Plasma Wind

ATR Applications: Tactical Missile Propulsion

The characteristics of the ATR as described previously give rise to a number of performance attributes that make this engine cycle uniquely suited for a number of potentially important applications.

In general, ATR applications break down into two main categories: tactical missile propulsion, and air/space craft propulsion.  In this part, we’ll discuss ATRs for Tactical missile propulsion.

For Tactical missile applications, mission requirements dictate what might be the best propulsion choice.  Performance-wise, the requirements most often take the form of a range requirement, i.e. the missile has to go x kilometers, or a time-of-flight (TOF) requirement, i.e. the missile has to reach a certain distance in x seconds.   For a TOF requirement, some sort of solid rocket motor almost always wins out because of their high thrust-to-weight (T/W).  For the range requirement, the turbojet may represent the best choice because of its low propellant flow rate per unit of thrust, i.e. its low thrust specific fuel consumption (TSFC) value.  A graphic of these constraints would look something like this:

As can be seen, the rocket can make the TOF constraint, and the turbojet can make the range constraint.  However, it’s only when you apply both of these constraints, that you make end up with a situation wherein neither the rocket nor turbojet can make both constraints simultaneously.  You need something that has a high enough T/W to make the TOF constraint, and something that has a low enough TSFC to make the range constraint.  You need an ATR.  Than, in a nutshell, explains, where the ATR may provide the best propulsion solution, and also why the ATR has yet to debut in any fielded missile system.   No missions have emerged that truly constrain the designer to both TOF and range constraints.

However, we have gotten close.  There are in fact, a few missions that apply both types of constraints. The first mission is the “take out a moveable target” mission.  Examples might include a mobile missile launcher that is positioned, prepped, fired, and then retreats to a bunker.  At long ranges, a rocket-powered missile can’t get there, and a turbojet-powered missile can’t get there fast enough.  Only an ATR-powered missile can get there in time: the simultaneous TOF and range constraint.  An equivalent mission is one which might require a missile to fly out rapidly, then loiter over the target area.  When a target appears, the missile throttles up and engages the target.  A second example of the simultaneous TOF and range constraint mission would be the stand-off engagement of incoming cruise missiles.  In this mission, the missile has to fly out to engage, perhaps throttle back to acquire the cruise missile target with sensors, then throttle back up to overtake and intercept.

One particular “niche” application that the ATR is uniquely suited for is one which requires a self-boosting airbreather.  Such an application might be a missile that must launch itself out of its container, then fly out to its intended target.  Turbojets in general lack a sufficiently high T/W ratio to allow them to self-boost, whereas the ATR can easily be designed to provide the necessary T/W.

We sometimes call the ATR the “decathlete “of the propulsion Olympics, because although it may not be number one in any particular event, it does the best over all the events.  Furthermore, because the compressor and turbine are decoupled aerodynamically (as discussed in previous posts), the ATR design can be “tuned” for a particular application by trading T/W against Isp.  If more range is needed, the Isp can be increased at the expense of a reduction in T/W, and vice versa.  The “tuning” of an ATR is achieved by the adjustment of the mixture ratio present in the combustor.  A leaner combustor MR allows for higher Isp and thus longer range. A richer combustor creates greater thrust.  This tenability advantage of the ATR remains a strength of the engine, but has yet to be fully appreciated and utilized for tactical missile propulsion applications.

In our next ATR installment, we’ll discuss the ATR for air and spacecraft propulsion applications

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.