Analysis of Vibratory Data Collected by the Space Acceleration Measurement System (SAMS) on Blue Origin, June 19, 2016

Blue Origin's New Shepard is a suborbital reusable launch vehicle. After liftoff, the vehicle ascends vertically for approximately 150 s before main engine cutoff in the upper atmosphere. Several seconds after engine cutoff, the capsule separates from the booster and is pushed away by mechanical springs. From this point, the capsule and booster coast and reenter separately. The vehicles experience microgravity for a period of about 3 min before returning to Earth. The booster maneuvers back to a landing pad, restarts its engine, and deploys landing gear to perform a rocket-powered vertical landing. The capsule reenters and lands under a three-parachute canopy with the assistance of a retro-thrust system to reduce landing loads. The entire flight typically occurs autonomously, without flight or ground crew intervention.

On January 11, 2016, the Space Acceleration Measurement System (SAMS) team was approached by NASA Headquarters (HQ) regarding flying an accelerometry unit on an upcoming flight on Blue Origin's New Shepard vehicle. Given the flight history of the Space Acceleration Measurement System as a microgravity measurement system, NASA HQ sought to use SAMS to understand the microgravity vibratory environment available on this suborbital vehicle.

The SAMS is primarily used for precision accelerometry aboard the International Space Station (ISS). This system was designed to characterize the vibratory regime, and therefore cannot measure accurately low-frequency, low-magnitude (residual) acceleration. Multiple triaxial sensor heads located throughout the ISS send acceleration measurements to a centralized control unit, which is used for configuration control, commanding, and data downlink. These data are continuously streamed to the ground. The Blue Origin flight configuration used the same SAMS sensor head without a centralized control unit, allowing for the relatively short turn-around needed to meet mission readiness milestones.

Logistically, the Blue Origin flight most closely resembled previous SAMS missions on parabolic aircraft, sounding rockets, and drop towers dating back to 1991. This flight on Blue Origin's New Shepard was the second time the SAMS had been used on a commercial vehicle, the first being its use on the Zero-G aircraft in 2008.

There can be widely differing objectives and requirements with regard to the platform on which a given payload's near-weightless investigations should be conducted. This paper is not intended as a comprehensive analysis to cover all of those conditions. Instead, we aim here to present a clear and incisive accounting of some of the key features and characteristics of the vibratory environment as measured by the SAMS on Blue Origin's New Shepard vehicle. We do so by describing three different periods during the flight as measured by the SAMS sensor in a representative payload locker location. The most intuitive period to describe is the Coast Period, between capsule stabilization and the start of reentry. This phase is quite well defined and delimited by the Blue Origin flight log's Coast Start and Coast End events, providing researchers with convenient trigger points. On the other hand, the Post-Separation Period and the microgravity period were defined more pragmatically on the basis of the flight data. A synopsis of all three periods is shown with three subplots of the SAMS sensor X-, Y-, and Z-axis measurements in Figure 2.

Figure 2

For a spectral overview of the vibratory environment during the New Shepard flight on June 19, 2016, we computed and plotted a spectrogram (Figure 3). This three-dimensional plot helps to show boundaries, patterns, and structure in time along the horizontal axis and frequency along the vertical axis. The third dimension, represented by color, only crudely shows magnitudes in the form of color-mapped PSD values. This spectrogram combines the three independent SAMS measurement axes via root-sum-squares of the per-axis PSDs.

Figure 3

We focused on three main periods most relevant to low-g researchers during this flight of New Shepard to give users an array of results showing a tradeoff between, primarily, the duration of quiescence and acceleration magnitude levels. Notably, the period of free fall was reduced on this flight compared to other launches of the system because of RCS firings to compensate for vehicle dynamics. The root cause has been identified and addressed, and future flight designs are expected to increase the duration of quietest flight and improve environments throughout free fall. Per the New Shepard Payload User's Guide (Blue Origin, 2017), on a nominal mission, nearly all RCS thruster firings will happen during the 10 to 15 s immediately following separation as the capsule nulls rates in pitch, yaw, and roll axes induced by any asymmetries in the mechanical separation spring force or residual aerodynamic disturbances. After this period, the frequency of RCS thruster firings is expected to drop significantly. After the start of the Coast Phase, approximately 35 s after separation, the RCS thrusters are not expected to fire again until reentry, when the sensed acceleration due to aerodynamic drag will be far greater than any RCS-induced acceleration.

A network interface and software supplied to Blue Origin Payload Controller users allows for real-time monitoring of vehicle mission data such as the Coast Start/End events mentioned earlier. This information can be used to trigger scripted actions based on these data. This can be leveraged by payloads to control various aspects of their operations using these triggered actions.

The longest quiescent period observed was the Post-Separation Period, which on this flight lasted about 2.8 min. The start of this period was selected to be 1 s after separation to allow recoil/ringing to damp out. At the reentry end of the analysis window, an alternative end point for this period might be when the X-axis acceleration (perhaps a sliding mean value) starts diverging from the Microgravity Period's mean value. Researchers interested in ultra-quiet environments to support their science goals should consider isolating payloads against vibrations greater than 50 Hz to significantly reduce g-jitter across the Post-Separation Period. SAMS measurements on this flight suggest an attenuation factor approaching an order of magnitude when using 50 Hz vibration isolation. Low-cost, passive isolation measures such as rubber grommets can provide attenuation at the higher-frequency portion of the vibratory acceleration environment, although, this will come at the cost of low-frequency amplification as passive isolators move vibration energy from higher frequencies to lower frequencies. Structural modeling or vibration lab testing could then be used to further hone the specifications for passive isolation hardware.

The Coast Period was similar in character and duration to the Post-Separation Period, with the only potential advantage for researchers being its better-defined Coast Start and Coast End event markers available for triggering at the cost of a somewhat shorter duration.

While the Microgravity Period was only 46.8 s, the vibratory acceleration environment in this span provides a much quieter vibratory environment, with acceleration magnitudes on all three measurement axes roughly an order of magnitude smaller. Blue Origin also predicts the duration of this period will improve in future flights.

When we consider a widening bandwidth in the form of a cumulative RMS acceleration versus frequency plot, we can potentially address various investigators’ concerns across the entire bandwidth measured by the SAMS sensor (Figure 4). When this is plotted, we see that the Post-Separation Period closely resembles the Coast Period, albeit for a longer duration. Furthermore, we see that the Microgravity Period stays well below 1 mg up to the sensor cutoff frequency of approximately 200 Hz. Comparing RMS acceleration levels below 10 Hz, see Figure 5, shows that the best New Shepard vibratory environment is superior to the vibratory environments of the complete test periods of parabolic flight, traditional sounding rockets, ISS, and even drop towers. Note too that the entire Coast Period in New Shepard is shown to be comparable to that of traditional more expensive single-use sounding rockets. Further study of aerodynamic drag data would round out characterization of the micro-g environment on New Shepard and other similar vehicles.

Figure 4

Figure 5