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Friday, October 3, 2014

Cargo to Crew

This is the ULA Delta IV Heavy that will launch the Orion spacecraft on its uncrewed maiden voyage this December.

This raises the issue of the difference between rockets for cargo flights into low earth orbit versus crewed flights to the same destinations, and specifically why we can't just use the same rockets for what is nearly the same job. It turns out that modifying an existing cargo rocket for crewed flight is a fairly involved exercise.

The reason is safety, mostly. Rockets have an unhappy propensity for exploding, so any crewed vehicle must be able to safely escape an explosion. Part of the solution is a Launch Escape System, which is simply a small rocket on top of the spacecraft to pull it away from the main engines and fuel tanks in the event of a catastrophic failure. Here is an example, with an Apollo space capsule.

Cargo rockets take the shortest, fastest and simplest (hence closest to vertical) route to orbit that their engines allow, with little consideration for the massive changes in g-forces that the cargo is subjected to. Humans need a slower and gentler ascent. Also, cargo rockets maximize the distance they coast upward unpowered between stages, and by launching essentially vertically the exhaust remains below them. Neither of these are problematical so long as the launch proceeds normally. However, if the LES needed to deploy at certain parts of the launch trajectory (such as at the top of one stage's coasting before the stage above it ignited, or the first few seconds of a launch when the huge exhaust fireball is below the rocket), the LES would be unable to safely extract the spacecraft. These time intervals in which the LES cannot launch the crew safely away from an exploding rocket are called "black zones." Every crewed rocket has some, but the goal is to minimize both the amount and duration of these. One method for accomplishing this, for example, is to launch the rocket in a lower parabola so that for most of its flight to orbit, the spacecraft does not have its own exhaust gasses below it.

The currently used Delta IV, Atlas V and SpaceX Falcon rockets are all presently being modified for commercial crewed flights. Each of these rockets will be discussed here in greater detail, as this series progresses.

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