3.4 A-3 STAGE ENGINE

The total thrust of 48000 pounds is subdivided into three engines of 16000 -pound thrust each. This effects a shorter overall propulsion system and a shorter, lighter vehicle interstage. Furthermore, the liquid nuorine/liquid hydrogen propellant combination was chosen because of the relatively stringent performance requirements for upper stages.

Fluorine is the most reactive and energetic chemical element. It has vigorous and reliable hypergolic ignition characteristics, and superior specific impulse capabilities with most fuels. The high density of liquid fluorine, combined with high performance with liquid hydrogen, results in maximum payloads. As mission requirements become more ambitious, payload advantages from the fluorine-oxidized propellant combination should compensate for handling problems caused by fluorine toxicity and corrosiveness. Past experience has indicated that the operation of a fluorine-oxidized engine is practical at this thrust level ( 50000 pounds or less). Fluorine when used for gaseous passivation of metals renders a metallic surface resistant to future chemical reaction. Thus, once a metallic fluoride film is formed, further action by the liquid fluorine is either prevented or significantly retarded, making handling or storing of liquid fluorine less of a problem. No known elastomer is completely compatible with fluorine; however, flow tests of liquid fluorine with Teflon have given satisfactory results.

General Engine System Description

The A-3 engine is a multiple-start, gimbaled, bipropellant system. The basic system includes a thrust chamber assembly using a combination of fuel-film ( $\mathrm{LH}_{2}$ ) and radiation cooling; propellant ducts; valves; and a control subsystem. Ignition is achieved by the hypergolicity of the propellants. These are fed directly from pressurized propellant tanks, through main propellant valves, to the thrust chamber inlets. The propellant tanks and their gas pressurization system are considered part of the engine propellant feed system. Gaseous helium supplied from a high-pressure helium bottle located inside the main fuel tank is used for main oxidizer tank pressurization. The main fuel tank is pressurized by gaseous hydrogen supplied from a liquid hydrogen bottle which is pressurized by helium and which is also located inside the main fuel tank. Both pressurants are heated in heat exchangers located at the thrust chamber nozzle extensions before they are expanded through pressure regulators and transferred to the propellant tanks. Helium gas is used to operate the main valves and the gimbal actuators, and to purge the propellant manifolds during start.

A-3 engine operating parameters, for vacuum conditions, are presented in table 3-4. The propulsion system schematic diagram is shown in figure 3-6.

The design of the entire propulsion system is governed by simplicity and minimum number of components. This is essential because of the highly reactive and toxic nature of fluorine. Welded joints are used extensively. No rotating seals are employed. Sliding seals are of the metal-bellows type.

A preliminary design layout of the A-3 propulsion system and dimensions are shown in figure 3-7.

The fuel tank is pressure stabilized rather than mechanically stabilized. The thrust loads are transmitted to the payload through the fuel tank. Both tanks are insulated, as are the ducts between tanks and engine systems.

Thrust-vector control is achieved by gimbaling the thrust chambers. Each basic engine weighs approximately 330 pounds dry and 365 pounds at burnout. It has a cylindrical space envelope of 5 feet 4 inches diameter by 7 feet 6 inches length. The propulsion system (including the three engines and the tankage) weighs

Figure 3-6.-A-3 stage propulsion and engine system schematic diagram.

Table 3-4.-16K A-3 Stage Engine Operating Parameters

[Vacuum conditions]
Engine (pressurized gas-feed):			Calibration orifice pressure drop	psi	17
Thrust	lb	16000	Oxidizer tank pressure	psia	170
Nominal total multiple-firing duration	sec.	300	Total oxidizer weight ( 300 sec duration for 3 engines, plus
			1 percent residual)	Ib	27950
Specific impulse	sec	446	Oxidizer tank volume (including
Oxidizer $\mathrm{LF}_{2}$ :				$\mathrm{ft}^{3}$	305
Density	$\mathrm{lb} / \mathrm{ft}^{3}$	94.16	Pressurant (helium) flow rate
Flow rate	lb sec	30.78	(assuming tank gas temperature
Fuel $\mathrm{LH}_{2}$ :	$\mathrm{lb} / \mathrm{ft}^{3}$		$400^{\circ} \mathrm{R}$ )	lb/sec	0.1555
Density		4.42	Total pressurant weight (including
Flow rate	lb/sec	5.13
Mixture ratio	O/F	6
			other requirements in the system) (assume storage bottle final pressure 350 psi , plus 2 percent reserve)
Thrust chamber (solid wall film cooled by fuel and radiation cooled on nozzle extension):
				Ib	60
			Pressurant storage tank, volume
Thrust	lb	16000	(assume $200^{\circ} \mathrm{R}$ storage temper-
Specific impulse	sec	446	ature, including 3 percent
Injector end pressure	psia	110	ullage volume)	$\mathrm{ft}^{3}$	7.35
Nozzle stagnation pressure	psia	100	Pressurant storage tank, initial
Oxidizer flow rate	lb/sec	30.78	pressure	psia	4500
Fuel flow rate	lb/sec	5.13
Mixture ratio	O/F	6
$c^{*}$ efficiency	Percent	98	Fuel side (pressurized by heated hydrogen):
$c^{*}$	ft/sec	7910	Injector pressure drop	psi	25
$C_{t}$ efficiency	Percent	102	Inlet manifold pressure drop	psi	10
$C_{t}$		1.817	Main valve pressure drop	psi	10
Contraction ratio	$A_{c} / A_{t}$	2	Line pressure drop	psi	5
Expansion ratio	$A_{e} / A_{t}$	35	Fuel tank pressure	psia	160
Throat area $A_{t}$	$\mathrm{in}^{2}$	88
L*	in	28	Total fuel weight ( 300 sec duration for 3 engines, plus 1 percent
Nozzle contour		70 percent bell	residual)	lb	4660
			Fuel tank volume (including 3 percent ullage volume)
Thrust vector control:				$\mathrm{ft}^{3}$	1087
Minimum acceleration	$\mathrm{rad} / \mathrm{sec}^{2}$	2	Pressurant (hydrogen) flow rate (assuming tank vapor temperature
Maximum velocity	deg/sec	15	$300^{\circ} \mathrm{R}$ )	lb/sec	0.346
Displacement	deg.	$\pm 7$	Total pressurant weight (assuming storage bottle final pressure 350
Oxidizer side (pressurized by heated helium):
			psia, plus 4 percent reserve)	lb.	108
Injector pressure drop	psi	25	Pressurant storage tank, volume
Oxidizer dome pressure drop	psı	5	(liquid hydrogen including 3
Main valve pressure drop	psi	8	percent ullage volume)	$\mathrm{ft}^{3}$	25.2
Line pressure drop.	psi	5	Pressurant storage tank, pressure.	psia	350

approximately 5130 pounds dry, 37900 pounds wet, and 5530 pounds at burnout.

System Operation

The A-3 propulsion system is designed for automatic start, on receipt of a signal from the guidance system. A similar signal effects automatic engine shutdown. One or more restarts can be made by merely sending additional start and shutdown signals to the propulsion system.

Figure 3-8 shows the operational sequence of the A-3 stage engine. In conjunction with figure $3-6$, this illustrates the system starting and shutdown operations.