A built-in hold is a
period in a
launch countdown during which no activities are scheduled and the countdown clock is stopped. The hold serves as a
milestone
A milestone is a numbered marker placed on a route such as a road, railway, railway line, canal or border, boundary. They can indicate the distance to towns, cities, and other places or landmarks like Mileage sign, mileage signs; or they c ...
in the countdown, an opportunity for non-launch activities (such as a shift change or meal break), and a chance to perform unanticipated activities such as equipment repair.
Most importantly, a hold provides an opportunity to synchronize the pre-launch activity schedule (concluding at T−0) with the desired wall-clock time of launch (L−0). Activities might take more or less time than planned, or the launch time might be moved, e.g. due to weather.
A planned hold may be of a fixed or variable duration. Criteria for exiting the hold and restarting the countdown may be based on a fixed time, the completion of a checklist of work items, or a
go/no-go
A go/no-go test is a two-step verification process that uses two boundary conditions, or a binary classification. The test is passed only when the ''go'' condition has been met and also the ''no-go'' condition has failed.
The test gives no inform ...
decision from
mission management.
For example, space shuttle launch countdowns begin at T−43
hour
An hour (symbol: h; also abbreviated hr) is a unit of time historically reckoned as of a day and defined contemporarily as exactly 3,600 seconds ( SI). There are 60 minutes in an hour, and 24 hours in a day.
The hour was initially establis ...
s and include seven holds at T−27 hours, T−19 hours, T−11 hours, T−6 hours, T−3 hours, T−20
minute
A minute is a unit of time defined as equal to 60 seconds.
It is not a unit in the International System of Units (SI), but is accepted for use with SI. The SI symbol for minutes is min (without a dot). The prime symbol is also sometimes used i ...
s, and T−9 minutes.
These holds total about 26 hours, so the launch countdown begins at about L−69 hours.
References
NASA
Spaceflight
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