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Why is Orbital Science's Minotaur V rocket a five stage design? I am aware this is derived from an ICBM design, but why would anyone want a five stage design for launching non-lethal payloads? It seems to me that additional stages only open more opportunity for problems and thus mission failure.

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It's always easier to create a new launcher by making a small modification to a reliable existing launcher. Small solid rocket stages are relatively simple and reliable, so building up a launcher from solids is less risky than the stage count might indicate. Small solid stages are also extremely cheap.

The Peacekeeper ICBM is a three-stage suborbital rocket. By replacing its warhead payload with a fourth stage which could insert a payload into low Earth orbit, the Minotaur IV was created. Two different solid rocket motors were used for the upper stage, the Orion 38 and the Star 48. The Star 48 in particular has a very long operational history going back to the 1980s; Orion 38 was 20 years old when Minotaur IV was first flown, so both of these are proven stages. Minotaur IV has 5 successful launches under its belt -- not a long track record, but validating the principle.

Minotaur V extended Minotaur IV by adding a Star 37 to propel a payload out of low Earth orbit to GTO or to the moon. The Star 37 has an even longer history than the Star 48, so again this is a low-risk modification to the launcher.

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