Safety requirements dictate that all liquid engines of the initial flight stage be at full power and healthy before releasing the vehicle from the launch pad. To meet this
requirement the vehicle is attached to the launch pad with large mechanisms with sufficient strength to counteract the thrust force pad environments with margin. As a result the total thrust (7.5 million pounds) of the Saturn V first stage is stored in the structure as strain energy (potential energy). When the engines are all declared healthy the vehicle is released from the pad to fly into space. This release of energy creates a very large structural dynamic response of the vehicle and increases the loads significantly in the vehicle first stage with some increases in the loads in the other vehicle stages. These load increases would result in performance loss due to the structural weight increase required to handle the loads. As a result a soft release was employed as illustrated on Figure 12-4. It consisted of a tapered bolt that was severed using a pyrotechnic device. The tapered bolt was then pulled through the smaller diameter skirt attachment hole, slowly releasing the energy and decreasing the loads by approximately 30%. Summarizing:
• Engines are started and are at full thrust in order to assure engine health, before releasing the vehicle from the launch facility.
• Engine thrust energy is stored in the thrust frame and other vehicle structures.
• The stored energy, that is suddenly released, creates a large dynamic response.
• Saturn V used a soft release mechanism in order to reduce the dynamic response (loads).
• The release was an extruding bolt through an orifice as shown on the next figure.
• Loads in the rear of the vehicle were reduced more than 30%.
Figure 12-4. Saturn V Hold Down Post Soft Release Mechanism
Space Shuttle Liftoff Loads
Space Shuttle had the same requirement as Saturn V with a much more complex dynamic system and with a very sensitive performance issue. Therefore it was mandatory that structural mass be as minimum as possible. The Shuttle is a five body system
connected with interfaces called struts and/or attachment mechanisms. The vehicle is unsymmetrical in the pitch plane with the orbiter attached on the side of the External Tank and Solid Rocket Motors. The vehicle is held to the Mobile Launch Pad (MLP) with four pyrotechnic bolts on each solid. See the following figure. The vehicle is first filled with the cryogenic liquid propellants which shrinks the External Tank longitudinal and in diameter. For example the aft SRB to ET attachment strut is at 7 degrees to perpendicular before filling the tank and becomes perpendicular due to the cryogenic shrinkage when the tanks are filled. In addition its diameter shrinks approximately 2 inches. This stores energy into the structure. At the start of the on pad liftoff cycle the SSME‘s are ignited and carried to 90% thrust to ensure engine health. The engines are canted for c.g. tracking and are offset from the vehicle center line several feet. This stores additional energy in the structure in two ways. The orbiter is lifted up bending the vehicle over the attached points introducing a large bending moment in the system. This can be illustrated visually by watching the launch of Shuttle observing the tank tip moving laterally approximately 36 inches. This sets up a dynamic motion of the vehicle on the pad about the mean moment introduced by the SSME thrust. Since the engines are offset, they also push the Orbiter and ET between the two motors that are holding the vehicle to the pad. [Ryan, 1996]
At SRB bolt release all this stored energy is released creating a large complex
dynamic response. The interaction of the four vehicle bodies plus the payload body requires approximatly 300 bending modes to simulate the response and calculated all the system
loads. At release two additional forces add to these dynamic responses, thus loads. These are the expansion of the Solid Rocket Motor due to the 960 psi of internal pressure created by the burning propellant. This pressure goes from zero to the 960 psi in 500 milliseconds after motor ignition. The ignition of the motors and the large thrust created moves through the launch pad thrust tunnel creating a overpressure wave that travels up the vehicle. Since the Shuttle is a 1½ stage vehicle it is very performance critical, thus these high dynamic loads had to mitigated to reduce structural weight.
As stated above the SSME thrust at engine start and buildup creates a dynamic motion laterally as shown on Figure 12-5. The vehicle motion of moving back to a minimum position has a minimum energy point at which time if the vehicle is released from the pad will have the lowest load. Shown as stored bending moment on Figure 12-6 is the time
dependency of the bending moment. As a result it was decided to burn the SSME‘s 2½ seconds beyond the point where they are declared healthy, greatly reducing the loads. This delay of liftoff from the time of verified SSME‘s to the minimum energy point reduces vehicle performance but at a much lower level that would be the impact of the increased loads.
Figure 12-7 is a plot of the predicted liftoff response versus a flight response.
Figure 12-5. Shuttle Liftoff Dynamics
Bending moment -inch lbs x 106 0 200 400
Shuttle Liftoff Signal,
SRM Ignition • The bottom figure
shows the SSME thrust history.
• The top figure shows the bending moment stored in the vehicle as a function of time after SSME‟s ignition.
• SRM ignition and liftoff is deferred about 2 seconds until the bending moment is minimum.
0 1 2 3 4 5 6 7
Time after SSME ignition signal - seconds
SSME thrust level -%RPL
0 100
SSMEs Verified at Thrust
Figure 12-6. Shuttle Liftoff Timing
In Figure 12-7 one can see the stored load and the minimum energy release point followed by the large dynamic response discussed above. The plot is for the ET to SRB strut load response. The difference in prediction versus flight is explained by the fact that the prediction was for 3-sigma conditions while the flight was near nominal conditions.
Stored Load
Dynamic Response
Liftoff
ET -SRB Strut P10 Load (K-LB-F)
Time from Start Signal