More about Air Turbo Rockets

I know John, He's a good guy and a smart guy, but, A very wise friend of mine once said "Hybrid concepts are very dangerous, they take what are two decent ideas separately and marry them together hiding the weakness in the corners between the two main concepts." He had examined a number of hybrid concept systems and found that under detailed analysis, those weaknesses come out and come out badly.

Posted by anon at 07/09/09 10:41:58

Obviously, Anon's very wise friend and me are not the same person!

I would be interested in knowing what hybrid systems I've examined, and what their weaknesses were.

One must be careful about confusing the notion of a "hybrid system" with that of a "compound system". I think it could be argued that the first successful private suborbital spacecraft (spaceshipone) used such a "compound system": turbojets on the carrier aircraft (whiteknight), and a rocket on the spacecraft. As one can see by the responses to these postings, there is interest in this approach, but I fully acknowledge the difficulty in trying to integrate different propulsion modes and systems. The probability of failure is large, and some would argue that they payoff is small and does not justify the potential benefits. But perhaps it may be just a small advantage that makes the difference between a viable approach and a dead end.
This area is a good place for guys like me to work: low-probability-of-success approaches, working alone in dusty garages, hoping to discover an interesting and useful synergy.

Posted by John Bossard at 07/09/09 12:13:08

a good idea that will NEVER become a "cheap" idea! "save" a few tons of LOX never will be cheap enough to worth the costs to develop and build a "turborocket" and the same will happen with the UK Skylon (that has several other design flaws)

Posted by one at 07/09/09 12:14:32

I would also like to point out, and this is important to me professionally and personally, that I make a firm distinction between "transition engine" concepts, which are notional and highly-speculative, and ATR engine development work, which is an active research and development area, has real hardware, and has genuine potential for a number of interesting propulsion applications. (see my latest post at Plasma Wind, and sorry for the plug).

Posted by John Bossard at 07/09/09 12:21:45

John it’s great to have another air breather on board. You’re probably wasting your time with this rocket deranged bunch. Anything like air breathers or combined cycle concepts that threatens the pure rocket private space paradigm will get a sound bashing. As a student of Paul Czysz MCDD I am profoundly air breather lifting body deranged. Great post keep’em coming.

Posted by Doug at 07/09/09 19:07:24

Jon
How about reusability, durability, dependability, servicing and turn-around time of the ATR? To me beyond performance these are key factors to consider in propulsion selection. What is the weakest link or most common mechanical failure mode of the ATR? I find the ATR rocket combination “transition engine” most intriguing.

Posted by Doug at 07/10/09 00:23:33

John

While we have met several times at conferences, I was not referring to you in the discussion of the ATR and the general issue of "Hybrid" systems.

I also try to distinguish between Staged systems and "Hybrid" systems. Air Launch is a staged method, While ATR or Air Augmented Rockets are Hybrid concepts.

In "Staged" systems, there are clear
interfaces, boundaries between operation and clear start and end points. The Saturn V had a clear interface and job for the S-1C, the S-IVB and S-II stages. Each interfaced with the gantry, with each other and had clear start and end points, and you could determine if they were going to work or do their job, independently. The X-15 was Staged off the NB-52, and the NB-52 had it's job, and when it was at the staging point, the X-15 had it's job. Same with an F-15 and a sidewinder missile.

a hybrid system blurs the lines between systems and it's very easy to hide defects between them. Consider the ROTON-A, which was using small tip jets to spin up the rotor, use the rotor to climb out of the atmosphere, and then swivel the tipjets to near vertical, use the tipjets to climb to orbit and reverse that for re-entry.

The Hybridization was hiding the fact that it was both a bad rocket and a bad helicopter.

The X-30 NASP was a bad rocket a bad SSTO and a bad air breather. But the hybrid approach hid that from casual inspection.

So Forgive me, but I am very cautious of hybrid systems.

Here's a quick test of any hybrid. Draw a decent concept up, apple weights, calculate performance using standard atmosphere, calculate trajectory,...

Then

take that same weight and figure out if you can do a similiar pure rocket.

try to remember when the Turbine is helping you and when it's hurting you.
Consider, lets say you can make a turbine survive rocket exhaust. in the atmosphere it's doing real work, bleeding energy from the stream, but, then it's adding thrust, so, that's cool.

At some point, the turbine starts seeing
supersonic flow, so you have start stalling the atmosphere in, then speed it back up, because rotating blades hate supersonic flow. so you need to figure out that drag penalty, then as you go exoatmospheric the turbine has to start dumping work into a dummy load. because it's still drawing power off the rocket, but there is nothing for it to work against and it will overspeed. calculate that work energy, figure out where you need to dump that work/heat to. calculate how much energy the turbine is dragging out of the rocket exhaust.

the great flaw of many conceptuals is people use an ideal system with zero loss, and simple design. If you want, take something like the j58 engine from the SR-71, use it's weights including inlets and exhausts, remove the weight of the combustors and add the weight of your rocket fuel. I think you will find it to be very unpromising then.

Then try this again using the weights of the F-14 F-100 engine and inlet system.

Posted by anon at 07/10/09 10:16:30

"One" AKA GM, quit polluting this site with your inane comments

Posted by me at 07/10/09 14:21:40

Doug:
As a recipient of many a’ rocket beat-down, I very much appreciate your kind words. Thanks for the support! I have always had a tremendous about of respect for Prof. Czysz, he’s done some really ground-breaking work over the years. It must have been great to have had him for a prof.

Regarding some of the “illities” of ATRs, I think the intrinsic simplicity of the engine helps in these regards. Much of my work on ATRs has been associated with tactical missile propulsion, and in that case, service life is measured in minutes. I believe that the weakest link for the ATR is the compressor. The adverse pressure gradient of the compressor flow demands careful attention to design, but is less of an issue for the ATR compared with an turbojet (see performance attributes: part II of my posts on ATRs at Plasma wind, and apologies for the plug).

Anon:
Thank you for the clarification and discussion regarding hybrid systems. And I fully agree with your comments regarding the difficulties and challenges associated with trying to combine different elements. Synergy can also can make a system perform worse that the sum of its parts! So one needs to be very aware of these interactions.

The J58 was (is) an amazing engine. If you’ve ever looked at even just the fuel control system for this engine, it is an incredible piece of engineering work. The engine is also incredibly complicated, and finicky because it combines turbojet operation with ramjet-like operation (and perhaps this is the point you’re trying to make). This engine was specifically engineered for use in a high-speed cruise aircraft. So the design constraints are fundamentally different than those for an engine that you would use as an airbreathing component to a suborbital or orbital launch vehicle concept. So I would agree with you that trying to use a J58 for some sort of launch vehicle concept may very well not trade well.

All engine start as conceptuals. Furthermore, the very basis of engine performance models are based on idealized thermodynamic behavior. We include efficiencies, “knock-down” factors, et al, in an attempt to understand system sensitivities, and help our theoretical predictions match expected, and then measured results. Despite these simplifications, we can learn a lot about system requirements from trying to quantify our concepts, however flawed may be our initial attempts. And, eventually, reality sets in. As one of my old bosses used to say “ the first thing you hear about something it the best thing you’ll hear”.

Can an airbreather make for a viable propulsion element for suborbital and orbital vehicles? Hard to say, and I don’t think anyone has the final word on this discussion. So we keep trudging along in our own little areas, creating the technologies that may (or may not) be part of the answer.

Posted by John Bossard at 07/10/09 15:29:11

John

All Powerpoints look good at first :-(

Now rockets tend to be accelerators not cruisers and air breathing is all about cruising. The longer the time in sensible air, the better for a cruiser.

Posted by anonymous at 07/11/09 18:16:03

Anon:
Calling the Roton-A a "bad-rocket/bad-helicopter" hybrid ignores a lot of factors inheirent in the system that synrgetically would have made the system work. But I suppose the MAJOR issue is "which" Roton are we talking about?

The ORIGINAL Roton concept and vehicle design took advantage of the known properties of a high pressure rotating rocket engine to proivde mechanically simple high thrust and ISP while adding aerodynamics to the 'rotor' added a very important atmosphereic 'assist' up to around 30,000ft or so. So the rocket portion of the vehicle was quite 'good' and actually well proven and understood. The ORIGINAL rotor/prop was shown to be able to give quite a high performance boost below supersonic speed AND (worth noting which is why I was asking WHICH model Roton :o) was a dynamically stable rotor configeration with well known and proven capability.

That is as long as you left the rotor on the BOTTOM of the vehicle. Flying platform research has shown that unlike a helicopter, which is dynamically UNSTABLE and requires constant control due to its high mount position of the rotor, the LOW mounted rotor is completly the opposite of the helicopter both in control authority and stability.

The major 'fault' (other than the size increase due to the percieved market need) with the Roton concept was moving the rotor from the bottom to the top of the vehicle. This turned the "hybrid" vehicle from a stable and flight capable concept into an unstable and dangerous "rocket-with-helicopter-blades" vehicle with all the faults of a helicopter and none of the advantages.

Since the ONLY flight model of the Roton that was demonstrated was the "rocket-helicopter" using that single example while ignoring historical data on the succesful series of flying platforms does not give a true picture of the capability of a Roton-type vehicle with a low mounted rotor design.

This ALSO applies to any attempts to "consider" hybrid propulsion systems. Your examples of using the J58 and F110 (not F100, that is the F-16 and early F-15, the F-15 is currently being switched over the F110 btw :o) F-14 "inlet" and engine system as a whole without considering the differences that are required for a combat fighter jet engine and inlet system (or high-speed recon aircraft) over those that would probably be used for air-breathing launch assist while highly conservative don't paint a true picture of the ATR type engine.

According to work done by Danni Edar for Boeing the F100 with a simple direct inlet was usable for launch assist as a vertical booster with minimum work up to Mach-2 and 50,000ft. The work assumed a fixed inlet geomety similar to that used in the F-16 fighter which also can fly at Mach-2, but unlike the larger and heavier F-14/15 did not require a heavy variable inlet system to achieve these speeds.

As such it would be easy to "assume" for arguments sake that even using a rather "simple" movable intake and exhaust cone system similar to that used on the SR-71 but NOT using the massive and complex J58 turbine bypass and hydralic system that ducted the majority of the airflow around and not through the J58 engine.

While the probable "top" usable speed for the ATR is somewhere around Mach-6 you're really wanting to leave the atmosphere behind at that speed anyway. The key here would be not having the compressor and turbine aerodynamically connected as John Bossard notes on his blog. Nor would the turbine be "dumping" power into a "dummy" load as noted in addition to powering the compressor the turbine can power the fuel pumps, along with running generators for electic power. And that doesn't take into account that you can always design the turbine section to be bypassed in pure-rocket mode if you really wanted too. (I'm too tempted to keep them running the propellant pumps though)

All in all: YES hybrid designs SHOULD be examined closely, but that requires that they hybrid designs be well understood as to the differences and similarties to systems one is familiar with be CLOSELY examined ALONG with ones assumptions about those system and the "hybrid" which one is looking at.
(Though I agree on the NASP/X-30 as whole. Anyone who thinks airbreathing all the way up to orbital speed is a GOOD idea isn't thinking clearly.. or at all :o)

Randy

Posted by Randy Campbell at 07/12/09 14:11:16

anonymous wrote:
>Now rockets tend to be accelerators
>not cruisers and air breathing is all
>about cruising. The longer the time in
>sensible air, the better for a cruiser

This is actually a bit of 'false-wisdom' that "everyone knows is true" but isn't.

Air breathing engines CAN be accelerators they just haven't been use much in that role. Glenn Olson had a HUGE amount of ramjet data on his site (alt-accel.com) that PROVED that ramjets and other air-breathing engine while traditionally USED in 'cruise' mode could and would equal rockets in acceleration mode with much higher ISPs as long as they were DESIGNED to do so!

The classic example is when a ramjet with a Mach-2 max inlet design suddenly accelerates to over Mach-4 JUST because of a stuck fuel valve, that's pretty strong evidence you might be missing out on engine performance in your design.
(ASLAM missile test)

Both the F100 series, and F110 series turbofan engines are considered 'acceleration' engines in that they are both designed to have acceleration margins that allow them to accelerate thier designed aircraft system VERTICALLY against gravity without the use of afterburners. This in addition to thier roles as cruise and combat engines.

Randy

Posted by Randy Campbell at 07/12/09 14:21:19

Your comment: Name: Website: E-mail:

Enter the string of characters appearing in the picture: Remember Me