Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering)


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Dr Srinivas Tangellapalli

Figure shows the staged combustion concept for hybrid motors. To demonstrate the effectiveness of this concept, a series of tests were performed at various thrust levels, ranging from 1, lbf to , lbf. The same motor was retrofitted with. LMSSC conducted several successful tests that advanced the state of the art for hybrid rocket motors. The most significant of these tests was a sec test firing of a 30,lb-thrust hybrid motor at AFRL on June 10, The company conducted six more tests in These tests demonstrated the use of a hybrid rocket fuel-rich motor in a gas generator to drive a pump turbine and tested new high-performance fuels, advanced nozzle materials, and the dramatic improvements in performance and stability enabled by the GOx-rich upstream staged-combustion preburner.

Enabling technologies for 1 storable oxidizers; 2 minimizing residual fuel grain; 3 motor insulation compatible with hybrid combustion products; and 4 materials for the nozzle throat that inhibit erosion will be important for hybrid propulsion systems. Filled ethylene propylene diene monomer EPDM insulators have been proved adequate for solid propulsion systems.

With hybrid systems, rubber-based insulators act as fuel and have been shown to erode relatively fast when exposed to hybrid exhaust constituents. Insulation materials compatible with hybrid combustion products need to be improved to accomplish the run-to-empty goal. Future hybrid motor insulators will need to serve as a structural element during the initial burn, when the chamber pressure loads are highest, and will need to withstand erosion when exposed.


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Material testing in a relevant environment will enable minimum fuel residuals and lower the overall mass of inert insulation material. Test data indicate that the nozzle throat materials typically used for solid propulsion systems, such as three-dimensional carbon-carbon and ATJ graphite, erode fairly quickly in a high-pressure environment.

Rapid nozzle throat erosion does not affect the hybrid fuel burn rate, but it does reduce the nozzle expansion ratio and chamber pressure as a function of time, which eventually degrades performance. Nozzle materials that are more compatible with hybrid propulsion systems will have to be identified and developed. This will be necessary for long-duration missile motor burns.

Residual hybrid fuel on an upper stage translates directly to payload mass, which contributes to the physical size of a hybrid missile propulsion system as compared with a solid or liquid stage. Reducing the amount of fuel residual will decrease the physical size of hybrid propulsion systems, making them comparable to other systems. Future hybrid fuel grains will need to be tailored for the fuel webs to merge as a function of time to drive the fuel residual share well below 2 percent of the total fuel on board.

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Also, fuel structural strength will need to be improved and long-duration testing will be required to advance the current state of the art. Gelled propellant motors can provide total missile thrust flexibility at high combustion efficiency and meet IM requirements by storing fuel and oxidizer in separate tanks. Controllable thrust can provide significant system benefits. It can provide extended range and shorter time-to-target at midranges in a single system and can reserve propellant energy for endgame performance. Gelled propellants can meet operational and handling requirements.

Gel propulsion for airborne missiles has the potential to be inherently insensitive to IM threats because the fuel and oxidizer are stored in separate tanks. Gel propulsion systems have passed bullet impact, slow cook-off, fast cook-off, and shaped-charge jet IM tests. Logistics costs can be reduced by substituting a single controllable-thrust missile for currently deployed single-use systems. They are flexible enough to meet evolving airborne missile system requirements. Solid propellant motors have been almost universally used in tactical aircraft-borne missiles.

Generally these solid motors are simple fixed-thrust booster stages. Flexible flight profiles and endgame maneuvers are not well suited to solid propellant motors. Although solid propellant motors with variable plug nozzles can be throttled and can sometimes be shut down and restarted, their operating profile throughout a long fly-out mission is quite limited. Storable liquid propellant rocket motors have much more flexibility in operating profiles.

However, handling and leakage have always been of concern. Also, liquid propellant systems tend to have lower density. Because aircraft-borne missiles are usually volume constrained by aircraft configuration, lower density can mean the aircraft is not able to carry as much total impulse.

Although the I sp of liquid propellant can be higher than that of the best solid propellants, their density impulse may only just match that of solids.

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The development of gelled-liquid propellants over the last 20 years has provided attractive options for very demanding flight profiles. Gel-propellant formulations with very-high-density specific impulse have been demonstrated. These gels have the physical consistency of heavy toothpaste and can be stored. In fact, they can be frozen at extremely low temperatures and then rapidly thawed out for use with no detrimental effects. They also do not have the time-dependent deterioration issues experienced by most solid propellant grains and case bonds.

Under high shear loads, gelled propellants behave like normal liquids. They can be throttled, shut off, and restarted when using the right type of injectors. They can be pulsed with variable off times up to almost any value demanded. Design criteria for gelled-propellant rocket systems for missiles of almost any size have been validated at Northrop Grumman Corporation.

The critical components of a system are these:. The key technology for enabling the flexible multistart and thrust profile tailoring is a face shutoff injector. For this application, a single central-element pintle injector turns out to be ideal. With a single sleeve, both the oxidizer and fuel can be throttled while maintaining absolute control of the mixture ratio at all thrust levels, and the engine can be shut off at the face so that neither gelled propellant can evaporate at shutdown.


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This injector is essentially the same as the type used for the Lunar Module descent engine. It delivers efficient combustion with absolute dynamic stability. A cutaway illustration of an injector with a face shutoff configuration is shown in Figure The very high packaging density and optimized thrust profiles possible with these gelled propellant systems can significantly increase the range for a given fly-out missile envelope. This can also be an advantage for in-space fly-out missiles or responsive space tugs.

There are a couple of opportunities for transforming the means by which certain responsive global reach and ABM missions can be achieved. Two concepts for space access launch vehicles were described and discussed in Chapter 4. Both air-based launch platforms could transport rocket-powered missiles to high-altitude launch points at desired geographic locations. Both would enable tremendous flexibilities in launch time and azimuth orbital inclination for missiles for ABM missions, tactical support, or long-range global strike. Such platforms could provide a faster response to emerging threats than is available today.

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Launching missiles from a flying aircraft platform could dramatically improve the delivery of a warhead and decrease the time to target. High-altitude air launch allows a rocket to bypass the initial parts of a ground-launch trajectory. These losses are compounded by the reductions in thrust and specific impulse that rocket engines experience at high ambient pressures.

In addition, the integrated drag losses between the ground and 40, feet are great.

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The combined effects could reduce velocity by about 3, feet per second. Also, even if a ground launch takes place in the best geographic location, which is not likely, the time to reach 40, feet can be 20 to 40 seconds. This time difference could be crucial for a boost-phase ABM mission. Furthermore, vertical launch vs. In the BAE concept, missiles are preloaded in a self-contained, installable vertical launch module. The module can accommodate many missiles of several types.

This could permit multiple targeting for strike and tactical battlefield support. Some potential mission capabilities of a joint strike air network architecture are depicted in Figure Much of the launch dynamics and environment of an air-based vertical launch is very different from a ground-based launch, a Pegasus launch, or the candidate FALCON AirLaunch vehicle, described in Chapter 4.

Characteristics for candidate missile propulsion systems including parallel booster or strap-on combinations , along with propulsion technologies such as propellant combinations solids, storable liquids, gelled combinations, and storable oxidizer hybrids and operating characteristics including assured start-up profiles, thrust vector control, and rocket plume impingement patterns need to be optimized to take full advantage of the potential new operationally responsive mission capabilities of ABVL for global strike and near-space military applications.

To exploit this launch concept, one of first technology demonstration efforts should obtain some data on launch environment dynamics needed to carry out system trades. The propulsion technologies that need to be evolved to take full advantage of such launch platforms should be identified and developed.

The airframe is designed in such a way that the centerline payload could be either a self-contained launching pod for multiple medium-sized missiles or a single large missile. Either configuration could also be equipped with folded rotor blades for emergency separation or self-transport.

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The MMMV concept could also provide a transformational missile launching capability for large or small missiles. The aircraft can be configured with a specialized missile pod. Like the airborne vertical launcher it could transport larger rocket-powered missiles to high-altitude launch points at optimum geographic locations.

Missile configurations and propulsion technologies would need to be optimized to take full advantage of the transformational potential of aircraft configured to launch missiles at high altitudes. The studies would identify the propulsion technologies modifications or new concepts that should be evolved in order to take full advantage of such air-based launch platforms for operationally responsive missions. Critical enabling technologies for substantial improvement of missile propulsion operational capabilities should be redoubled to find new energetic propellants and heat- and chemicals-resistant.

These two areas are discussed below. Prospects for the search for energetic yet insensitive propellants in the near term seem poor. Monopropellants with higher density I sp may evolve first, but even if one is validated it can be expected to take many years to establish a reliable industrial product capability at an acceptable cost. A major problem limiting the future use of any new energetic propellants even if they become available is the lack of materials that are resistant to chemical attack and to erosion at high temperatures.

The high temperatures achieved by energetic propellants will produce the same molecules as are produced by other fuels, including CO 2 , H 2 O, N 2 , and CO. The requirement for low erosion materials is a result of the higher temperatures achieved by these propellants. Findi n g If the DoD and the Air Force are going to realize any transforming options in the specific performance profiles of tactical missiles in the far-term, a well-funded, continuous effort in energetic fuels and resistant materials is required.

DoD and the Air Force should fund the search for new high-energy propellants and development of very-high-temperature, chemical-attack-resistant, low-erosion-rate materials. Actual funding levels for technology programs such as IHPRPT and for sustaining and improving the engineering on tactical and strategic missiles have dropped well below the original planned funding levels.

Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering) Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering)
Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering) Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering)
Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering) Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering)
Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering) Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering)
Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering) Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering)
Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering) Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering)
Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering) Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering)
Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering) Advances in Chemical Propulsion: Science to Technology (Environmental & Energy Engineering)

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