available for download at Plasma Wind.

This is some really great discussion.

[Randy Campbell]

I'd be interested in the slides :o)

Google "SpaceCub" at some point by the way :o)

I wasn't actually suggesting contacting XCOR by-the-way nor do they even offer said "plans" or kits anymore. My point was that THEY had at one point intersected the idea of a 'home-built' rocket plane as an informational note.

Though of note they (XCOR) are now offering piston-pumps for rocket engines which from my understanding are much sturdier and operationally more reliable than tubro-pumps and from a maintenance point-of-view much easier to work on. Just an FYI thrown out no actual suggestion intended :o)

Your quite right the BD-5J is a good candidate airframe, I'll note that models have been flown under pressure jet power.
http://www.rqriley.com/gluharef.html
Example I've seen lited used two of the 130lb thrust engines.
Pics a bit blurry but here:
http://www.bd5.com/ulthm52.htm

As a 'note' since I'm on the subject (sort-of):
I'm kind of assuming here and should get some clarification.
Your talking carrying oxidizer aboard so are we talking a GG-ATR? I actually have a set of plans for a version of the above mentioned pressure jet engine, and it is what got me interested in using propane as both fuel AND pumping energy for a jet-type engine because the technical data gave me my first real understanding of propane's "duel-energy" ability.
Specifically, by pressurizing, (heating the tank is suggested in the technical data but discussions with the company also confirmed that the fuel can be directly pressurized in the tank similarly to a rocket pressurization system) the propane tank and using a liquid feed system and running the liquid through a heat exchanger in the combustion chamber the propane achieves a super-sonic flow rate once it leaves the heat exchanger. This would seem to me to be ideal for an expander cycle ATR use and very simple to impliment into the engine design. (My details on injecting the propane into the combustion chamber along with the compressor air are a bit fuzzy though :o)
Technical details of how the pressure jet works are here:
http://www.tipjet.com/tech_data.htm

I only half jokingly suggested on selenianboondocks that the FMX-4 airframe could be used. In actuality the frame would have to be covered by something other than cloth as the original versions was but having discussed a rocket powered version at one point with the designer that's probably not a major fault.
http://www.facetmobile.com/

Work on the follow on larger FMX-5 is on hold, but the design has already flown as a "near-space" payload return vehicle listed here:
http://www.wainfan.com/topper.htm

The concept of a tubular structure and sheet metal/cloth or composite panel covering is interesting and robust and the idea of interlocking composite panels is genius.

Randy

[Randy Campbell]

As an 'aside' comment:
John it would only take 10 of your 80lb thrust ATRs to lift the FMX-4 at a T/W of 1/1 :o)

(Oh and that should be a correction, the covering was cloth over aluminum sheeting not JUST cloth :o)

Randy

[John Bossard]

Randy:
The spacecub was a really novel idea and ahead of it’s time.

Your mentioning XCOR was a logical reference, and now I understand your meaning better. They have some talented people working there.

The Gluhareff Pressure-jet engine was a very interesting propulsion motor indeed! I purchased the instruction and operating manuals year ago. I actually got to meet Mr. Gluhareff back in the late 1980’s when he was still alive. He lived way out in desert, southeast of Mojave. We dropped by for part of a day. He fired up a few engines, and told some great stories about working as an aerospace engineer in CA.

In general, when I speak of ATRs, I’m talking about GG-cycle ATRs, rather than regen (sometimes called expander-cycle) ATRs. All of my testing and hardware experience is with GG-cycles.

For a regen ATR, propane would not be the best choice, in my opinion. I believe it would function, but the problem with propane is that it has a large molecular weight (about 44), giving it a small R, so as an enthalpic gas, it won’t be able to transfer as much energy to the turbine as a lower MW gas would (that’s why hydrogen is so great).

Propane makes a better choice for a gg-cycle, and the notion of using sub-cooled propane to improve density, as suggested by Jon Goff and Jon Hare, could be attractive as well.

Alternatively, methane would probably work well as a regen turbine drive gas, its MW is quite low (about 16).

Using the FMX-4 as a flying test bed is a crazy idea, so crazy…it…just…might….work…..

Wonder how it handles at high speeds.

[jsuros]

I'm interested in your slides, too! Please post them.

I discovered today as a result of looking that one of my company's divisions, Kyocera Industrial Ceramic Corp, makes silicon nitride turbine blades. Small world. I guess we are trying to sell the monolithic ones for use in small electrical generators (3kW). That translates to 9-12 kW of mechanical work, I suppose.

I'm guessing your 80 pound engine develops more than 9 kW of turbine work in its gas generator.

[Randy Campbell]

John B wrote:
(You'll note some of this stuff is out of order, apologies since I'm shooting this off "real-quick" I hope :o)
>Using the FMX-4 as a flying test bed is a crazy
>idea, so crazy…it…just…might….work…
>Wonder how it handles at high speeds.

First and foremost I hope your serious because "I" am :o)

Since it had only a "2-stroke's from dying" Rotex for power, (which died in a climb-out on take off and wrapped the prototype up in a barbwire fence but no injury due to the construction used) I'm guessing about average IF you actually put a power plant that works in it :o)

Depends on how 'high' speed you're talking, I had an email discussion with Barnaby Wainfan on a possible rocket powered version of the FMX-4. It was pointed out right away that modifications would need to be done to allow an aft mounted power plant instead of the current tractor, I pointed out that basically installing a 'reversed' and elongated copy of the frameing of the cockpit area would probably work as a thrust structure. In the case of the ATR though I'd probably bottom mount them (I'm assuming multiple engines because even if the 80lb thrust model can be worked up to 100lbs, Gross-Take-Off-Weight with only the Rotex was almost 800lbs, besides it gives one 'spares' to work with :o) in sort of a 'modular-pod' to allow engines to be swapped out as work progress.
Control authority is going to go to hell quickly above certian speeds because the majority of your control surfaces will begin to be 'shadowed' at higher speeds due to the high front face of the aircraft. But it was suggested that adding out-board movable horizontal surfaces would allow the design to "probably" work up above Mach-1. In addition I'd suggest elongating the nose by extending the line of the three (3) forward panels and framework and semi-reclining the pilot since the propellant load will be larger than the original. (I have a "Cyrano" version I've done in paint if anyone is interested once I've finished the "3-View" version :o)

Jordan (the 'volunteer' pilot I mentioned :o) actually hates the FMX-4 and thinks it's ugly and not 'cool-looking-enough' for him to fly, but mention it being a possible ATR test bed aircraft (oh and him getting to paint it himself... he's an art major what can I say :o) and he suddenly can 'stomach' flying it :o)

>The spacecub was a really novel idea and ahead
>of it’s time.

Way ahead, but a good example of what an internet team can acomplish research and organization wise if they put their minds to doing something. There was also a "home-built" X-Prize entry called the "Kitten" but not much information to go on with it.

>The Gluhareff Pressure-jet engine was a very
>interesting propulsion motor indeed!

"Is" is more accurate I suppose, both Mr. Gluhareff's original company (now "tiprotor.com") and R. Quigly enterprises sell licensed versions of the plans while a dozen other companies are selling 'rip-offs' of various versions.

>I purchased the instruction and operating
>manuals year ago.

I bought a plan set for the 20lb thrust motor myself :o)

I guess what I'm not understanding is why a similar fuel set up as that of the GPJ (Gluhareff Pressure jet) won't spin the turbine at high speed?

>In general, when I speak of ATRs, I’m talking
>about GG-cycle ATRs, rather than regenerative
>sometimes called expander-cycle) ATRs. All of
>my testing and hardware experience is with
>GG-cycles.

Thanks I get that feeling now :o)
I guess I'm not understanding or making myself clear here in that while I understand you working with what you feel most comfortable with, but I keep reading you as insisting the ATR needs an oxidizer supply on-board to run the gas-generator/combustion chamber when you HAVE a source of high pressure air you can tap off to burn with the fuel in the GG?
Let me caviat that with my own experiance which for the last 4 years has been working with cruise missiles and their engines.

While you mention in the initial articles that the ATR CAN have "only" one compressor stage there is nothing overly complicated about having two. The small turbofans in the missiles have multiple compressor and turbine blade sets they also have two (2) "component" sets: An axial section that bypass' the air around combustror section and a "centrifugal" high-pressure section that feeds only the combustor section.

For an initial test set up and engine configuration I'd suggest something similar to feed 'bypass' air into the after burner section and the other section feeding the "rocket engine" combustion chamber.

(I have to go and will have to finish this reply later)

Randy

[john hare]

For an early revenue app, the backpack rocket seems appropriate. Low thrust-400lb, low altitude, the few users are professional showmen, and demonstrates the tech in front of large crowds. The extra Isp compared to H2O2 monoprop should make for a better, and more valuable show.