The Space Shuttle SRB

Table 4.1 shows that the SRB is about twice the size of the MPS. It develops a thrust of 10 MN. The casing consists of eight steel segments flow-turned with the appropriate flanges. The fore and aft sections are fitted with the igniter and nozzle respectively. The casing sections are joined in pairs by factory joints which are then thermally protected by thick rubber seals, and the inner walls of the casing are protected by insulating material to which the propellant will be bonded. The propellant is mixed and cast into each pair of segments, with a mandrel of the appropriate shape to form the hollow core. The booster pairs are thrust-matched by filling the appropriate segments of each pair together, from the same batch of propellant. Insulating material and inhibitor are applied to the faces, which are not

Table 4.1. Two modern solid boosters.

SRB (Space Shuttle) MPS (Ariane 5)

SRB (Space Shuttle) MPS (Ariane 5)

Table 4.1. Two modern solid boosters.

Thrust (individual)

10.89 MN

5.87 MN

Thrust (fractional)

71% at lift-off

90% at lift-off

Expansion ratio

11.3

«10

Exhaust velocity

2,690 ms—1

2,690 ms—1

Temperature

3,450

3,600

Pressure

65 bar

60 bar

Total mass

591 T

267 T

Propellant mass

500 T

237 T

Dry mass

87.3 T

30 T

Burn-time

124 s

123 s

Charge shape

Upper section

11 point cog

23 point cog

Middle section

Truncated cone

Cylinder

Lower section

Truncated cone

Truncated cone

Propellant

Ammonium perchlorate 69.6%

Ammonium perchlorate 68%

Aluminium powder 16%

Aluminium powder 18%

Polymeric binder 14%

Polymeric binder 14%

Additive iron oxide 0.4%

Factory joints

4

6

Field joints

3

2

Length

45.4m

27 m

Diameter

3.7m

3.0m

Casing

Steel 12 mm

4SCDN-4-10 steel

to burn. The filled sections are then transported to the launch site where they are assembled, using the field joints, into the complete booster.

The nozzle is made of layers of glass and carbon fibre material bonded together to form a tough composite structure which can survive temperatures up to 4,200 K. This composite is then bonded onto the inside of the steel outer cone, which provides the structural support. Rings attached to the cone provide the anchorage for the hydraulic actuators. Inboard is the flexible joint which allows the nozzle to be tilted.

In addition to the propellant in its casing, each booster has a redundant hydraulic system to displace the nozzles by ±8°. There are two actuators, one for each orthogonal direction of displacement. The nose cap of the booster also contains avionics and recovery beacons.

The casing segments are reused. After recovery from the sea the casings are cleaned, inspected and pressure tested to ensure they are sound. Some are discarded because of damage, which is mostly caused by impact with the ocean. The segments are then dimensionally matched, as the combustion chamber pressure can permanently increase the diameter by several fractions of a millimetre. Once cleared for reuse they are refilled with propellant. The sections are rated for 20 re-uses.

The propellant core shapes are intended to produce a 'sway-backed' thrust curve rather than a constant thrust. This produces a period of lower thrust some 50 seconds into the flight, while the vehicle is passing through the region of maximum dynamic pressure (see Chapter 5). This is the period when the product of velocity and air pressure is at a maximum and when the possibility of damage by aerodynamic forces is greatest, and the risk is minimised if the thrust is reduced for a short time. The Space Shuttle main engines are also throttled back for this period.

At launch the two boosters provide 71% of the total thrust, effectively forming the first stage. Their short nozzles are adapted for sea-level operation, and the huge mass flow rate—almost five tonnes per second—provides the high thrust necessary for lift-off.

4.7.2 The Ariane MPS

The MPS (Moteur a Propergol Solide) is similar in many respects to the SRB, and about half the size. It has a 3-metre diameter compared with the 5 metres of the SRB. The thrust, at nearly 6MN, contributes 90% of the Ariane 5 lift-off thrust. The propellant is very similar, with percent level differences in the composition of ammonium perchlorate oxidant and aluminium powder fuel. The binder—a poly-butadiene rubber—may well be somewhat different in detail from the Thiokol rubber used for the SRB. The additives are not specified.

The booster consists of seven sections. The forward section contains the igniter and the aft section the nozzle, and both the forward and aft domes are protected with ablative insulating material. The forward section, which has a rough forged bulkhead to contain the pressure, is charged with 23 tonnes of propellant with a cog-shaped inner void, in Europe, prior to shipment to the launch site at Kourou. The remaining segments are charged, at the launch site, with locally manufactured propellant. The middle segment consists of three casing elements pinned together with factory joints. The MPS joints are overlapping joints with transverse pins, and the inner walls are protected with silica and Kevlar fibre insulator (GSM 55 and EG2) before the propellant is cast into the segment. This thermal protection covers the factory joints. The mandrel for the casting produces a shallow truncated cone shape to the grain void. The lower segment is constructed in the same way, and the mandrel here produces a steeper conical form to the void, opening towards the nozzle. The forward grain burns away in the first 15 seconds, while the two lower grains, each of 107 tonnes, burn for 123 seconds. This produces the 'sway-backed' thrust profile which reduces the thrust around maximum dynamic pressure.

The nozzle is made of composite materials incorporating carbon-carbon and phenolic silica materials. It is supported by a lightweight metallic casing, to which the 35-tonne servoactuators are connected by a strong ring. The nozzle can be traversed by ±6° for thrust vector control.

The boosters are recovered after launch, but presently there is no plan for reuse of casing segments. The main purpose of recovery is post-flight inspection of seals and components to ensure that they are functioning correctly throughout the flight.

Nozzle control system

Flexible Nozzle Joints Solid Rockets

Segmented propellant grain

Flexible bearing

Figure 4.4. The Ariane MPS solid booster.

Segmented propellant grain n

Flexible bearing

Figure 4.4. The Ariane MPS solid booster.

Post-flight inspection, for example, studies the seals and thermal protection to confirm that it remains intact. Some 10-12 mm of the aft dome protection ablates away during flight, in the region directly adjacent to the nozzle. The throat diameter is 895 mm, and 38 mm of the thermal protection was ablated away during a test firing. This is allowed for in the design, and demonstrates how thermal protection is provided by this technique.

The Space Shuttle and Ariane 5 represent the state-of-the-art heavy launcher capability presently available. The booster technology is mature, as evidenced by the similarity in techniques. Solid propellant boosters of this power represent the best way of increasing the in-orbit payload capability of large expendable launchers. Because of their tough construction and early burn-out, boosters are eminently recoverable, and their reuse is a factor in the economics of launchers. The Space Shuttle has shown the way in reuse, but caution still exercises a strong restraint, as the cost of quality assurance needed to ensure safe reuse is a significant fraction of the cost of new components. The next step in making space more accessible will come from the development of fully reusable launchers and the single stage to orbit. For these, liquid propellants are appropriate, but it is likely that hybrids and intermediate developments will still utilise solid propellant motors. Small launchers make considerable use of all solid propellant propulsion, and this is a market where cost and reliability indicate its continued use.

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