Dino Sciulli

Development and transition of low-shock spacecraft release devices

The Air Force Research Laboratory (AFRL) has been actively developing low-shock, non-pyrotechnic spacecraft release devices to mitigate problems with traditional pyrotechnic devices. Specifically, pyrotechnic devices produce high shock, contamination, and have costly handling requirements due to their hazardous nature. AFRL has provided funding for development and test of several shape memory alloy (SMA) actuated release devices. Another type of device is based on fuse link technology. Through both ground testing and on-orbit performance, these devices have been shown to reduce shock by at least an order of magnitude, while remaining comparable in size and mass to pyrotechnic devices. Flight heritage includes the U.S. Air Force MightySat I Shape Memory Alloy Release Device (SMARD) experiment that successfully fired and tested two SMA devices. The success of the first low-shock devices is expected to pave the way for numerous applications, such as picosats, large spacecraft release, and fairing and stage separation. A discussion of low-shock technology, current projects, and future potential is presented along with on-orbit test results.

Whole-spacecraft vibration isolation for broadband attenuation

Launch vehicles impart high levels of vibration to spacecraft during launch. The vibration environments are defined over several frequency bands: (1) transient vibration <80 Hz, (2) random vibration 20 to 2000 Hz, and (3) pyrotechnic shock 100 to 10000 Hz. Loads from transient vibration define spacecraft design of primary structures such as spacecraft bus, solar panel and antenna supports, instrument mounts, etc. Loads from random vibration define the design for spacecraft light structures such as antennas and solar panels, and shock loads define the design of electronic components and instruments. The spacecraft must survive the combination of all vibration environments. This requires spacecraft structures, instruments, and components to be designed to minimize vibration across a broad frequency range. Spacecraft are designed for the short launch to orbit, which is well beyond the requirements for on-orbit performance. A better choice is to reduce the magnitude of the high launch loads across all frequency bands and design smaller and less costly spacecraft. SoftRide systems are under development for the first and second OrbitaVSuborbital Program (OSP) launches and for the TaurusNTI launch. Additionally, isolation systems are being designed for larger liquid-fueled launch vehicles. This isolation system technology will greatly further the goal of better, faster, cheaper, and lighter spacecraft.

Hybrid structural/acoustic control of a subscale payload fairing

During launch, spacecraft experience severe acoustic and vibration loads. Acoustic loads are primarily transmitted through the shroud or payload fairing of the launch vehicle. In recent years, there has been a trend towards using lighter weight and extremely stiff structures such as sandwich construction and grid-stiffened composites in the manufacturing of payload fairings. While substantial weight savings can be achieved using these materials, the problem of acoustic transmission is exacerbated. For this reason, the Air Force Research Laboratory has been actively engaged in vibroacoustic research aimed at reducing the acoustic and vibration levels seen by payloads during launch. This paper presents experimental results for the simultaneous structural and acoustic cavity mode control of a sub-scale composite isogrid payload fairing structure. In this experiment, actuation is performed through the use of both an internal speaker as well as piezoceramic strain actuators located on the outer skin of the composite structure. Sensing is accomplished using a microphone as well as a piezoelectric strain sensor. The control approach presented in this paper is a decentralized frequency domain approach which makes use of a series of independent control loops. One loop uses the microphone and speaker, while additional loops use the piezoelectric sensors and actuators. The control algorithm consists of independent second-order Positive Position Feedback (PPF) controllers tuned to reduce the magnitude of each cavity mode. A PPF filter in conjunction with an extremely sharp bandpass filter is used on the structural mode of limit spillover. This approach leads to a substantial reduction in the acoustic transmission in the range of 0 - 800 Hz. Transmission coincident with the primary cavity modes of the system are reduced in magnitude by 26 and 9 dB respectively while the structural model that is responsible for the majority of transmission is reduced by approximately 7 dB.

Dynamics and Control of Slewing Active Beam

that for all cases of the D-model simulation the matrix enclosed by a broken line in Eq. (2) is almost singular, suggesting that the D model is the correct structure. On the other hand, Fig. 4b shows that the singular value ratio takes significant values in the frequency range of interest if the inner-loop gain Kpe is large enough. A close look at Fig. 4b and an analysis can point out the following: a peak exists around the short period mode natural frequency, the peak magnitude increases as the magnitude of Yph increases, and the frequency band where the singular value ratio takes significant values shrinks as the pilot remnant intensity increases.

Whole-spacecraft vibration isolation on small launch vehicles

Small launch vehicles historically provide a very rough ride to spacecraft during launch. This is particularly true of solid-fueled launch vehicles. In order for the spacecraft to survive such a trip to orbit, one of two choices must be made: (1) design all structure, payloads, and systems on the spacecraft to be strong enough to survive the high launch loads, or (2) reduce the magnitude of the high launch loads. The former is not a good choice because it typically requires additional cost, schedule, and weight. The latter is the preferred choice because it allows the focus of the spacecraft design to be primarily for on-orbit performance rather that for launch survival. Under a number of contrasts from the Air Force Research Laboratory, Space Vehicles Directorate, whole- spacecraft vibration isolation systems have been in development since 1993. This work has resulted in two whole- spacecraft isolation systems (SoftRide) that have been flown on Taurus launch vehicles, the first in February 1998 with the GFO spacecraft and the second in October 1998 with the STEX spacecraft. Both of these isolation systems were designed primarily to reduce axial dynamic responses on the spacecraft due to resonant burn excitations from the motors of the solid- fueled booster. Full coupled-loads analyses were used to predict the performance of the SoftRide systems. Using the isolation requirements derived from these analyses, hardware having the correct damping and stiffness was designed to implement the isolation system. All isolation system components were extensively tested and characterized. Typical results show 85% attenuation (i.e., only 15% of original) for the worst case resonant burn condition and 59% attenuation for a combination of static plus worst case resonant burn condition in the axial spacecraft c.g. location. No detrimental effects from the SoftRide system were observed. Limited flight data from the two flights agree with the predictions. SoftRide systems are now under development for the first and second OSP launches and for the Taurus/MTI launch. Additionally, isolation systems are being designed for larger liquid-fueled launch vehicles. This isolation system technology will greatly further the goal of better, faster, cheaper, and lighter spacecraft.

Advanced isolation design for avionics on launch vehicles

Research to create advanced vibration isolator designs and practical design techniques for Launch Vehicle (LV) manufacturers is discussed. Avionics of launch vehicles have unique requirements for isolation since many generate heat and cannot use convection cooling for dissipation. Nearly all isolation systems are ineffective thermal conductors unless expensive custom modifications are performed. The cost of a custom isolation design can rarely be justified, particularly with expendable vehicles. While viscoelastic isolators offer simplicity and affordability, such materials with high loss factors (greater than 0.25) also exhibit aggressive changes in stiffness with both temperature and frequency. Materials having new and unique formulations are introduced which have an order of magnitude higher thermal conductivity than todays materials of similar stiffness. This enables appreciable heat conduction with nominal temperature increases to isolated packages. The formulation of nearly all elastomeric vibration isolators creates heavy coupling between their loss factors and the rate of change in their storage moduli. High loss factors result in an aggressive temperature-dependent shift in the resonant frequencies of an isolated element. New compounds introduced in this paper address this limitation. A software utility has also been developed that greatly simplifies isolation design. The utility solves the equations of motion for a rigid body on flexible mounts and allows performance predictions using base vibration inputs. New progress in material technology and design techniques enables LV manufacturers to implement affordable designed vibration isolation systems on avionics and similar systems.

Hybrid launch isolation system

An innovative new hybrid isolation system has been developed to significantly increase the performance over a passive whole-spacecraft isolation design. The design builds upon the passive design and incorporates active components in parallel to the passive design. This means that if the active system fails, the passive system would be able to handle the isolation requirements. Preliminary results show that significant attenuation can occur using the hybrid isolation system over the passive isolation system. Also, it has been determined that the performance gained by the hybrid isolation system will be dependent on the stiffness of the launch vehicle. As this stiffness now becomes an important design parameter when developing a whole- spacecraft launch isolation system.

Isolation design for systems with a flexible base and equipment

Current literature has not fully explored the choice of isolator mount frequency or isolator placement for flexible systems. It is commonly suggested that isolators should be designed with a low-mount frequency. It is shown that these isolators tend to perform best in an overall sense; however, mount frequencies designed between system modes tend to have a coupling effect. That is, the lower frequencies have such a strong interaction between each other that when isolator damping is present, multiple system modes are attenuated. Also, for low-mount frequency designs, the first natural frequency can shift as much as 15.6%. For a mid-mount frequency design, the shift of the first three modes can be as high as 34.9%, 26.6%, and 11.3%, respectively. Also, when the base and system are flexible, isolator placement becomes a critical issue. There can be as much as 16% difference in the first mode for low-frequency mount design and as high as 25% for a mid-frequency mount design.