Design Evolution and Verification of the A-3 Chemical Steam Generator

Casey K. Kirchner1 Engineering and Test Directorate, NASA Stennis Space Center, MS, 39529

The following is an overview of the Chemical Steam Generator system selected to develop vacuum conditions for a new altitude simulation test facility, the A-3 Test Stand at Stennis Space Center (SSC) in Bay St. Louis, MS. A-3 will serve as the National Aeronautic and Space Administration’s (NASA’s) primary facility for altitude simulation testing of the J-2X rocket engine, which will be used as the primary propulsion device for the upper stages of the Ares launch vehicles. The Chemical Steam Generators (CSGs) will develop vacuum conditions in the test cell through the production and subsequent supersonic ejection of steam into a diffuser downstream of the J-2X engine nozzle exit. The Chemical Steam Generators chosen have a long heritage of operation at rocket engine altitude simulation test facilities since the days of the Apollo program and are still in use at NASA White Sands Test Facility (WSTF) in New Mexico. The generators at NASA WSTF have been modified to some degree, but are still close to the heritage design. The intent for the A-3 implementation is to maintain this heritage design as much as possible, making updates to substitute for obsolete parts and to increase maintainability and reliability. Reliability improvements are especially desired because the proposed system will require 27 generators, which is nine times the largest system installed in the 1960s. Improvements were suggested by Reaction Motors, LLC, by NASA SSC and NASA WSTF engineers, and by the A-3 test stand design contractor, Jacobs Technology, Inc. (JTI). This paper describes the range of improvements made to the design to date, starting with the heritage CSG and the minor modifications made over time at NASA WSTF, to the updated configuration which will be used at A-3.

Nomenclature O2 = chemical formula for oxygen C3H8O = chemical formula for isopropyl alcohol CO2 = chemical formula for carbon dioxide H2O = chemical formula for water O/F = oxidizer to fuel ratio, by mass

I. Introduction Chemical Steam Generator (CSG) is a combustor used for the purpose of rapidly producing large quantities of steam. (In this paper, the terms “CSG” and “generator” will be used interchangeably.) CSGs have long been

used in the aerospace industry to drive vacuum diffusers for altitude simulation chambers. CSGs used in altitude simulation rocket testing applications are essentially rocket combustors themselves. A CSG can be considered to have a longer-than-typical rocket combustion chamber, as the throat is comprised of multiple ejector nozzles typically located at the test cell diffuser interface, which may be located far away from the generators. An exothermic chemical reaction is quenched with water by direct spray injection, resulting in a flow of products of combustion in steam. The mass flow rates of the propellants and water can be adjusted to produce steam at varying temperatures, pressures, and flow rates. The initial capital cost to install a CSG system is far less than to build a commercial steam plant. CSGs provide the further advantages of being able to quickly generate superheated steam at the high flow rates required to develop vacuum conditions, and do not require a staff of licensed steam plant operators. Rocket engine test conductors already employed at a ground test site have the ability to operate the CSGs.

1 Mechanical Design Engineer, Engineering Division, EA32, AIAA Member.

A

45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit2 - 5 August 2009, Denver, Colorado

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Stennis Space Center (SSC) plans to use this technology to develop vacuum conditions in the rocket test cell for the A-3 test stand, which is under construction at the time of this paper’s publication. The precursors to the A-3 Chemical Steam Generators were a set of CSGs made by the Reaction Motors division of Thiokol Chemical Corporation for TRW, Rocketdyne, Bell, and the National Aeronautics and Space Administration’s (NASA’s) White Sands Test Facility (WSTF) in the 1960s. These generators were used to provide steam for altitude simulation testing of the Lunar Excursion Module (LEM) engines for the Apollo program (for which contracts these companies were competing). These generators use Liquid Oxygen (LOX) and Isopropyl Alcohol (IPA) as propellants. A LOX/Propane version of this generator was used for the NERVA test program at Jackass Flats, NV, and Reaction Motors developed earlier versions which ran with hydrogen peroxide and hydrogen, as well. The LOX/IPA version is most appropriate for A-3. First, LOX is less expensive and easier to handle than hydrogen peroxide. When compared with the LOX/Propane generator, there is a much greater experience base operating and making incremental improvements to the LOX/IPA version. Finally, the drive pressure at the steam ejectors is higher with LOX/IPA generator, and meets the vacuum level requirements for A-3. NASA WSTF is the sole remaining facility still operating their original CSG steam plant, which began service in 1965. This particular set of CSGs serves as the basis for the design of the A-3 CSGs, with improvements recommended by the WSTF operations staff as well as the original designer. These improvements and others have been incorporated by the design contractor, Jacobs Technology, Inc. (JTI) of Tullahoma, TN, with some further changes requested by NASA SSC personnel as customers.

II. CSG Basics Each CSG is supported by a complement of valves, instruments, and flow control devices, the assembly of which is called a “module.” The generators will be installed in groups of three, historically denoted as “units”. A module is

so called because of its modular ability to be replaced or serviced without affecting the other two generators installed on the same unit. A module is pictured in Fig. 1, shown with its generator secured by white bands prior to shipment. The heritage system at NASA WSTF is composed of a single unit (three generator modules), depicted in Fig. 2 as it was installed in 1965.In contrast, A-3 will have nine

Figure 1. A Chemical Steam Generator Module.4

Figure 2. A CSG Unit. Photo Courtesy of Reaction Motors, LLC. Figure 3. The CSGs at the A-3 test stand.1

________________________________ * “LOX / Alcohol Chemical Steam Generator: Modification History and Recommendations,” personal communication from Carl Kastner of Reaction Motors, LLC, 2007.

units operating in parallel to achieve vacuum conditions appropriate for testing the J-2X engine (Fig. 3). Each generator burns LOX and IPA in three successive concentric stages, designed to ensure reliability in a

repeatable startup sequence. Ignition of the first stage is achieved by exciting a spark plug; the second stage and main stage are lit by the flame front of the previous stage. Each stage is oversized in terms of its ability to ignite the next stage. The main stage burns approximately 97% of the total propellant flow and uses the heat energy to vaporize water into superheated steam. The main combustion chamber is a double-walled chamber lined with water injection elements. After first being used to cool the walls of the chamber, the water is then injected and turned into steam.

The CSG first stage igniter is often mentioned in connection with the X-15 aircraft. According to the original designer of the CSG, the first stage igniter is based on a developmental version of the igniter for the LR-99 rocket engine, which powered the X-15. The igniter ultimately used in the LR-99 is not the version that was developed into the CSG igniter.

The chemical equation for the reaction of IPA and LOX is

Note that this reaction assumes 100% pure IPA, giving a stoichiometric oxidizer to fuel (O/F) ratio of 2.4 by

mass. In practice, the IPA used for the CSG is diluted with water, and since water is inert, the effective O/F ratio is somewhat higher than the stoichiometric ratio. Therefore the CSG operates at an overall O/F ratio lower than 2.4 to maximize efficiency. Another useful ratio is the mass of the cooling water injected to the mass of the total propellants; this ratio is typically about 2:1. Changing this ratio increases or decreases the degree of superheat present in the steam product.

III. Design Evolution of the CSG Design changes were implemented to solve problems of obsolete components, vibration, excessive water

consumption, unreliable spark ignition, premature shutdowns, maintenance burdens, and poor ergonomics.2-4,*

A. Drawings JTI obtained the heritage design drawings from NASA WSTF

and Reaction Motors, LLC in hand-drawn hardcopy. A significant drafting effort was undertaken to recreate these designs in CAD format. Standards and specifications which have been superseded in the intervening years were updated to the latest versions. Material lists were updated to replace obsolete components with parts commercially available today. NASA SSC drafting standards were applied, and drawings were renumbered to comply with the configuration control system in place. Most significantly, some dimensions were slightly altered to ensure that the generator could meet the latest revision of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel code. Additionally, three “pathfinder” generators were constructed at NASA SSC to further test the design for ease of fabrication. The lessons learned from this effort were passed on to the CSG manufacturing contractor for A-3, with the drawings modified as necessary.

B. Feed System The heritage CSGs at NASA WSTF are fed with IPA and water

by diesel-powered pumps and with LOX by electric pumps, from ambient pressure storage tanks. It is easy for WSTF operators to trim the steam temperature by adjusting O/F ratio with pump speed. However, the cost of this flexibility is a considerable maintenance burden for the diesel engines and pump skids. At the A-3 test

Figure 4. A CSG main stage combustion chamber with water jacket and injection elements. 1

9O2 + 2C3H8O → 6CO2 + 8H2O (1)

stand, propellants and water will be pressure-fed from run tanks (which will also serve as storage tanks, in the case of water and IPA). Pump-fed and pressure-fed engines tend to have different startup transients, but pressure-fed engine operation is within the experience base of NASA SSC personnel. It will still be possible to make changes in the O/F ratio and Water/Propellant ratio with use of the run tank pressure control valves.

C. Water Conservation Each of the generators operates in two modes and achieves the so-called “full-steam” mode after all three of its stages ignite. While the main stage remains unlit, the combustor is in “idle” mode. In the NASA WSTF system, this idle mode is not optimized for water usage, and does not need to be, as the water is pumped from a large “million gallon” reservoir. The water supply at A-3 will be pressure-fed from tanks with finite volume, so water optimization is preferred for this updated configuration. Multiple solutions for this problem have been proposed, with the leading concept being a change to the operational definition of “idle mode.” Traditionally, the CSGs have always idled with the first and second stages lit, but because of the three-stage design, it can also idle with just the first stage lit. Redefining “idle mode” as first stage combustion only, with the generator running in a lower heat flux condition, should reduce the amount of water necessary to keep it cool. The water supply is divided into two streams – one which flows around the first and second stage combustion chambers to remove heat, and one which cools and is injected into the main stage. Traditionally, both water systems are flowing before ignition takes place in the first stage. With the new concept of “idle mode”, only the igniter cooling water (which represents about 3% of the total) flows while the first stage is lit. Second stage ignition is delayed until after main stage water has started. The second and main stages have a history of igniting reliably at NASA WSTF, once the first stage is lit.

D. Spark Ignition System NASA WSTF has sometimes had problems with first stage ignition due to damage to the spark plug,

transformer, and ignition cable assembly. An aircraft spark plug designed for operation in a high pressure chamber is necessary. Through careful material selection and testing, NASA WSTF has done a lot of development to produce a system much less susceptible to insulation cracking, moisture intrusion, and internal arcing – all of which have the potential to cause ignition system failures. NASA SSC has taken this development further by specifying a much higher voltage power source to ensure the spark always jumps the gap. Additionally, a spark plug without an internal resistor is used, and it is connected to a race car ignition system to ensure near-continuous sparking.

E. Vibration Vibration is produced by the heritage system while the generators are firing, due to the inherent combustion

noise and also the use of cavitating venturis to control the flow of propellants and water. This vibration contributes to two resulting problems: there are several joints which must be re-tightened with much higher frequency than other joints – a rate that may be unsustainable at a larger-scale facility like A-3; and pressure and temperature switches connected to the relay control system are sometimes “unlatched,” initiating undesirable shutdowns of the steam system. This vibration has never been measured quantitatively, but the NASA WSTF main water venturis were relocated several years ago to a new location which resulted in a noticeable decrease in the frequency of such pressure-switch shutdowns. The water venturis were removed from the module-mounted location immediately downstream of an elbow and reinstalled underneath the Unit structure, close to a concrete anchor. NASA SSC plans to similarly locate the main water venturis away from the generators, anchored to the slab beneath the Unit platform. This position will allow for the ASME-recommended number of diameters of straight pipe upstream and downstream of each venturi. Similarly, the main LOX and IPA venturis have been relocated into straight pipe runs. All three main venturis have been redesigned as one-piece flanged venturis, instead of inserts cantilevered from a flange. This change should improve pressure recovery, leading to increases in performance and decreases in induced vibration. Finally, grooved pipe segments joined by flexible couplings will be installed downstream of the main water venturis to decouple any remaining vibration from the CSG itself.

F. Instrumentation and Control Another key design problem is the method of control. Historically, the CSGs have been controlled by relay logic

with inputs from pressure and temperature switches. Snubber orifices have been used in the pressure sense lines to prevent pressure spikes from prematurely latching switches, with a corresponding maintenance burden involved in keeping the orifices and sense lines (in the water and IPA systems) clear of rust. In addition to the aforementioned undesirable shutdowns, another consequence of using switches is that there is not a large data set of operational parameters available to fully characterize performance of the CSGs. In order to better monitor the operation and

health of such a large system at A-3, an update to transmitter instruments and a Programmable Logic Controller (PLC) has been implemented. During system activation and operation, controllers will have the advantage of being able to quickly change the firing sequence and logical parameters through programming, rather than changes in hardware settings.

G. Ergonomics Based on years of operating in cramped quarters, WSTF engineers recommended spacing the CSG modules

further apart to provide easier access to components. Two additional feet were added between each module, which resulted in a lengthening of the manifolds which feed propellant to, and carry steam from, the modules. This presented additional problems in terms of thermal flexibility design. Specifically, the steam plenum supports the weight of all three generators, but not of the main valves which supply the propellant. With a greater expansion of the steam manifold under temperature due to extra material, the flexible pipe segments which connect the main valves to the generators had to be redesigned. In the case of the propellant connections, flex hoses were lengthened. In the case of the water inlets, flexible couplings were added to join grooved hard pipe segments. These changes produce an additional benefit of further vibration damping.

IV. Verification The E-2 test facility at NASA SSC was configured to hot fire a full-scale production LOX/IPA Chemical Steam

Generator. Risk mitigation testing of the A-3 configuration continues possibly throughout 2009 and into early 2010 at this test facility to verify that the CSGs operate as expected. The generator(s) undergoing this testing is (are) instrumented in excess of what is normally required for operation. The extra data will allow for easier troubleshooting and more complete knowledge of expected performance. In addition, the early testing will give SSC personnel experience in operating the CSG systems, which will expedite the process of installation and activation at A-3.

This risk mitigation test facility provides propellants and water for a maximum run time of about 90 seconds for one generator or 30 seconds for three, based on run tank capacity. The generators exhaust into a long pipe which simulates the approximate length of pipe from the generators to the diffuser at A-3. This pipe has a variable position valve at the end of it which can provide an equivalent backpressure in the pipe to that which will be induced by the steam ejector nozzles at A-3. There is a section of pipe teed downward to catch any water not vaporized into steam, to prevent the ill effects of two-phase flow, and a rupture disk teed upward to protect the system. There are several bosses studding the length of the pipe for instruments and for drawing steam samples for chemical composition analysis.

Two phases of testing are in progress. From a hardware perspective, the first phase verifies the combustor only.

Figure 5. The E-2 Test Stand, configured for risk mitigation testing of the CSG. Photo Courtesy of NASA SSC.

CSG

One of the three SSC-built pathfinder generators is the basis for this testing, outfitted with valves and instruments already in reserve locally. The firing sequence and expected operating parameters are being proven during this phase, with some “off-nominal” testing scenarios being pursued – in some cases, for the first time in the history of the generator due to the unique requirements of the A-3 test stand and J-2X engine. The second phase will test an entire Unit of three generators as fabricated by the A-3 CSG vendor. This phase will not only verify the workmanship of the contractor, but also expand the control logic to three generators and provide new insight into the function of a complete Unit. The E-2 risk mitigation facility is designed to handle the flow rate of three generators operating together at full steam.

A. Drawings As stated before, the CSG drawings were converted from the original hardcopies to an updated CAD format

using the latest specifications and making some minor dimensional and tolerance changes. The generator assembly process at NASA SSC was successful, and the CSG did fit up to adjacent hardware easily. As of this writing, the SSC-fabricated generator built to these CAD drawings has been fired dozens of times with no problems. A progressive test sequence was implemented, firing just the first stage initially, then the first and second stages, and firing the main stage only after data from the initial tests were thoroughly analyzed. After each day of test, the generator hardware was inspected by borescope, and although some discoloration and wear is evident, it is within past experience at NASA WSTF. No hardware failures have been observed.

B. Feed System To verify that a pressure-fed CSG will produce steam within the required pressure and temperature envelope, the

CSGs were connected to the E-2 test stand run system. Three run tanks – one each for LOX, IPA, and water, provided fluid at the desired cavitating venturi inlet pressure by means of a specifically tuned pressure control valve with closed-loop feedback from the tank bottom pressure transducer. No detrimental startup transients were observed. O/F ratio control was demonstrated by changing the O/F over the course of individual hot fire runs with preprogrammed pressure control valve set points.

C. Water Conservation To verify the new definition of “idle mode,” two types of tests were performed with the first stage of the CSG.

Since the intention is to ignite the first stage with only 3% of the total water flowing instead of 100%, thermocouples were inserted near vulnerable internal parts to check for excessive heating. Steady state temperatures were reached at a level deemed acceptable for the material selected within a period of time consistent with the planned operational concept. Also, gaseous oxygen (GOX) fed to the first stage igniter is produced through coils of copper tubing wrapped around the main stage water supply. In the past, full-flow water was always flowing through this main pipe before first stage ignition, making it a heat exchanger which produces GOX. Without the main stage water flow to exchange heat with, it was feared that the LOX may stop vaporizing before the planned duration of idle mode was reached. The danger in this is that the O/F (and with it, flame temperature) might rise high enough to damage components of the main stage chamber, which during this mode has no water flow available for cooling. Preliminary tests of this heat exchanger – without full-flow main stage water– indicate that the oxygen remains in the gaseous state long enough for safe first stage combustion.

D. Spark Ignition System At the time of this writing, the new spark ignition system has proven itself with dozens of successful starts and

no detectible damage to either the plug or the transformer. A few off-nominal starts were attempted to find the limits of the system, and trouble was encountered in some tests with applied backpressure. It is theorized that these ignition failures have more to do with inefficient mixing of propellants due to lower injection speeds than with the performance or configuration of the ignition system. All nominal starts have been immediate and unmistakable.

E. Vibration Although troublesome vibration has been experienced to date in risk mitigation testing, some changes to the

location of the 6” water cavitating venturi have helped to lessen its effect. The initial departure from the heritage design was to move it upstream of the water prevalve where it could be located in straight pipe. Since the vibration at NASA WSTF has never been quantified, it is unclear whether this resulted in a decrease in harmful vibration. To be sure, nuts were still found loose several times over the course of testing, and in one case the generator shut down prematurely due to a loose nut on some valve actuation tubing, which prevented a valve from opening. After this

shutdown, the venturi was relocated and configured with two different anchoring arrangements. An additional pipe support was fastened to the water pipe near the venturi, below the CSG support platform. For some tests, it acted as a damping weight, but for other tests, it was bolted to the concrete pad below as an anchor. Acting as a weight, the magnitude of the vibration energy (as measured on triaxial accelerometers) was reduced by orders of magnitude. As an anchor, there appeared to be little change. Further high speed data analysis is in work at the time of this paper’s writing.

F. Instrumentation and Control No difficulty has been encountered firing the CSG under PLC control. Over the course of risk mitigation testing

done to date, it has been possible to quickly develop a new control sequence for each type of test being run, with its attendant go/no go and test cut limits on instrument measurements. The test bed was instrumented with static, delta, and high frequency pressure transducers, as well as thermocouples and accelerometers. Mass flow rates were calculated from pressure and temperature measurements taken in the vicinity of flow control devices (rather than using turbine flow meters). At A-3, temperature and pressure transmitters are planned. The CSGs will be test-fired with these instruments in a future phase of risk mitigation testing. At A-3, all 27 CSGs will be controlled from a common PLC, so that their firing sequences can be integrated for maximum reliability and efficiency.

G. Ergonomics To verify that the anticipated greater expansion of the steam manifold under temperature would not place too

much stress on the propellant water inlet spools, the CSG was installed in the furthest position from the steam pipe anchor, where it would experience the greatest deflection of the three CSG modules. Neither damage to the inlet spools, nor leaks have been observed. Only one of the three CSGs was installed for the first phase of risk mitigation testing, so the full benefit of the extra spacing was not be realized. Three CSGs will be installed and fired in the second phase, offering an enhanced experience more comparable to true conditions at A-3.

V. Conclusion The Reaction Motors CSG has a long record of high flow rate steam production for altitude simulation rocket

test facilities. Improvements to the heritage design have been incorporated to improve the reliability, maintainability, and performance.

Risk mitigation testing is being performed in 2009 and early 2010 at NASA SSC’s E-2 component test facility to verify that the updated A-3 CSGs operate as expected. Preliminary results indicate that the design meets requirements for the new implementation at A-3.

Acknowledgments The author thanks Carl Kastner of Reaction Motors, LLC, the original designer of the Chemical Steam

Generator, Perry Waller of JTI-Tullahoma, and Kevin Farrah of NASA White Sands Test Facility. The author also thanks the A-3 Risk Mitigation test team, and project managers Barry Robinson, Lonnie Dutreix, Robert Ross, and Gary Benton.

References 1”A3 Test Facility Systems Requirements Review,” NASA SSC EA50 Personnel, May 2007. 2 Barrett, Mike, “WJI PROP-ALT-0106.G - LARGE ALTITUDE SIMULATION SYSTEM OPERATIONS,” NASA White Sands Test Facility, 2005. 3Barrett, Mike, “WJI PROP-ALT-0036.B - ALTITUDE SIMULATION SYSTEM DETAILED MAINTENANCE PROCEDURE,”, NASA White Sands Test Facility, 2004. 4“Operation and Maintenance Manual, Altitude Simulation System for White Sands Missile Range – Publication No. 0724-M1,” Thiokol Chemical Corporation, Reaction Motors Division, Denville, NJ, year unknown.