Vanguard Stage Two

Soon after the army deployed Corporal I, Aerojet had occasion to develop its storable-propellant technology further with stage two of the Vanguard launch vehicle for the navy. The firm’s Aerobee sounding rockets, building on the WAC Corporal engine technology, had led to the Aerobee-Hi sounding rocket that provided the basis for the projected stage two. As requirements for that stage became more stringent, though, Aerobee-Hi proved deficient, and Aerojet had to return to the drawing board. The firm charged with designing Vanguard, the Martin Company, contracted with Aerojet on Novem­ber 14, 1955, to develop the second-stage engine. Martin had deter­mined that the second stage needed a thrust of 7,500 pounds and a specific impulse at altitude of 278 lbf-sec/lbm to provide the required velocity to lift the estimated weight of the Vanguard satellite.20

The development of an engine to meet these specifications proved to be difficult. Martin’s calculated thrust and specific im­pulse would not meet the vehicle’s velocity requirements without severe weight limitations. Aerojet engineers selected unsymmetri­cal dimethyl hydrazine (UDMH) and inhibited white fuming ni­tric acid (IWFNA) as the propellants because they were hypergolic (eliminating problems with ignition), had a high loading density (reducing the size, hence weight, of propellant tanks), and delivered the requisite performance. Another advantage of hydrazine and acid was a comparative lack of problems with combustion instability in experimental research.21

The history of the evolution from the aniline-nitric acid propel­lants used in the WAC Corporal (specifically, red fuming nitric acid with 6.5 percent nitrogen dioxide plus aniline with the addition of 20 percent furfuryl alcohol) and in the first Aerobee sounding rocket (35 instead of 20 percent furfuryl alcohol) to the UDMH and IWFNA used in Vanguard is complicated. But it illustrates much about pro­pellant chemistry and the number of institutions contributing to it. The basic aniline-RFNA combination worked as a self-igniting pro­pellant combination. But it had numerous disadvantages. Aniline is highly toxic and rapidly absorbed via the skin. A person who came into contact with a significant amount of it was likely to die rapidly from cyanosis. Moreover, aniline has a high freezing point, so it can be used only in moderate temperatures. RFNA is highly corrosive to propellant tanks, so it has to be loaded into a missile or rocket just before firing, and when poured, it gives off dense concentra­tions of nitrogen dioxide, which is also poisonous. The acid itself burns the skin, as well. Two chemists at JPL had discovered as early as 1946 that white fuming nitric acid (WFNA) and furfuryl alcohol with aniline were just as poisonous and corrosive but did not pro­duce nitrogen dioxide.

But WFNA turned out to be inherently unstable over time. A complicated substance, it was hard for propellant chemists to ana­lyze in the early 1950s. By 1954, however, researchers at the Na – 154 val Ordnance Test Station and at JPL had thoroughly investigated Chapter 4 nitrogen tetroxide and nitric acid and come up with conclusions that were to be used in the Titan II. Meanwhile, chemists at the Naval Air Rocket Test Station, Lake Denmark, New Jersey; JPL; the NACA’s Lewis Flight Propulsion Laboratory; the air force’s Wright Air Development Center in Dayton, Ohio; and Ohio State Univer­sity, among other places, had reached a fundamental understanding of nitric acid by 1951 and published the information by 1955. In the process, the Naval Air Rocket Test Station was apparently the first to discover that small percentages of hydrofluoric acid both reduced the freezing point of RFNA/WFNA and inhibited corrosion with many metals. Thus were born inhibited RFNA and WFNA, for which the services and industrial representatives under air force sponsorship drew up military specifications in 1954. In this way, the services, the NACA, one university, and the competing indus­tries cooperated to solve a common problem.

During the same period, chemists sought either replacements for aniline or chemicals to mix with it and make it less problematic. Hydrazine seemed a promising candidate, and in 1951 the Rocket Branch of the navy’s Bureau of Aeronautics, issued contracts to

Metallectro Company and Aerojet to see if any hydrazine deriva­tives were suitable as rocket propellants. They found that UDMH rapidly self-ignited with nitric acid, leading to a military specifica­tion for UDMH in 1955.22

Despite the severe weight limitations on the second-stage en­gine, the Vanguard project engineers had decided to use a pressure – fed (rather than a pump-fed) propellant-delivery system. The pumps produced angular momentum as they rotated, and for stage two, this would be hard for the roll-control system to overcome, especially after engine cutoff. Concerns about reliability led to a decision to use heated helium gas as the pressurant in the feed system. Aerojet convinced the Martin Company and the navy to use stainless-steel instead of aluminum propellant tanks. Because steel had a better strength-to-weight ratio than aluminum, Aerojet argued that the lighter metal would, paradoxically, have had to weigh 30 pounds more than the steel to handle the pressure.

Moreover, a “unique design for the tankage" placed the sphere containing the helium pressure tank between the two propellant tanks, serving as a dividing bulkhead and saving the weight of a sepa­rate bulkhead. A solid-propellant gas generator augmented the pres­sure of the helium and added its own chemical energy to the system at a low cost in weight. Initially, Aerojet had built the combustion chamber of steel. It accumulated 600 seconds of burning without corrosion, but it was too heavy. So engineers developed a lightweight chamber made up of aluminum regenerative-cooling, spaghetti-type tubes wrapped in stainless steel. It weighed 20 pounds less than the steel version, apparently the first such chamber built of aluminum tubes for use with nitric acid and UDMH.23

During 1956 there were problems with welding the stainless – steel tanks despite Aerojet’s experience in this area. Martin recom­mended a different method of inspection and improvements in tool­ing, which resolved these problems. The California firm also had to try several types of injector before finding the right combination of features. One with 72 pairs of impinging jets did not deliver suffi­cient exhaust velocity, so Aerojet engineers added 24 nonimpinging orifices for fuel in the center portion of the injector. This raised the exhaust velocity above the specifications but suggested the empiri­cal nature of the design process, with engineers having to test one design before discovering that it would not deliver the desired per­formance. They then had to use their accumulated knowledge and insights to figure out what modification might work.24

The development of the combustion chamber and related equip­ment illustrated the same process. Despite the use of inhibited

white fuming nitric acid, the lightweight aluminum combustion chamber—which could be lifted with one hand—gradually eroded. It took engineers “weeks of experimenting" to find out that a coating of tungsten carbide substantially improved the life of the combus­tion chamber. There also were problems with the design of valves for flow control, requiring significant modifications.25

A final problem lay in testing an engine for start at altitude. At the beginning of the project, there was no vacuum chamber large enough to test the engine, but according to NRL propulsion engineer Kurt Stehling, “Several tests were [eventually?] made at Aerojet with engine starts in a vacuum chamber." In any event, to preclude prob­lems with near-vacuum pressure at altitude, the engineers sealed the chamber with a “nozzle closure" that kept pressure in the cham­ber until exhaust from ignition blew it out.26

The original Vanguard schedule as of November 1955 called for six test vehicles to be launched between September 1956 and Au­gust 1957, with the first satellite-launching vehicle to lift off in Oc­tober 1957.27 It was not until March 1958, however, that the second stage could be fired in an actual launch—that of Test Vehicle (TV) 4. TV-4 contained modifications introduced into the stage-one engine following the failure of TV-3 (when stage one exploded), but it did not yet incorporate the tungsten-carbide coating in the aluminum combustion chamber of the stage-two engine. And it was still a test 156 vehicle. On March 17, 1958, the slender Vanguard launch vehicle Chapter 4 lifted off. It performed well enough (despite a rough start) to place the small 3.4-pound Vanguard I satellite in an orbit originally esti­mated to last for 2,000 (but later revised to 240) years.28

On TV-5, launched April 28, 1958, the second stage provided less-than-normal thrust, but the first stage had performed better than normal, compensating in advance for the subpar second stage. Then an electrical problem prevented ignition of the third stage, pre­cluding orbit. On the first nontest Vanguard, Space Launch Vehicle (SLV) 1, apparent malfunction of a pressure switch also prevented orbiting a 21.5-pound satellite on May 27, 1958. Here, the second stage performed normally through cutoff of ignition. On SLV-2, June 26, 1958, the second-stage engine cut off after eight seconds, probably due to clogged filters in the inhibited white fuming nitric acid lines from corrosion of the oxidizer tank. The Vanguard team flushed the oxidizer tanks and launched SLV-3 on September 26, 1958, with a 23.3-pound satellite. Despite the flushing, second – stage performance was below normal, causing the satellite to miss orbital speed by a narrow margin. This time, the problem seemed to be a clogged fuel (rather than oxidizer) filter.

On February 17, 1959, however, all systems worked, and SLV-4 placed the 23.3-pound Vanguard II satellite in a precise orbit ex­pected to last for 200 years or more. This did not mean that Aerojet had gotten all of the kinks out of the troublesome second stage. SLV-5 on April 13, 1959, experienced a flame oscillation during sec­ond-stage ignition, apparently producing a violent yaw that caused the second and third stages with the satellite to tumble and fall into the ocean. Engineers made changes in the second-stage engine’s hy­draulic system and programmed an earlier separation of the stage, but on SLV-6 (June 22, 1959), a previously reliable regulating valve ceased to function after second-stage ignition. This caused helium pressure to mount (since it could not vent), resulting in an explo­sion that sent the vehicle into the Atlantic about 300 miles down – range. At least the problem-plagued Vanguard program ended on a happy note. On September 18, 1959, a test vehicle backup (TV-4BU) version of the launch vehicle placed a 52.25-pound X-ray and envi­ronmental satellite into orbit.29