The FAA usual procedure is "prove to us that it's safe". You can point to other certified craft using the same or similar technologies or systems, and say "it works there, so we'll be okay". This is what most of the aircraft manufacturers do. For novel systems you have to demonstrate for them that it will work as intended and safely. How you do that is negotiated with them. Plenty of time on flight tests is likely required. (Though the rocket guys mostly do this by showing that when it does fail, it can't possibly hit anything interesting, due to flight path and abort limits.)

FAA generally won't force you to do it a specific way. Which is both good and bad from a development standpoint. You might take a look "at Airworthiness Certification of Unmanned Aircraft Systems and Optionally Piloted Aircraft":

http://www.faa.gov/documentLibrary/media/Order/8130.34B.pdf

Thanks that document is somewhat helpful.

> FAA generally won't force you to do it a specific way.

I don't know if they "force" but practically you cannot certify commercial aircraft without adhering to DO-178 and a bunch of other standards.

So my question is what mandates or recommends use of those standards and will the same apply to UAS?

edit: Found it: http://www.faa.gov/documentLibrary/media/Advisory_Circular/A...

Turns out you are correct, using DO-178B is optional but I can't imagine anyone implementing alternative means given the complexity of DO-178.

The question that remains for me is do UASes have or will have the same or similar airworthiness requirements as other aircraft?