There are a number of stressing environments that payloads are subjected to during a rocket launch, and g-loads, often referred to as quasi-static loads/accelerations, are just one of them, and often not the most stressing. Other key environments include:
- random vibration
These environments differ greatly between launch vehicles, and are typically published in Payload User's Guides as general guidance and provided to a payload during the launch systems integration process, where compatibility of the payload and rocket is established.
Typically a combination of test and analysis is used to ensure the payload can survive the environments and it can be one of the most expensive and time-consuming parts of developing spacecraft.
Quasi-static loads can be thought of as constant accelerations. They are a result of the bulk acceleration of the entire rocket as well as the contribution of any random vibration significantly below the natural frequency of the payload, where it will respond as a rigid body. QS loads are typically specified as axial (along the central axis of the vehicle) and lateral (perpendicular to axial). Axial loads tend to be highest at stage burnout, just prior to separation events. Several g's is typically, above 10 is not uncommon. Vehicles are designed with factors of safety, so a vehicle may need to withstand, for example, above 20-g of acceleration. Lateral g's are typically driven by aerodynamic loading or other vehicle maneuvers. They tend to be less (horizontally launched and spin-stabilized rockets are a notable exception because they have notably different trajectories and/or body dynamics than vertically launched rockets).
Random vibration is typically depicted as as a power spectral density function, which is a visualization of vibrational energy in frequency space (e.g. an fast Fourier transform of the time history random vibration), and as a gRMS (g, root-mean-square) value.
These are often the most stressing environments because amplification at specific natural frequencies can result in the effective load at certain locations in the vehicle to be many tens of g's equivalent (I've seen things as high at 70 or 80 g's easily).
Shock is also depicted as a PSD, but at much higher frequencies than RV. Shock loads are a result of events like stage or fairing separation and is caused by things like explosive bolts. It results in a high-frequency ring through the system. Structures tend to be less susceptible to shock than RV or QS loads, but they can be murder on mechanisms, electronics, and finely calibrated instruments (including optics).
Acoustic is sound loading. It tends to put energy into large area, lightweight structures like panels or antennas. It excites these structures which feed random vibration into the rest of the structure.
Thermal environments can be extreme because of heating on the fairing during ascent, which then radiates the payload.