Pyroshock Analysis
Pyroshock: Theoretical Foundations
What is Pyroshock?
Professor, what is pyroshock?
It is a high-frequency shock generated by the operation of pyrotechnic devices. It occurs during rocket stage separation, satellite separation, bolt cutters, etc.
Characteristics:
- Extremely high frequency components — 100 Hz to 100 kHz
- Short duration — Several ms or less
- Very high acceleration — Thousands to tens of thousands of G
- Little structural damage but destroys electronic equipment — Relays, crystal oscillators, HDDs, etc.
Tens of thousands of G acceleration! The structure doesn't break, but electronic components do?
Because pyroshock is dominated by high-frequency components, the structural response itself (low-frequency deflection) is small. However, electronic components are sensitive to high frequencies, leading to solder joint detachment or relay malfunctions.
SRS (Shock Response Spectrum)
SRS (Shock Response Spectrum) is used to evaluate pyroshock. It is a plot of the maximum response of single-degree-of-freedom systems at each natural frequency.
So it lists the maximum response at each frequency.
Using SRS, we determine "whether the response in this frequency band exceeds the allowable value." Allowable SRS values are specified in NASA-STD-7003.
Analysis with FEM
FEM for pyroshock requires accurately tracking high frequencies (on the order of kHz), necessitating very fine meshes and small $\Delta t$.
- Explicit FEM — $\Delta t$ is automatically small. Suitable for high frequencies
- SEA (Statistical Energy Analysis) — Statistical response for high frequencies
- FEM-SEA Hybrid — Low-frequency FEM + High-frequency SEA
Summary
Key Points:
- High-frequency shock (100 Hz to 100 kHz) — Generated by pyrotechnic device operation
- Evaluated with SRS (Shock Response Spectrum) — NASA-STD-7003
- Electronic components are the primary damage target — Structure usually remains intact
- FEM requires high-frequency meshing — Consider hybrid with SEA
Pyrotechnic Shock is the Greatest Challenge for Spacecraft
When a satellite's separation pyrotechnic device (bolt cutter/explosive bolt) operates, a shock acceleration of tens of thousands of G occurs within a few μs. This pyrotechnic shock is evaluated by SRS (Shock Response Spectrum), and the harsh environment exceeding 10,000 G at 2000 Hz is a major cause of electronic equipment destruction. It became a serious problem during the Apollo program in the 1960s-70s and was systematized as NASA HDBK-7005.
Computational Methods for Pyroshock
Pyroshock FEM
Mesh requirement to resolve high frequencies: $\lambda_{min} / 6$ or less. For 10 kHz elastic wave (steel: $c = 5000$ m/s):
If 80 mm elements are fine, then it's not that detailed.
That's true up to 10 kHz, but if 50 kHz is needed, then $h < 17$ mm. For 100 kHz, $h < 8$ mm. Covering the entire pyroshock frequency range with FEM alone incurs high computational cost.
SEA (Statistical Energy Analysis)
At high frequencies (above 1 kHz), modal density is high, making it less meaningful to track each discrete FEM mode individually. SEA is a method that statistically calculates the average energy flow between subsystems, optimal for high frequencies.
Solver
| Tool | Method |
|---|---|
| LS-DYNA | Explicit FEM. Up to mid-frequencies |
| VA One (ESI) | FEM-SEA Hybrid. Standard for pyroshock |
| Wave6 (Free Field Tech) | FEM-SEA Hybrid |
Summary
- For high frequencies, use SEA or FEM-SEA Hybrid — FEM alone has limitations
- VA One is the industry standard for pyroshock analysis
- Evaluate results with SRS — Compliant with NASA-STD-7003
SRS was conceived by Shepard in 1932
The Shock Response Spectrum (SRS) concept was proposed by Charles Shepard in 1932 for earthquake motion evaluation. It plots the maximum response of single-degree-of-freedom systems with various natural frequencies to a shock input as a function of frequency. Test and analysis methods are specified in MIL-STD-810H Method 516.8 and ECSS-E-HB-32-25A. Standard settings for SRS calculation are 1/12 octave frequency steps with Q (quality factor) = 10.
Pyroshock in Practice
Pyroshock in Practice
In spacecraft equipment design, evaluate the integrity of electronic components under pyroshock environments.
Practical Checklist
- [ ] Are the location and type of shock source defined?
- [ ] Is the SRS within the allowable limits of standards (e.g., NASA-STD-7003)?
- [ ] Have you compared with the shock resistance specifications of electronic components?
- [ ] Does the FEM mesh resolve the frequency of interest?
- [ ] Are high-frequency bands complemented by SEA?
- [ ] Is damping set? (Damping at joints is important)
How JAXA Satellites are Protected by SRS Analysis
For JAXA's "Daichi 2" (ALOS-2, launched 2014), the satellite separation shock SRS environment from the H-IIA rocket was pre-evaluated using modal superposition response analysis with MSC Nastran. Simulation confirmed that the satellite's received shock environment (SRS) fell within component test specification values, and this was used to set vibration test input conditions. The industry acceptance criterion for FEM prediction of pyrotechnic shock is agreement within ±6 dB of actual measurements.
Pyroshock: Software & Solver Comparison
Pyroshock Tools
- VA One (ESI) — Industry standard for FEM-SEA hybrid analysis
- LS-DYNA — Explicit FEM for mid-frequency analysis
- Wave6 (Free Field Tech) — FEM-SEA hybrid specialized tool
- MSC Nastran — Modal superposition and transient response
Related Topics
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