The Aerospike drag reduction mechanism was designed and developed for use on the Trident I submarine launched ballistic missile. This mechanism encounters a unique combination of environments necessitating unique design solutions to ensure satisfactory operation over its design life. The development of the Aerospike is reviewed emphasizing the unique and interesting problems encountered and their solutions

Richter, B. J.

Waterman, M. D.

English

NASA Technical Reports Server

DEVELOPMENT OF THE TRIDENT I 
AERODYNAMIC SPIKE MECHANISM 
By Mark D. Waterman and B. J . Richter 
Lockheed Missi les  and Space Company 
ABSTRACT 
The Aerospike drag reduction mechanism was designed and developed for use on 
the Trident I submarine launched ballistic missile. This mechanism encounters a unique 
combination of environments necessitating unique design solutions to ensure satisfactory 
operation over its design life. This paper traces the development of the Aerospike 
emphasizing the unique and interesting problems encountered and their solutions. 
INTRODUCTION 
The Aerodynamic Spike (Aerospike) is a deployable drag reducing mechanism 
stowed within the nose fairing of the Trident I submarine launched ballistic missile. This 
mechanism maximizes missile performance within the limited envelope available by trans- 
.forming the aerodynamic characteristics of a blunt, space efficient nose fairing into a 
more streamlined shape. 
The Aerospike mechanism is comprised of three subsystems which perform three 
separate and distinct functions: The mechanical inertial initiator (I2) discriminates the 
correct missile flight acceleration and initiates a gas generator; the gas generator pro- 
vides the energy for deployment, and boom segments which once deployed and locked, 
establish a region of separated flow which provides the induced drag reduction. The 
Aerospike is required to be maintenance- and service-free over a ten year design life. 
Initiation and Deployment of the Aerospike occurs just after the missile is launched; 
after the missile leaves the atmosphere the Aerospike is jettisoned along with the nose 
fairing approximately 2 minutes after missile launch. The major components of the 
Aerospike are shown in Figure 1. 
The Aerospike is now in production after a four year development program culmin- 
ated by eighteen consecutive successful flight tests. 
ACCELEWTION SENSING 
The first task of the Aerospike is to detect the correct flight acceleration profile 
and to ignite the gas generator at the moper time. An all-mechanical approach was 
selected for this function a s  the Aerospike is completely isolated from the missile elec- 
trical system due to its position in the nose fairing, which is subject to numerous 
installation cycles. The inertial initiator must also remain safe when subjected to a 
variety of transportation/handling environments of short term acceleration, vibration, 
39 
https://ntrs.nasa.gov/search.jsp?R=19790014372 2020-03-21T23:46:56+00:00Z
shock, and temperature/humidity extremes as well as the long term low level vibration 
encountered in the submarine launch tube. The inertial initiator must possess good 
aging characteristics a s  it is not tested or  serviced once it is installed in the Aerospike; 
it must be flight ready over its 10 year design life. 
The components of the inertial initiator a r e  shown in Figure 2. A spring mass 
system senses missile acceleration and releases a timer once the correct acceleration 
level has been reached, Torque generated by a watch-like mainspring drives a timer 
escapement, After the correct time interval has been achieved, a spring-driven firing 
pin is released to ignite the gas generator. E the acceleration is removed before 
firing pin release, the unit will reset in less than one second. Figure 3 deiineates the 
"must fire'' regime of the inertial initiator; the inertial initiator will not fire when sub- 
jected to any acceleration outside of the "must fire" environment. 
Correct operation of this mechanism is ensured by a series of in-process tests 
and a final acceptance test in which the "must fire" and "no fire'! limit are checked. 
During development testing it was discovered that the unit would cease to operate properly 
after only three months of storage. The problem was traced to the lubrication system 
used and to the low design torque in the mainspring. Various design changes were in- 
corporated to increase the torque margin of the 12 so that the mit was much less 
susceptible to friction variations, and a change in the lubrication method was made. 
The lubrication system originally used was a combination of the MIL-L-6085 oil 
for the linear and ball bearings, a dry film lubricant on the timer, and a combination of 
dry film lubricant and grease for the firing pin release mechanism. Examination o€ the 
aged units revealed that this system was unacceptable, mainly due to oil migration which 
'made the unit susceptible to contamination and interaction of the oil and dry film lubricant, 
The dry film lUbricant used in the timer also was a problem due to process variations and 
the extremely tight tolerances of the timer gears. 
The aging problem was solved by reducing the amount of oil to a minimum by 
centrifuging the bearings, elimination of the grease on the firing pin release mechanism, 
and a change in the timer lubrication to a Lockheed Missiles & Space Co. (LMSC) developed 
process of applying dry moly disulfide powder to the part to be lubricated (Molykote 
This method was chosen after testing showed it to be superior in terms of friction reduc- 
tion, aging, and ability to withstand repetitive cycling (service life). Table 1 shows the 
results of the test. No inertial initiator timing anomalies have occurred in the two years 
after the new lubrication system was incorporated. 
Table 1 e Inertial initiator timer lubrication evaluation 
Lubricant Friction Aging Service Life 
Low 
Dry Film 
o Everlube 620C 
o Electrofilm 77s Low 
No Lubrication High 
Molykote Z Low 
Moderate Moderate 
Moderate Good 
Good Poor 
Good Good 
40 
DEPLOYMENT 
The Aerospike is deployed shortly after the missile has reached a sustained 
acceleration and the deploying boom sections are subjected to a combination of aero- 
dynamic loads , acceleration, and vibratory loadings; in addition to deploying under these 
flight loads, the Aerospike is ground tested on a centrifuge which imparts a large side 
load due to Coriolis effects. Once deployed, the Aerospike must meet stringent dimen- 
sional and structural requirements. 
Deployment is accomplished by internal pressures created by the solid propellant 
gas generator, which acts on the tube sections to overcome static and dynamic friction 
and external environments. The momentum and forces imparted by the gas force the 
joint/locking mechanism into the engaged position. Gas pressure initially builds up 
rapidly until a set of retention bolts fracture, releasing the tube sections. A s  the tube 
sections begin moving, the rapidly expanding volume within the Aerospike causes the 
pressure to decay; after the Aerospike is fully extended, the gas generator continues to 
build up pressure to ensure that all joints are in the locked position. Under large external 
loadings the deployment process is slowed sufficiently to require joint engagement due to 
internal pressure forces alone Two related problems occurred during early deployment 
testing: Under ground test conditions the deployment was taking place too rapidly, causing 
joint structural damage and rebound, and the gas generator was extinguished by impact 
induced acoustic resonance, These two problems were solved by a complete redesign of 
the gas generator and modifications to the joint/locking mechanism ~ 
. The gas generator extinguishment problem was especially perplexing because the 
N-5 propellant is used throughout the industry (2.75 inch rocket) and was thought to be 
well characterized; additionally, the extinguishment phenomena was temperature related, 
occurring only at low ambient temperatures. The extinguishment problem was traced to 
acoustic resonances induced by the shocks produced by the joints impacting into place. 
This resonance caused pressure excursions outside the region of stable burning and 
extinguishments occurred. Although extinguishment occurred before final extension, the 
tube sections had sufficient momentum to complete the deployment. Two methods of 
solutions were pursued: The first was to alleviate the impact shocks through addition of 
an energy absorbing device in each joint, o r  to slow down the deployment by reducing 
gas generator flow rate, The second method was to mechanically break up the resonance 
in the gas generator through grain design and resonance rods. 
Although a number of solutions could eliminate the extinguishment, complete 
elimination of the pressure fluctuations at joint impact could not be achieved and it was 
decided that the N-5 propellant was unsuited to this application. A low metalized pro- 
pellant, Arcite 386 M with 2% aluminum for resonance suppression, was selected and 
the grain geometry was tailored to deploy the Aerospike more slowly thereby reducing 
the shock induced resonance as well as the related joint impact damage. 
The second major problem was that the deployment was  too rapid, causing damage 
to the joint/locking mechanism and rebound past the locking area. The most severe 
damage occurred at the innermost joint which is the last joint to impact and carries the 
greatest kinetic energy; internal gas pressure was  insufficient to re-engage the lock 
mechanism on this joint which has the smallest cross sectional area for the gas to react 
against. The solution to this problem took three forms; the strength of the joint/locking 
41 
mechanism was increased to provide more resistance to impact damage and the 
locking €inger strength was  increased to reduce the chances of rebound. The deploy- 
ment was slowed by decreasing the initial gas generator flow rate and reducing the 
strength o€ the retention bolts by SO%, and, finally, the propellant loading was in- 
creased by 7Q% to increase peak pressure to ensure pressure lockup. 
Incorporation of the gas generator and the modifications have proven completely 
successful in solving the deployment problems. Aerospikes have since been tested 
under a large variety of conditions ranging from static gromd tests to missile flight 
test without an anomaly. Figure 4 compares the two deployments and Table 2 gives a 
history of Aerospike deployments. 
Table 2, Aerospike deployment 
Peak Peak 
Deploy Velocity Pressure Joint 
Test # Time, ms  in/sec psi Lock 
1 75 - 240 No 
2 82 - 175 No 
3 92 960 17 7 NO 
4 138 5 60 470 Yes 
5 118 695 500 Yes 
6 135 540 575 Yes 
7 136 550 550 Yes 
8 137 - 500 Yes 
9 123 - 500 Yes 
10 120 - - Yes 
11 138 - Yes - - Yes 12-18 - 
Note: 1. Changes introduced at  Test 4 
2. Tests 12-18 were flight tests 
STRUCTURAL 
The Aerospike becomes a part of the missile structure after it has been de- 
ployed and locked and it is subjected to severe environments during powered missile 
flight. The primary requirement for the Aerospike is to maintain a first bending 
mode frequency of 25 Hz to assure overall missile stability. During its two minutes 
of operation, the Aerospike is subjected to a combination of aerodynamic heating 
loads that raise parts of the aerospike above 1000°F, compressive axial aerodynamic 
loads of up to 2380 pounds, and vibratory loads that results in tip accelerations of up 
to 60 G in the first lateral bending moda. 
42 
The Aerospike carries these loads through its joint/locking mechanism as 
shown in Figure 5.  This mechanism, the primary determinant of natural frequency, 
carries the lateral bending loads through the two interference lands (this loading 
mechanism has been described in a previous paper, "Structural Evaluation of Deploy- 
able Aerodynamic Spike Booms, '' by B * J * Richter, 9th Aerospace Mechanisms 
Symposium), while the locking fingers provide the required axial strength. These 
fingers had to be redesigned after an early test revealed a design deficiency. The 
locking fingers were fastened to the tube section by pins; the early design had only 
one pin for every four lock fingers which allowed the fingers to rotate about the re- 
taining pins and slip out of the lock groove. By doubling the number of pins, this 
rotation was minimized and the axial capability was doubled to about 16,000 pounds - 
more than five times the flight requirement. Other changes, detailed in Figure 5,  
also helped increase the axial capability and provided additional capability to withstand 
the deployment shock and subsequent rebound. 
The Aerospike posed a challenge in testing the structural capability because 
each of the major environments -heat, vibration, and axial loads - affects the natural 
frequency and axial strength of the Aerospike, the prime determinants of Aheraspike 
acceptability. A combined environments test was devised to apply simutaneously 
simulated flight loads to the Aerospike. A shaker is used to apply a simultaneous 
combination of sine and random vibration, radiant heat lamps provide simulated aero- 
dynamic heating, and an air cylinder/cable arrangement applies the simulated aero- 
dynamic compressive load. The Aerospike is subjected to vibration in both the axial 
and lateral directions, although not simultaneously. Figure 6 shows the results of a 
typical lateral combined environments test; natural frequency can be seen to decrease 
as both the vibration level is increased and the skin temperature increases. Data 
from wind tunnel tests and two instrumented flight tests have shown the combined 
environments test to be a reasonable simulation of flight environments and the 
Aerospike has been proven to have structural capability far exceeding its flight loadings 
due to being designed for natural frequency and heat loadings at the end of flight. 
CONCLUDING REMARKS 
The Aerospike has proven to be an extremely successful method of improving the 
range of the Trident I at minimum cost. The unique nature of the Aerospike highlighted 
the importance of overall system design in mechanisms of this type. 
43 
/ 5 v) w 2 0 E 0 0 m w v) 
44 
Figure 2 Aerospike inertial Initiator 
MINIMUM TIME FOR BOOM MOTION 
8 4.0 
U 
/ 
MAXIMUM ACCELERATION 
MAXIMUM DEPLOYMENT TIME 
MINIMUM ACCELERATION 
" MUST FIRE" 
REGION 
01 I I I I I I I I 
0 2 4 6  8 IO 12 14 16 
TIME (SEC) 
Figure 3 Aerospike Acceleration Initiation and Deployment Requirement 
45 
GAS GENERATOR 
. 1  .2 .4 .5 .6 .7 
OLD DESIGN N-5 GAS GENERATOR 
DEPLOYMENT 
PRESSURE FLUCTUATION 
AT JOINT IMPACT 
. 1  .2 . 3  .4 .5 .6 .7 
TIME - sec 
Figure 4 Aerospike Deployment 
46 
OLD 
DESIGN 
TUBE STRENGTH 
HOOK HEIGHT 
GROOVE RAKE 
- FINGER STRENGTH 
NEW 
DESIGN 
Joint/Loc k Design Improvements 
17 INTERFERENCE FITS 
JOINT DESIGN 
LATERAL 
LOAD 
\-LOAD AXIAL 
\ LOCK FINGERS 
Figure 5 Joint/Lock Mechanism 
47 
48 


Development of the Trident 1 aerodynamic saike mechanism

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server