PLEASE have the rocket not use liquid hydrogen as fuel for especially the first stage main engines. Kerosene/LOX (aka kerolox) is so much safer than LH2/LOX (aka hydrolox), and so much less of a technical hurdle and a point of failure for these early rockets. Early hydrolox engines also tend to have a much lower thrust than kerolox engines due to the much lower density of liquid hydrogen compared to kerosene. A kerolox first stage is also likely to be cheaper in early rockets than an equivalent hydrolox first stage too due to the much-reduced technical challenges and the very expensive cost of creating the liquid hydrogen, despite the kerolox stage being larger due to the lower specific impulse (aka efficiency, usually counted in seconds- higher is always better). If you absolutely have to use hydrolox for some literary reason, have the rocket use hydrolox second or third-stage sustainer engines to provide the final push into orbit like what hydrolox stages had been limited to until the Space Shuttle; sustainer engines are where the main advantage of hydrolox (better specific impulse and lower weight) is actually useful in earlier launch vehicles.
(Though on the earlier rockets like this one, is it is still safer to stick with kerolox sustainers due to the major technical challenges of early hydrolox; the Soviet R-7 rocket family- which included the Vostok-K rocket that brought Yuri Gagarin into space as well as the Soyuz launch systems, used kerolox engines for both their lower and upper stages, the modified Atlas D launch vehicles used in the OTL Mercury program used kerolox for lower and upper stages, while the Titan II launch vehicles used in the OTL Gemini program used Aerozine-50/nitrogen tetroxide, described below. It was only in 1963 IOTL that a hydrolox engine, the American RL-10, had a successful flight)
And I don't know if you had planned this or not, but NEVER have the rocket's orbital maneuvering systems and/or engines for in-orbit work use hydrolox; or kerolox for that matter. Anything with a cryogenic propellant or oxidizer is out for in-orbit work (unless you are only sitting in orbit for up to like three orbits or so before launching a payload out from a low earth parking orbit (which is between like 165km or so to like 225km or so- the lower the better except for some hydrolox stages as boiloff is somewhat less with reduced atmospheric density) to try to achieve a geostationary/geosynchronous/Molniya/whatever orbit or more likely even further away, like for trans-lunar injection or for going out of the Earth's sphere of influence) for many decades at least due to the boiloff that occurs with cryogenic systems due to the heating of the spacecraft caused by solar energy (Even LOX has this problem).
Your only real bet for the orbital maneuvering systems, RCS systems, and in-orbit (vacuum) engines is to use bipropellant hypergolics, which are liquid at room temperature and crucially do not require external ignition to ignite, only needing the oxidizer and fuel to mix for the reaction to occur. The best options to use for fuel would be hydrazine-based fuels, including MMH (monomethylhydrazine, a derivative of UDMH, which was used in the Space Shuttle OMS), UDMH (Unsymmetrical dimethylhydrazine, which the Russians, Chinese, and Indians use IOTL; slightly lower specific impulse than MMH but cheaper), and Aerozine-50 (a combination of half hydrazine and half UDMH which has slightly better specific impulse than UDMH alone though has a somewhat shorter shelf life, used for the Apollo Lunar Module and the Titan and Delta rocket systems; replacing the UDMH in Aerozine-50 with MMH also could possibly work and is slightly higher specific impulse than regular Aerozine-50 or MMH alone). Plain hydrazine alone as one component of a bipropellant fuel is another choice that has an even higher specific impulse of ~343,5s with typical nitrogen tetroxide oxidizer (though only like ~0,7% better in vacuum than the ~341,5s specific impulse of MMH or Aerozine-50 propellant and N2O4 oxidizer, and only ~0,3% better in vacuum than the ~342,5s of a hypothetical modified version of Aerozine-50 that uses MMH instead of UDMH as propellant and N2O4 oxidizer), though I wouldn't really recommend hydrazine due to the storability problems it has; especially at high temperatures.
There is only one real choice for oxidizer for bipropellant hypergolics; nitrogen tetroxide. The two other options that were used IOTL, red fuming nitric acid and hydrogen peroxide, are far inferior to nitrogen tetroxide. Using RFNA as an oxidizer results in an ~8% or so lower specific impulse compared to N2O4 oxidizer, while hydrogen peroxide has storability problems as it slowly spontaneously decomposes into water and oxygen gas, making it very hard (along with other factors) to reach the purities needed to reach high specific impulses, which even still max out at less than that of nitrogen tetroxide even at 100% purity. (Don't use chlorine trifluoride or chlorine pentafluoride as oxidizers. Just don't.)
Bipropellant hypergolics are also a relatively good option for first stage main engines and second and third stage sustainers; the ~285s sea level and ~333s vacuum specific impulse of UDMH/nitrogen tetroxide, used in the Soviet Proton family of rockets, and the ~289s sea level and ~341,5s vacuum specific impulse of Aerozine-50/nitrogen tetroxide, used in the American Titan family of rockets, are only somewhat worse than the ~300s sea level and ~358s vacuum specific impulse of kerolox. (which still is definitely much lower than the ~389s sea level and the ~455s vacuum specific impulse of hydrolox, but still.) This lower specific impulse of hypergolics compared to kerolox could probably be considered an acceptable tradeoff due to the reduced infrastructure required for the launch facility; especially since it is in Vietnam, so far away from Germany. Germany also probably has a pretty good base of technical skills and understanding for hypergolic main engines due to the fact that it is very probable that the German ICBMs use the same type of N2O4 and hydrazine-based hypergolics for their engines (a lot of OTL mid-era ICBMs between used N2O4/UDMH or Aerozine-50-fueled engines as the propellants were storable); lots of OTL launch vehicles used rocket engines originally designed for intercontinental ballistic missiles, and it wouldn't be that much of a stretch for the Germans ITTL to do the same.
Another option for in-orbit work is to use a monopropellant of most likely just plain hydrazine running across a heated iridium/alumina catalyst (used IOTL in many space probes and landers on other planets); no oxidizer required. Hydrazine monopropellant has a lower specific impulse than bipropellant hypergolics, but is simpler. Another monopropellant option is a blend of nitrous oxide and a lighter hydrocarbon, which is initially run through a heated catalyst causing the nitrous oxide to decompose into nitrogen and oxygen gas (modern-day work is being done on ethane/ethene/ethyne mixed with N2O); which can have a potentially much higher specific impulse than hydrazine monopropellant; though the specific impulse still would most likely be less than the bipropellant hypergolics; I personally wouldn't recommend a mixed monopropellant due to the inherent dangers of oxidizer and propellant being in the same mixture.
Sorry for my long word-spewing rant; I hope that it wasn't too onerous. (My god, that was a lot longer than I expected it to be)
SOmeone's been reading Ignition. I agree whole heartedly on this by the way, having read the book.
Is he going for a water or landing on the ground?
