Plasma Wind

The Air Turbo Rocket: Performance Attributes (Part II)

As discussed in Part I, fundamental performance differences accrue to the ATR relative to the turbojet because the ATR’s turbine and compressor are coupled thermodynamically, but not aerodynamically.  In this second part, we will continue discussing ATR performance attributes which arise from this coupling arrangement.

Inlet Distortion

As airbreathing engines fly through the air, they have occasion to pitch up and down, yaw side to side, and to roll, as the vehicle that they are propelling maneuvers around or changes its orientation.  When this happens, the flow of air into the engine can become disturbed, and may not be flowing into the engine uniformly or even symmetrically.  The non-uniform flow of air into an airbreathing engine is known, generically, as inlet distortion.  It can occur, for example, when a flight vehicle yaws to one side or the other or pitches up or down.  Inlet distortion presents a non-uniform air flow distribution to the compressor, and if the distortion is severe enough, in can induce the onset of instabilities in the compressor's operation.  In a turbojet, it can upset the flow in the combustor, reduce power output from the compressor, and even cause a flameout.  In an ATR, the power to the compressor is independent of the airflow, whatever it’s doing.  Because of this, the ATR is much less susceptible to the bad effects caused by inlet distortion, compared to that of the turbojet.

Surge/Stall stability

Turbocompressors can encounter several types of instability modes.  Compressor stall is the condition wherein flow over the compressor blades stalls, just like in an airplane wing.  This stalled condition can be passed on the to next compressor blade, rotating around the compressor at some integer value of the rotor speed.  The next effect of stall is a reduction of compressor performance in terms of delivered pressure rise and air flow rate.  Compressor stall is not usually destructive, however it can be a precursor to compressor surge. Compressor surge is a more severe instability mode because it can result in mechanical damage or destruction of the compressor itself.  Compressor surge occurs as a dynamic coupling between the compressor pressure/flowrate output and the ability of downstream components to pass the airflow.  In compressor surge, the dynamic coupling is unstable and grows until it reaches a limit-cycle behavior, which is often the dynamic disassembly of the compressor.

Greitzer provided one of the classic treatments of surge/stall characteristics of axial compressors, and their downstream components.  He developed the dimensionless parameter, B, to quantity when the  onset of surge/stall may occur in a turbocompresor system.  Using Grietzer’s B number, Bossard analyzed the compressor stability characteristics of ATR systems.  Based on this analysis, Bossard concludes that  the operating conditions under which a compressor may enter into surge or stall are basically similar between the turbojet and the ATR.  However, the difference between the two is which instability mode the compressor will enter into, surge or stall, once the compressor departs into instability.  Whereas a turbojet may more easily depart into the stall mode, the ATR is more likely to depart into the surge mode.  This finding was a surprising and significant result of Bossard’s paper. Furthermore, the ATR lacks the self-limiting behavior of the turbojet, once the compressor enters an instability mode.  On the positive side, this means that the ATR’s thrust output will not degrade as severely as that of the turbojet.  On the negative side, it means that if a departure into surge occurs, it is more likely to lead to damage or catastrophic failure of the ATR.

Engine Throttling

The aerodynamic decoupling of turbomachinery components within the ATR also has ramifications regarding the throttleably of the engine.  This decoupling allows the ATR to throttle up and down much more quickly than that of the turbojet.  When increasing the thrust output of a turbojet, there is always some amount of delay, or thrust lag, between commanded throttle-up and the actual thrust increase.  In the ATR, the delay is largely negligible. And lastly, the turn-down ratio of the ATR is much larger than that of a turbojet.  The turn-down ratio is the ratio of the maximum to minimum thrust produced by a propulsion system.  Turn down ratios of 22:1 and even 30:1 have been demonstrated with ATRs.  Turbojet turn-down ratios are typically 10:1 or less.

Implications for High Speed Flight

At its most basic, the ATR is an accelerator engine.  It is light-weight, can produce a great deal of thrust for a given size, is simple, and can be relatively low-cost.  It can operate over a very large speed and altitude range, produces static thrust, and can use a wide variety of propellants.  One of the advantages of turbomachinery for propulsion systems is that the compressor provides a shock-isolation system for subsequent combustion processes.  This has implications for supersonic/hypersonic flight where inlet shock fluctuations can have deleterious effects on downstream combustion such as flameouts and unstarts.  From that standpoint, the ATR offers superior protection against inlet shock fluctations compared to that of the turbojet due to its aerodynamic decoupling, as previously discussed. 

In our next installment (Part III), we'll discuss possible applications of the ATR, and where it might make sense to use an ATR, and where it doesn't. 

The Air Turbo Rocket: Performance Attributes (part 1)

As mentioned in the previous ATR post, the fundamental difference between the ATR and the turbojet lies in the relationship between the compressor and turbine.  In a turbojet, the turbine and the compressor are coupled thermodynamically, i.e. power output of the turbine is the power input into the compressor, and aerodynamically, i.e. air flow through the compressor equals airflow through the turbine.  In the ATR, the turbine and compressor are coupled thermodynamically, but are not coupled aerodynamically.   This coupling difference is the source of the major performance differences between the turbojet and the ATR.

For a turbojet, the limitation on its achievable flight speed is determined by its maximum allowable turbine inlet temperature.  As you fly at higher and higher flight speeds, the stagnation temperature of the incoming air increases roughly as the square of the flight Mach number.  As the air passes through the compressor, compression work on the air raises its stagnation temperature further.  Adding fuel in the combustor section, the combustion process raises the gas temperature even higher, nearing the allowable temperature limitations on the turbine.  As you fly at ever higher flight speeds, the amount of fuel you can add before you reach the turbine temperature limit decreases continuously, until it reaches the point where you can’t add any more fuel.  This becomes effectively the upper flight speed of a turbojet.  Advances in materials and cooling techniques have continuously pushed the turbine temperature limitation upwards, but it still remains the flight speed limiting characteristic of the turbojet.  With current materials and cooling techniques, turbojets can now reach into high Mach-two range.  Advanced engines now in the development phase should push this number well above Mach-three.

For the the ATR, the situation is different.  Because no atmospheric air ever sees an ATR’s turbine, the flight speed limitation of the turbojet is by-passed. Instead, the flight speed limitation of the ATR is passed on the material limits of the compressor.  With current materials and cooling techniques, the ATR should be able to operate in the high Mach-five range.

Further advantages accrue by aerodynamically decoupling the turbine and compressor.  In the ATR, as well as an afterburning turbojet, the majority of the net forward thrust produced by these engines comes from that produced by the combustor (for an ATR) or afterburner (for a turbojet).  Because there is no pressure drop through a combustor or across a turbine, the pressure within the combustor of the ATR is higher than that of the turbojet, for a given compressor pressure ratio.  This feature allows the ATR to produce considerably more thrust than a comparably-sized turbojet.  In addition, it allows the ATR to fly at a much higher altitude than that of the turbojet.

It is this higher combustor pressure that allows the ATR to have the highest thrust-per-unit-frontal area (sometimes called the “thrust density”) of any airbreathing engine.  In addition to the high combustor pressure, the relatively simple geometry of the ATR allows this engine cycle to be relatively light, and gives the ATR a very high thrust-to-weight ratio.

Further discussion of ATR performance attributes will continue in subsequent posts.  For further reading of ATR attributes, I recommend this paper (which I wrote in 2000).

     

The Air Turbo Rocket: An Introduction

The Air Turbo Rocket, also known as the Air Turbo Ramjet or by its acronym, the ATR, is an airbreathing propulsion engine which combines elements from both turbojets and rocket engines.  The ATR belongs to a general class of propulsion engines known as Turbine-Based-Combined-Cycle, or TBCC, engines. 

The basic operation of the ATR is as follows.  A source of hot, high energy, fuel-rich gas is used to drive a turbine, which in turn drives a compressor. The compressor compresses incoming atmospheric air, like a turbojet, which then flows into a combustion chamber, which is much like an afterburner (or reheater) on a turbojet.  The fuel-rich gas which was used to drive the turbine also flows into the combustion chamber.  Within this combustor chamber, the compressed air from the compressor and the fuel-rich turbine drive gas are mixed, burned, and then expanded through a nozzle to produce thrust. 

The source of the hot, high energy fuel-rich gas is generically known as the gas generator, and it can be configured in a number of different ways.  The hot gas can be produced by mixing and burning a liquid or gaseous fuel and oxidizer combination, by the use of a mono-propellant gas generator, or by the burning of a solid propellant.  These configurations are known as gas-generator, or gg-cycle ATRs.  Figure 1 below shows schematically the basic gg-cycle ATR configurations.

figure 1  Gas Generator-Cycle ATR Configurations

Alternatively, the hot, high energy gas can be produced by running a fuel through a heat exchanger located in the combustion chamber of the ATR. This last configuration of the ATR is known as an expander-cycle ATR.  In figure 2 below the expander-cycle ATR configuration is shown.

figure 2 Expander-Cycle ATR Configuration

The essential feature that characterizes the ATR is the fact that, unlike a turbojet, the gases which pass over the turbine contain no atmospheric air.  All of the constituents of the turbine drive gas come from some on-board propellant supply, and it is this characteristic which is the defining characteristic of the ATR, and differentiates it from the turbojet.  In most other respects, the ATR behaves much like an afterburning turbojet.

ATR Operating Characteristics

Although functionally very similar to the turbojet, the ATR possesses a number of operating characteristics that are significantly different from that of the turbojet.  The fundamental difference between the ATR and the turbojet lies in the relationship between the compressor and turbine.  In a turbojet, the turbine and the compressor are coupled thermodynamically, i.e. power output of the turbine is the power input into the compressor, and aerodynamically, i.e. air flow through the compressor equals airflow through the turbine.  In the ATR, the turbine and compressor are coupled thermodynamically, but are not coupled aerodynamically.   This coupling difference is the source of the major performance differences between the turbojet and the ATR.

It allows the ATR to fly much faster and higher than the turbojet, and to have the highest thrust-per-unit frontal area of any airbreathing engine over its operational speed range.

In the next posting, we’ll be discussing performance attributes of the ATR which emerge as a consequence of its operating characteristics.  We’ll then give some consideration to propulsion applications which might be able to best take advantage of these attributes.  

The Air Turbo Rocket: Its a New Day

Perhaps you’ve looked at the pictures in the sidebar, or read the description on the “about” page, and wondered just what an Air Turbo Rocket is.  Perhaps you’ve just googled the word “air turbo rocket”, and you clicked up this blog.  Well, you’ve come to the right place if you want to find out more about this engine cycle.   

The air turbo rocket, or ATR, has been around for a long time, but has remained a solution looking for a problem.  Despite its significant advantages, the ATR has not yet found a home.

In the interest of promoting a wider understanding and appreciation of the ATR concept, I’ve started writing a series of posts about the ATR: how it works, a bit of its history, and its potential in various propulsion applications. 

So stay tuned, and come on back to get the latest ATR postings. I promise to make it worth your while.

small turbojets: the turbomachinery

Here's a look at the turbomachinery:

The compressor is a single-stage, radial compressor possessing 5 blades, and 5 splitters.  It's got about an 8 deg. forward lean, and about 15 deg. of sweepback.  I don't have direct measurements of its delivered pressure ratio, but my power balance calcs. suggest it should be around 3.8.  At a air flowrate of 0.58 lbm/sec, and an efficiency of 82%, the compressor would absorb around 70 hp at full power.

Small Turbojets

Here's some pictures of a small turbojet that I use as an experimental testbed.  By using a small, low cost turbojet (this one cost me less than $1200, including all hardware, electric starter, FADEC, fuel pump, and batteries), I can run many tests very quickly and for very low costs.  I can also try out different, advanced technologies that are too risky and/or too expensive with a bigger machine.
This is an example of using many trial-and-error tests to development technology, as discussed in a previous post. 
Here's a look "under the hood": This turbojet makes about 12 lbf thrust at full power.  It consumes about 0.007 lbm/sec at full power, resulting in a TSFC of around 2.19 lbm/hr-lbf.  The turbojet masses about 1.87 lbm, yielding a thrust-to-weight of about 6.42.  At full power, the nominal rpm is approximately 160,000 rpm.
(if you're interested in other specs, please let me know via comments).

For the Love of Technology

We love technology (from the Greek, literally “know-how”). It’s so…so… American.  We’re a culture evolved from all the do-it-yourselfers, early adopters, pioneers, independent thinkers that left Europe a few centuries back, so its only natural that we want to know how to do something better, easier, or maybe so that we can make a buck, or maybe just because we like knowing how to do something that other people can’t.  This is something we seek out and embrace.

 But the implementation of new technology is a different story.  New technology faces the challenge of displacing the incumbent technology.  When people are faced with this decision, they make a reasonable assessment: does the new technology and its claimed benefits outweigh the difficulty and risk of replacing the old technology, especially when the incumbent way is working, making money, is can be counted on?  (Way) More often than not, the incumbent wins, and the new technology is not implemented. The difficulty of implementing new technology and innovations is well-documented and analyzed in Clayton M. Christensen’s books, The Innovator’s Dilemma, and The Innovator’s Solution.  Christensen basically shows that new technology has to enter “from the bottom up”, by providing innovation for the needs of customers that aren’t being served by the incumbent. 

 Recently, considerable discussion has arisen from John McGowan’s article in the Space Review regarding technological innovations that might make cheap access to space possible.  I think the significant contribution of McGowan’s article is the reiteration of the fact that technological development requires many trial-and-error attempts in order to work out all the kinks. 

 I provided some comments to the article, in an attempt to shed some light on specific approaches to technical innovations for low cost space access.  In my opinion, there is almost no better reference to this issue than John London’s seminal book, LEO on the Cheap, which can be downloaded here.  London points out that simplicity and design choices can radically alter development and operational costs, and that there are cultural issues involved as well. He provides excellent examples as well as recommendations.  Col. London (ret) is a rather amazing person, and is one of the few people I know to have served in the Air Force (as an officer), and in NASA and the DoD (as a civilian), and has remained a tireless supported of the "New Space" Industry.  Not too long ago this was considered fringe territory, and it was John who coined the term the "radical rocket underground".  So he's not just talking out of his hat, he's been there, done that.

 LEO on the Cheap essentially provides a roadmap on how to develop low cost space launch systems.  And in fact, many of his recommendations and observations have found their implementation in some of the current space launch systems, such as SpaceX, Armadillo Aerospace, XCOR, and others.

 Finally, I think its worth noting, as I did in my comments to McGowan, that we may eventually rediscover the advantages provided by spaceplane-type launch vehicles which take off from a runway, fly an envelope-expansion type flight, and then return to land. Despite their larger dry mass fractions, aircraft-type flight operations allow incremental flight testing, and hence, many more trials for a given development budget.

 But the question remains as to why this has taken so long.  This is not a simple question to answer, because there have been, in my opinion, quite a number of contributing factors.  But, my best attempt at putting it in a sentence or two goes something like this:  the Space launch industry emerged largely as the product of post-World War II industrial practices and the success of large, government-led projects.  Subsequently, the aviation and aerospace industries were largely usurped by cold war imperatives.

 Further discussion and explanation of what I mean above will require additional posts, which I’ll provide later.  But the upshot of this, at least for Americans, is that launch vehicle design, development, and the selling of launch services got wrapped up with only one customer, the U.S. Government.  The U.S. Government became the incumbent, and it’s taken a long time to create and find markets for these alternative launch services.

 I have to remind myself that new markets have to be created.  They are created when there is an exchange of things between someone who wants and/or needs something, and someone who has something to offer.  Usually, when someone wants something that doesn’t currently exist, creative people scramble around to provide that something.  However, if a creative person has something that they are offering, and no one wants it at the moment, then it sometimes takes a while to find someone who does want it, and sees its value.  Oftentimes, people don’t even know what they want except more/better/faster/cheaper of what they currently have. 

 How does this apply to our current launch markets?  We have people who need launch services, and we have people who provide those launch services.  The people who need launch services want more/better/faster/cheaper launch services than they have now.  Incumbents, like Boeing, LockMart, Orbital, have the advantage.  This is the innovator’s dilemma.  New launch providers, SpaceX, Blue Origin, TGV Rockets, et al, are seeking ways to “enter from the bottom up” by providing lower cost services to customers who aren’t being served by the incumbents.  This is the Innovator’s Solution.  And I think it’s going to be successful beyond our wildest reckonings.

Propulsion Technology Development for Low Cost Space Access

John McGowan has an interesting article in The Space Review regarding ways to develop low cost space access via improvements to propulsion and power technology.  The article can be found here.  This is my cup o' tea.

Basically, if you can do many trial and error testing, then you have a much better chance of creating better technology.  To do many trials, you need to make sure that each trial doesn't cost very much.  Then you can do more trials for a given amount of funding. 

This is why I built my own Air Turbo Rocket (ATR), so that I could do multiple, low-cost test runs for engine development. (here is my  ATR). 

Why I got into Propulsion

People become engineers for all sorts of different reasons.  For some, one or both of their parents were engineers, and they're familiar with that world.  Others like technology, and some find the prospect of a being able to land a good-paying job the attraction.

 For me, I was always interested in space travel.  Seeing Star Wars the night of my graduation from 9^th grade had a huge impact on me:  I was the right age, and saw the right movie.  I never looked back.

 I got into Mechanical Engineering in College, and really liked fluid mechanics and heat transfer.  But when I took a course on turbomachinery and propulsion, that’s when the “wheels started spinning”.  

 My interest in propulsion, both airbreathing and rocket, was further kindled in graduate school, and it was there that I became acquainted with the engine concept known as the Air Turbo Rocket, or ATR.  I took my first job at a rocket engine company, and actually got to work on an ATR program that was currently underway there.

 As I came to better understand propulsion, and its role in making space travel happen, I felt that propulsion was a good fit for me, and a good place to make useful contributions.  The rocket equation became a way to parse out who was a propulsion person (look at the specific impulse), and who was a structures person (look at the mass fractions).

 I definitely fell into the propulsion camp, where I remain to this day.  I’ve always felt that improvements to the specific impulse was the “high leverage” route to making space vehicles more practical, and by more practical, I mean a fully-reusable, get-in-and-fly-up-to-orbit-and-then-fly-it-back-down kind of craft; an SSTO vehicle to borrow from the world of staged launch vehicles. 

 Currently, reusable launch vehicles (RLVs) are definitely the goal, and the current bevy of launch vehicles try to reuse as much as possible, with the idea of bringing recurring costs down.  But the idea of aerospace vehicle that operates like a aircraft today still seems pretty far away.  Improvements to dry mass fractions are only going to get you so far.  In the end, if you don’t have very high specific impulse (Isp) propulsion, you’re going to remain in the staged launch vehicle, low structural margin kind of design envelope that we’re in currently.  

 High Isp propulsion can really be the game changer there.  But I think we’ve sort of abandoned that path because we want to do what we can right now, and not invest the time, money, and energy that it will take to develop these new types of high Isp propulsion systems.  We accept the limitations of rocket propulsion, and build our launch systems accordingly.  And really, that's probably the correct course of action.

 But somebody’s got to be doing the basic research and development work on high Isp propulsion now, and laying the groundwork for the propulsion advances that are to come.  Advances that will carry us to that dream machine, where we will be able to fly out into space, do our business, and then return.  So that’s what I do.  I work on weird, impractical, complicated, propulsion concepts, that somehow may offer vastly improved propulsion options.  Most will fail.  Maybe one concept will succeed.

 It’s a lonely world.  Folks think rockets are the only answer, and we’re wasting time on other approaches.  And in a way, that’s true, if you constrain yourself to the rocket-only approach.  But if the current launch vehicles are successful, and we are able to develop a thriving commercial launch industry, then we will be wanting, needing, and demanding high Isp propulsion.  Perhaps this is the way it all should be.

Force, Mass, and Acceleration

Newton's second law, F = ma, is ubiquitous in science, technology, and just about anything that involves the consideration of forces, mass, and accelerations, in other words, dynamics.

F = ma is also the basis for the definition of units of force, and in particular, for the relationship between units of force, and units of mass.  There is occasionally some confusion over the relationship between force, and mass, or more specifically between the units of force and mass.  A particular area of irritation concerns English units of force and mass, and the need to include 32.2 in the relationship between these two units. 

To clear up this situation, I've prepared a little chart that I hope will shed some light on the important relationship between force, mass and acceleration.  Here's the chart:

 In all cases, Force equals mass times acceleration.  In each column, a particular unit of force is presented and what it relates to in terms of mass and the appropriate magnitude of acceleration.  Here, one can see that the value of 32.2 arises in the relationship between pounds of force and pounds of mass is because this relationship is defined by a specific value for of acceleration, which in this case is 32.2 and is, in fact, the acceleration due to gravity here on Earth.  It turns out that it is very handy to have a unit of force have the same magnitude (not the same units) as a unit of mass when you’re on the surface of the Earth.  This is why, perhaps to the surprise of American engineers, that there is a commonly used measure of force used in Europe known as the kilogram force, and for the exact same reason.

Another interesting observation is that, if you want to do away with that pesky 32.2 value when using English units, then simply switch over to using the poundal, which is simply one pound of mass accelerated at 1 ft/sec^2, completely analogous to the realationship between  Newtons and kilograms. 

There are other even more obscure units of force and mass, which are not shown here.  As I come across others, or you readers have suggestions, I will include them in future updates of this charts.

However, the point of this chart is to simply organize units of force, mass, and acceleration, based on their origin, which is Newton’s Second Law.  I hope that by so doing, it will be easier to visualize the actual relationships between these units, and make it easier to keep them straight in your head.