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I am writing a report on CubeSat construction, and while reading Cal Poly's CDS I encountered the concept of ascent venting (or more specifically, the constraint that 'ascent venting per ventable volume/area [must be] less than 50.8 meters (2000 inches)' - section 2.1.9 of the CDS).

What is ascent venting? And how does 'ascent venting per ventable volume/area' have a dimension of length?

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Ascent venting is the venting of gases during ascent. Spacecraft are typically assembled and integrated with the launch vehicle on Earth, at standard atmospheric pressures. Therefore, any gas that is trapped inside the spacecraft will be at Earth ambient pressure.

During the ascent, the pressure environment in the payload fairing very quickly changes from Earth ambient pressure to near vacuum within minutes. Any gases that are trapped inside the spacecraft must be vented in order to avoid damage to the spacecraft.

However, the vented gases need to go somewhere: they will thus increase the pressure in the payload fairing (or whatever container the spacecraft is contained in, e.g., a space tug or satellite dispenser). This container itself only has limited capabilities to vent the gases. Therefore, the venting needs to be limited.

The limit is set based on the surface area of the spacecraft, i.e., the spacecraft is allowed to vent a certain volume of gas per surface area of the spacecraft.

Volume divided by area is length.

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  • $\begingroup$ Ah yes, I understand now - however, I still don't get the dimensions. If the quantity is ascent venting per ventable volume/area, shouldn't it be in units of $m^{-1}$? $\endgroup$
    – moooligan
    Commented Nov 22, 2023 at 23:44
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    $\begingroup$ In this case, "per" means "based on". The venting that occurs during ascent is based on volume/area which gives the answer in units of length which must be smaller than 50.8 m. $\endgroup$
    – JohnHoltz
    Commented Nov 23, 2023 at 0:50
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    $\begingroup$ It's not the surface area of the spacecraft, but the surface area of the openings between the interior and exterior. For a 1U CubSat, enclosed volume at most 1000 cm^3, an area of 0.2 cm^2 suffices. No a difficult requirement: few are so thoroughly enclosed. $\endgroup$
    – John Doty
    Commented Nov 24, 2023 at 18:51
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    $\begingroup$ I'm not quite following. Both the fairing cavity and the satellite start out at 1 atm. During ascent the outside pressure decreases, so the fairing vents and the pressure inside the fairing cavity decreases. This causes the satellite to vent. Pressure decreases in the satellite as it tries to match the cavity pressure. It might have trouble keeping up with the fairing's venting, which could cause a problem for the satellite, thus fairing venting is purposely limited. But if the satellite vents easier than the fairing does, then the satellite pressure will always equal the fairing pressure. $\endgroup$
    – Steve Pemberton
    Commented Nov 24, 2023 at 19:27
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    $\begingroup$ @moooligan - that's what I am assuming also. If the fairing cavity pressure drops faster than the satellite is able to vent, then the pressure differential between the satellite internal pressure and the cavity pressure increases, creating sort of a ballooning effect inside the satellite that can overstress the structure. However if the satellite vents as well as or better than the fairing, then there is no pressure differential. Or if the satellite vents at least well enough to keep the differential within limits. So I'm still not understanding why satellite venting is being constrained. $\endgroup$
    – Steve Pemberton
    Commented Nov 28, 2023 at 15:18
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Answer: PLF Volume / Vent Area = linear

“Volume” is the payload fairing (PLF) – volume of payload

“Area” is the area of the vent(s)

For instance,

  • if the PLF is 2m diameter and 2m long, its volume is just over 6m^3
  • and the vent(s) have a combined area of 0.1 m^2
  • then the ratio would be 60m

A previous answer stated the “area” was the “surface area of the spacecraft” but it makes more sense that it would be vent area.

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