Freezing point depression is also a reason to use Aerozine-50 vs straight hydrazine. Straight hydrazine freezes at +2C (even worse than N2O4 at -11.2C) while UDMH freezes at -57C. (I don't know offhand if they form a eutectic that freezes at a temperature below either pure compound.)
Straight hydrazine also cannot be used in regeneratively cooled rocket engines (i.e, most bipropellant engines) because it would decompose.
UDMH cannot be used in monopropellant rockets, so I guess the high freezing point of straight hydrazine is one reason to switch to a more complicated bipropellant engine. On the other hand, some spacecraft with large bipropellant engines use hydrazine-fueled monopropellant thrusters for attitude control so at least one set of tanks still has to be kept warm.
U.S. military specifications for UDMH (1955) came out about than same time as IRFNA (1954). Both were developed for the primary purpose of providing stability and storability over a wide temperature range. The specific impulse of UDMH/IRFNA is not nearly as good as other fuel/oxidizer combinations, but when you’re on a freezing battlefield, that’s not always to most important factor.
As a fuel in a bipropellant engine, straight hydrazine has the best performance of the hydrazine derivatives, but as you say, its poor freezing point and stability usually regulates to use only as a monopropellant. However, it’s superb in that application and has become the standard in catalytic decomposition engines.
Bipropellant engines are more common because they provide far better specific impulse than monopropellant engines. Monopropellant hydrazine has a specific impulse of only about 230-240 seconds, versus better than 300 s for a bipropellant engine. Monopropellant hydrazine is typically used only when simplicity is more important than high performance, such as RCS thrusters. These types of systems also have a small fuel load, so the trade off of having to keep the hydrazine warm is usually worth it.
Hydrazine is also sometimes used in dual-mode systems, where the same fuel supply is used in both monopropellant RCS thrusters and a bipropellant main engine. I have a vague memory that
Surveyor might have used a dual-mode system, but I could be wrong about that.
MMH was discovered about the same time as UDMH, but it’s not as stable as UDMH in applications where regenerative cooling is used. However, MMH gives a better specific impulse, so that’s why we often see the switch to MMH in applications where ablative or radiation cooling is used, such as small pressure-fed spacecraft systems (Space Shuttle’s OMS for example). I think MMH is also less toxic and a safer alternative for a manned vehicle.
So why not just use straight UDMH in bipropellant engines? Some rockets do (or did), notably the original Ariane 1 design. Its second launch failed due to a combustion instability, an event I remember very well because my group had a payload on it. One of the design modifications was to switch to UH-25, 75% UDMH + 25% hydrazine. I'm not sure why it helped.
Some rocket’s still use straight UDMH. I’m pretty sure that Russian Proton rocket and the Chinese Chang Zheng (Long March) rockets use UDMH.
One of the main reasons to use UDMH-hydrazine blends it to improve performance. Hydrazine, when used as a bipropellant, produces a higher specific impulse than UDMH. Thus, by using a blend we obtain a fuel that has a higher specific impulse than UDMH alone, and that still has enough stability to use in a regenerative-cooled engine. The only blends in use that I know of are Aerozine 50 and UH25.
Another reason to add hydrazine to UDMH is to increase its average density. Hydrazine is 1.021 g/cc while UDMH is only 0.79 g/cc.
Yes, that’s another good point.
BTW, ka9q, thanks for the nice explanation about enthalpy of formation.