[cpgxiii]

>> Are we talking HTP+Kerosene or UDMH+N2O4 here?

> I’m thinking kerosene or even methane. UDMH is a toxic mess.

Methane is absolutely out because of pressures/temperatures involved in keeping it liquid for any useful length of time. It would require both significant on-site gas storage as well as a gas compression plant to produce liquid methane on demand, both considerable additional targets (and very fragile ones at that). Kerosene + additives (e.g. RP-1 and JP-8) is the only really viable storable "friendly" hydrocarbon.

In practice, Hydrazine/UDMH + N2O4 is going to be much easier to handle safely than you think (Hydrazine itself is probably safer to handle than Ammonia, and there's a lot of Ammonia handled out there). You fill and seal the tanks at assembly, and outside of rare leaks the handling is fairly benign. Most notably, basically every major military has, as some point in the 20th century, done so - including aboard USN carriers, famously a very skeptical customer from a handling standpoint. Even today, every single F-16 (about 2000 in active service with about 25 operators around the globe) carries Hydrazine in the onboard EPU, and those operators are prepared to service (and handle leaks) safely.

While they aren't pleasant chemicals by any stretch, Hydrazine/UDMH + N2O4, by virtue of its immediate hypergolic behavior, is actually quite well-behaved from a fire/explosion safety standpoint, which is practically a larger concern than toxic fumes (consider how many other things in and around a combat environment can produce toxic gasses). Hydrazine/UDMH + N2O4 ignite promptly on mixing, and thus don't accumulate in an explosive mixture to be ignited later like alternative fuels and oxidizers.

[nixon_why69]

I feel like the whole point is what's most economical/practical. Liquid engines are better than solids on specific impulse, throttling/relighting, this is great if you're going to orbit but solids are better for transport/storage and the downsides are minimal for interceptors or boost-stage for ramjets.

What liquid situation changes that? What liquid would be more economical? My instinct is towards minimizing operations outside of the US, keep it simple stupid and just go solid, but could it actually be reasonable to have, say, sealed containers of fuel/oxidizer of whatever type inside the missile and you "pull the pin" to uncap them and start mixing in the ignition chamber?

[JumpCrisscross]

> could it actually be reasonable to have, say, sealed containers of fuel/oxidizer of whatever type inside the missile and you "pull the pin" to uncap them and start mixing in the ignition chamber?

No. The economics really shine if you can mass produce dry engines and then fuel them on site, ideally with locally sourced or even on-based manufactured oxidizer (if you’re going LOX).

Barring that, though, not being limited by AP would make the last comment’s hydrazine path acceptable.