Hesp

1.35m diameter

Making some assumptions based on Falcon 9 launch profiles, the rocket velocity at 20km altitude is around Mach 2 / 2.5 It's safe to assume that most of the air augmentation will happen below that altitude, so exotic intakes (J58 variable geometry spike) are not needed. A proper designed diverterless supersonic intake could suffice.

Well, if there is a system that allows you to relight your engine an indefinite number of times, why not use it over "old style" solutions that impose a hard limit? (see Falcon heavy splashdown and TEA/TEB exhaustion) The capability of repeated/precisely timed ignition is only an advantage from my point of view, complexity of the system doesn't change much. If the trend is to go towards rocket engine reusability and fast turnaround, a technology (and mindset) transfer from automotive sector would be a great step forward. Cost isn't a barrier for sure, with aerospace budgets you can pick the most advanced solutions and still have a bargain.

I think they will use something comparable to the TJI (Turbulent Jet Ignition) currently applied in Formula1 engines. Basically, a small prechamber with a spark plug and injector. Gaseous methane would work flawlessly with it. At ignition, shaped flame jets are injected into the main combustion chamber. http://www.mahle-powertrain.com/en/experience/mahle-jet-ignition/

Shouldn't GEO be enough for that? Anyway, a dollar bill is 0.11mm thick. 9B$ = 990km

Cheap or not, what I was pointing out was the uncertain mixture and the ability to work with it. Car engines are more omnivorous because they've been conceived and developed to be like that. A F1 (Formula 1) engine without its bespoke fuel won't even start up. Oh, and if a car stops when using appropriate fuel, car manufacturer gets in trouble and loses money -> it's a critical failure as well from an engineering point of view.

Well, commercial LPG is mostly a propane-butane mixture in uncertain proportions, and cars work reliably with it. As told before, if the engine can accept a variable HC mixture some of the ISRU complications can be ignored. What's easier, develop a more tolerant engine or deliver a "perfect" fuel refinery to Mars? I don't have an answer to this. Pure propane at -40°C has a vapour pressure of 1 bar, 0.1 bar at -85°C, 0.03 bar at -100°C. If you can keep it reasonably cool you don't need pressurized tanks at all. It's a tradeoff between cryo CH4 and Kerosene.

The first is a problem that could be solvable using automotive techniques (i.e. a UEGO sensor in closed loop control to adjust Fuel-LOx mixture). Needs to be engineered for a rocket engine, yes, but it removes the need for a laboratory quality fuel and allows to settle for a "good enough" mixture. Boiloff... not sure why nobody is evaluating propane as ISRU fuel. Way more easier to keep liquid than methane, denser and more densifiable when subcooled, almost same ISP. The biggest fraction of the propellant weight will be oxygen anyway, but you can obtain it in a "easier" way from atmosphere. Maybe it's worth to liquify it, keep LOx cool by evaporation, use the GOx for other applications (fuel cells?) instead of going for a zero-boiloff solution.

As an European, my opinion is that they could do way more for the space industry. Close to where I live there are Thales-Alenia and Avio, but they are using only a minimal portion of the engineering, scientific and manufacturing resources available in the area.

About time! Next step would be realizing that big and heavily structured organizations are slow to adapt to changing conditions...

Just having a Launch Escape System decreases dramatically the LOC probability. Regarding Shuttle, risk assessment for STS-114 put a SSME catastrophic failure at 1:610

I'm wondering if they ever considered Propane instead than Methane. It has a higher densification potential when subcooled (less tanks on the vehicle) and storage on Mars should be way simpler (boiling point at 1 bar: -42°C, basically Mars' mean temperature)

It was just a simple excel file answering the question: "what V_excess do I need at Mun SOI exit to have the correct V_excess at Kerbin SOI exit?". Nothing much than some basic calculations to see if it was convenient compared to a LKO launch.

Honestly I did not perform further calculations, but my gut feeling is that could be a plausible explanation. I have to remark that @HebaruSan wrote a great tool to plan this kind of manouvers with his Astrogator. After completing that Duna mission i restarted a career in a 2.5x scale "real" system, and I should perform soon some calculations on where to place the Lunar fuel depot. The current Lunar station for surface operations is in a 100x100km orbit at 28.5° inclination from Earth equator (no plane changes needed if the launch is correctly timed). For the fuel depot I'm planning to keep an higher orbit (tentative 300x300) and aligned to the Ecliptic plane for interplanetary missions.