Flight Vehicles

As mentioned, the vast majority of rocket propelled vehicles are simple, single stage, and use solid propellant rocket motors. Most are used in military applications, as described in the next section. This section discusses more sophisticated multistage space launch vehicles and mentions others, such as large ballistic missiles (often called strategic missiles) and some sounding rockets. All have some intelligence in their guidance and navigation system. The total number of multistage rocket vehicles produced world wide in the last few years has been between 140 and 220 per year.

A single stage to orbit (LEO) is limited in the payload it can carry. Figure 4-2 shows that a high-performance single-stage vehicle with a propellant fraction of 0.95 and an average Is of 400 sec can achieve an ideal terminal velocity of about 12,000 m/sec without payload. If the analysis includes drag and gravity forces, a somewhat higher value of Is, maneuvers in the trajectory, and an attitude control system, it is likely that the payload would be between 0.2 and 1.4 percent of the gross take-off mass, depending on the design. For a larger percentage of payload, and for ambitious missions, we use vehicles with two or more stages as described here.

Multistage Vehicles

Multistep or multistage rocket vehicles permit higher vehicle velocities, more payload for space vehicles, and improved performance for long-range ballistic missiles. After the useful propellant is fully consumed in a particular stage, the remaining empty mass of that expended stage is dropped from the vehicle and the operation of the propulsion system of the next step or stage is started. The last or top stage, which is usually the smallest, carries the payload. The empty mass of the expended stage or step is separated from the remainder of the vehicle, because it avoids the expenditure of additional energy for further accelerating a useless mass. As the number of steps is increased, the initial takeoff mass can be decreased; but the gain in a smaller initial mass becomes less apparent when the total number of steps is large. Actually, the number of steps chosen should not be too large, because the physical mechanisms become more numerous, complex, and heavy. The most economical number of steps is usually between two and six, depending on the mission. Several different multistage launch vehicle configurations have been used successfully and four are shown in Fig. 4-14. Most are launched vertically, but a few have been launched from an airplane, such as the three-stage Pegasus space vehicle.

The payload of a multistage rocket is essentially proportional to the takeoff mass, even though the payload is only a very small portion of the initial mass. If a payload of 50 kg requires a 6000-kg multistage rocket, a 500-kg payload would require a 60,000-kg rocket unit with an identical number of stages, and a similar configuration with the same payload fraction. When the operation of the upper stage is started, immediately after thrust termination of the lower stage, then the total ideal velocity of a multistage vehicle of tandem or series-stage arrangement is simply the sum of the individual stage velocity increments. For n stages, the final velocity increment Auj is n

The individual velocity increments are given by Eq. 4-6. For the simplified case of a vacuum flight in a gravity-free field this can be expressed as

Auf = C\ ln(l/IVRi) + c2 ln(l/Mt2) + c3 ln(l/]VR3) + • • • (4-36)

This equation defines the maximum velocity an ideal multistage vehicle can attain in a gravity-free vacuum environment. For more accurate actual trajectories the individual velocity increments can be determined by integrating Eqs. 4-15 and 4-16, which consider drag and gravity losses. Other losses or trajectory perturbations can also be included, as mentioned earlier in this chapter. Such an approach requires numerical solutions.

For two- or three-stage vehicles the overall vehicle mass ratio (initial mass at takeoff to final mass of last stage) can reach values of over 100 (corresponding to an equivalent single-stage propellant mass fraction f of 0.99). Figure 4-2 can be thus divided into regions for single- and multistage vehicles.

Sustainer

Sustainer stage (contains propeilant for booster thrust)

Sustainer stage (contains propeilant for booster thrust)

Dropable booster engine ring package (without propeilant)

Staging in series or tandem

Partial staging

Staging in series or tandem

Partial staging

Sustainer

Winged sustainer stage z

First / stage

Four strap-on boosters

Dropable booster engine ring package (without propeilant)

Parallel staging

Parallel staging

Winged sustainer stage

First / stage

Four strap-on boosters

Booster

Piggy-back staging

Piggy-back staging

FIGURE 4-14. Simplified schematic sketches of four geometric configurations for assembling individual stages into a launch vehicle. The first is very common and the stages are stacked vertically on top of each other, as in the Minuteman long-range missile or the Delta launch vehicle. Partial staging was used on early versions of the Atlas; it allows all engines to be started at launching, thus avoiding a start during flight, and it permits the shut-off of engines on the launch stand if a failure is sensed prior to lift-off. The two booster engines, arranged in a doughnut-shaped assembly, are dropped off in flight. In the third sketch there are two or more separate "strap-on" booster stages attached to the bottom stage of a vertical configuration and this allows an increase in vehicle performance. The piggy-back configuration concept on the right is used in the Space Shuttle.

For multistage vehicles the stage mass ratios, thrust levels, propulsion durations, and the location or travel of the center of gravity of the stages are usually optimized, often using a complex trajectory computer program. The high specific impulse rocket engine (e.g., using hydrogen-oxygen propellants) is normally employed in upper stages of space launch vehicles, because a small increase in specific impulse is more effective there than in lower stages.

Example 4-3. A two-stage planetary exploration vehicle is launched from a high-orbit satellite into a gravity-free vacuum trajectory. The following notations are used and explained in the diagram.

m0 = initial mass of vehicle (or stage) at launch mp — useful propellant mass of stage m, = initial mass of stage(s)

rrif = final mass of stage (after rocket operation); it includes the empty propulsion system with its residual propellant, the structures of the vehicle and the propulsion system, the control, guidance, and payload masses. mpi = payload mass; it includes the guidance, control and communications equipment, antennas, scientific instruments, research apparatus, power supply, solar panels, sensors, etc.

or booster

Subscripts 1 and 2 refer to first and second stages. The following are given:

Flight and velocity increment in gravity-free vacuum 6200 m/sec

Specific impulse, Is 310 sec

Effective exhaust velocity, c (all stages) 3038 m/sec

Initial launch vehicle mass 4500 kg

Propellant mass fraction, £ (each stage) 0.88

Structural mass fraction, (1 — £) (each stage) 0.12

Determine the payload for two cases: (1) when the two stage masses are equal, and (2) when the mass ratios of the two stages are equal.

SOLUTION. For launch the takeoff mass (m0) equals the loaded first-stage mass (mi)[ plus the loaded second-stage mass (m,)2 plus the payload (mpl). The propellant mass fraction £ is 0.88. For case (1) the first and second stages are identical. Thus

K)i = 0.88^)! (m0)l = 4500 kg = Irrii + mp, gAu/c _ g6200/3038 _ -j gggg _ __(mo)2

From these relationships it is possible to solve for the payload mass mpl, which is 275 kg.

rtii = (4500 - 275)/2 = 2113 kg each stage mp = 0.88m, = 1855 kg each stage

For case (2) the mass ratios of the two stages are the same. The mass ratio (1/1VR) was defined by rno/m/ = (moh/Kmo)i ~ {{mp)x] = (m0)2/[(m0)2 - (mp)2] (mo) i = 4500 = (mi)l + (m,)2 + mpi eAu/c = ? 6968 = (4500/[4500 - (m^}}2

Solving for the first-stage propellant mass gives (mp)l = 2878 kg.

(m0)2 = (mi)2 + mp, = 4500 - 3270 = 1230 kg eA"/c = 7.6968 = (1230/[1230 - (mp)2]}2; (mp)2 = 786.6 kg

The payload mp/ is 1230 - 894 = 336 kg. This is about 22% larger than the payload of 275 kg in the first case. When the mass ratios of the stages are equal, the payload is a maximum for gravity-free vacuum flight and the distribution of the masses between the stages is optimum. For a single-stage vehicle with the same take-off mass and same propellant fraction, the payload is substantially less. See Problem 4-13.

If a three-stage vehicle had been used in Example 4-3 instead of a two-stage version, the payload would have been even larger. However, the theoretical payload increase will only be about 8 or 10%. A fourth stage gives an even smaller theoretical improvement; it would add only 3 to 5% to the payload. The amount of potential performance improvement diminishes with each added stage. Each additional stage means extra complications in an actual vehicle (such as a reliable separation mechanism, an interstage structure, joints or couplings in a connecting pipes and cables, etc.), requires additional inert mass (increasing the mass ratio IVR), and compromises the overall reliability. Therefore, the minimum number of stages that will meet the payload and the Au requirements is usually selected.

The flight paths taken by the vehicles in the two simplified cases of Example 4-3 are different, since the time of flight and the acceleration histories are different. One conclusion from this example applies to all multistage rocket-propelled vehicles; for each mission there is an optimum number of stages, an optimum distribution of the mass between the stages, and there is usually also an optimum flight path for each design, where a key vehicle parameter such as payload, velocity increment, or range is a maximum.

Launch Vehicles

Usually the first or lowest stage, often called a booster stage, is the largest and it requires the largest thrust and largest total impulse. All stages need chemical propulsion to achieve the desired thrust-to-weight ratio. These thrusts usually become smaller with each subsequent stage, also known as upper stage or sustainer stage. The thrust magnitudes depend on the mass of the vehicle, which in turn depends on the mass of the payload and the mission. Typical actual configurations are shown by simple sketches in Fig. 4-14. There is an optimum size and thrust value for each stage in a multistage vehicle and the analysis to determine these optima can be quite complex.

Many heavy launch vehicles have two to six strap-on solid propellant motor boosters, which together form a supplementary first stage strapped on or mounted to the first stage of the launch vehicle (Space Shuttle, Titan, Delta, Atlas, Ariane). This is shown in the third sketch of Fig. 4-14. The Russians have used liquid propellant strap-on boosters on several vehicles, because they give better performance. Boosters operate simultaneously with the first stage and, after they burn out, they are usually separated and dropped off before completion of the first stage's propulsive operation. This has also been called a half stage or zero stage, as in Table 1-3.

There is a variety of existing launch vehicles. The smaller ones are for low payloads and low orbits; the larger ones usually have more stages, are heavier, more expensive, have larger payloads, or higher mission velocities. The vehicle cost increases with the number of stages and the initial vehicle launch mass. Once a particular launch vehicle has been proven to be reliable, it is usually modified and uprated to allow improvements in its capability or mission flexibility. Each of the stages of a space launch vehicle can have several rocket engines, each with specific missions or maneuvers. The Space Shuttle system has 67 different rockets which are shown schematically in Fig. 1-13. In most cases each rocket engine is used for a specific maneuver, but in many cases the same engine is used for more than one specific purpose; the small reaction control thrusters in the Shuttle serve, for example, to give attitude control (pitch, yaw, and roll) during orbit insertion and reentry, for counteracting internal shifting of masses (astronaut movement, extendible arm), small trajectory corrections, minor flight path adjustments, docking, and precise pointing of scientific instruments.

The spacecraft is that part of a launch vehicle that carries the payload. It is the only part of the vehicle that goes into orbit or deep space and some are designed to return to earth. The final major space maneuver, such as orbit injection or planetary landing, often requires a substantial velocity increment; the propulsion system, which provides the force for this maneuver, may be integrated with the spacecraft, or it may be part of a discardable stage, just below the spacecraft. Several of the maneuvers described in Section 4-6 can often be accomplished by propulsion systems located in two different stages of a multistage vehicle. The selection of the most desirable propulsion systems, and the decision on which of the several propulsion systems will perform specific maneuvers, will depend on optimizing performance, cost, reliability, schedule, and mission flexibility as described in Chapter 17.

When a space vehicle is launched from the earth's surface into an orbit, it flies through three distinct trajectory phases. (1) Most are usually launched vertically and then undergo a turning maneuver while under rocket power to point the flight velocity vector into the desired direction. (2) The vehicle then follows a free-flight (unpowered) ballistic trajectory (usually elliptical), up to its apex. Finally (3) a satellite needs an extra push from a chemical rocket system up to add enough total impulse or energy to accelerate it to orbital velocity. This last maneuver is also known as orbit insertion. During the initial powered flight the trajectory angle and the thrust cut-off velocity of the last stage are adjusted by the guidance system to a velocity vector in space that will allow the vehicle to reach the apogee of its elliptic path exactly at the desired orbit altitude. As shown in Fig. 4-9, a multistage ballistic missile follows the same two ascent flight phases mentioned above, but it then continues its elliptical ballistic trajectory all the way down to the target.

Historically successful launch vehicles have been modified, enlarged, and improved in performance. The newer versions retain most of the old, proven, reliable components, materials, and subsystems. This reduces development effort and cost. Upgrading a vehicle allows an increase in mission energy (more ambitious mission) or payload. Typically, it is done by one or more of these types of improvement: increasing the mass of propellant without an undue increase in tank or case mass; uprating the thrust and strengthening the engine; more specific impulse; or adding successively more or bigger strap-on boosters. It also usually includes a strengthening of the structure to accept higher loads.

Figure 4-15 and Table 4-7 illustrate the growth of payload and mission capability for the early Titan family of space launch vehicles and the effect of the orbit on the payload. The figure shows the evolution of four different multistage configurations of the launch vehicle and their principal propulsion systems; the table defines the increase in payload for the four vehicle configurations and also how the payload is reduced as more ambitious orbits are flown. When each of these vehicles is equipped with an additional third stage, it is able to launch substantial payloads into earth escape or synchronous orbit. The table describes the propulsion for each of the several stages used on those vehicles and the payload for several arbitrarily selected orbits.

Table 4-7 shows the effects of orbit inclination and altitude on the payload. The inclination is the angle between the equatorial plane of the earth and the trajectory. An equatorial orbit has zero inclination and a polar orbit has 90° inclination. Since the earth's rotation gives the vehicle an initial velocity, a

Launch vehicle

Configuration

Major configuration modifications

First flight

Titan II SLV

Titan I

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