Hydrolox Engine

Since the early 1960s, hydrogen/oxygen rockets have been the sin qua non of high performance for space-borne maneuvers where you can afford its tradeoffs. Other higher performance engines, from nuclear thermal to the Fluorine or tripropellant mixtures get measured against Hydrolox. Lower performing combinations like methane or kerosene/oxygen had to justify lower performance for orbital applications by making up for some of hydrogen’s drawbacks.

The drawbacks for that performance are serious though: liquid hydrogen is one of the hardest of common cryogens, requiring extensive cooling and thermal management, and its low density makes for heavier tanks. Even so, some of the best-performing stages for interplanetary launches have been Hydrolox, among them the Centaur and the Saturn V’s S-IVB.

Hydrolox is well suited for near-future rockets that can’t or won’t use nuclear thermal, or for cislunar maneuvering vehicles where Hydrolox has a dry mass advantage over heavier nuclear options. It is poorly suited to long-duration spacecraft that need performance above 500 seconds and mission durations measured in years.

Engine Performance

Name	Hydrogen-Oxygen Chemical Rocket
ISP or Exhaust Velocity	459s <-> 4500 m/s (vacuum)
Thrust	1,000,000 N
Efficiency	98%
Drive Power	2205 MW
T/W	102
Energy Source (Fuel)	Hydrogen + Oxygen at 6:1 ratio
Propellant	Water exhaust
Reactor	Combustion chamber
Power Density	2205 kW/kg

Introduction

Since the early 1960s, hydrogen/oxygen rockets have been the sin qua non of high performance for space-borne maneuvers where you can afford its tradeoffs. Other higher performance engines, from nuclear thermal to the spicier Fluorine or tripropellant mixtures get measured against Hydrolox. Lower performing combinations like methane or kerosene/oxygen had to justify lower performance for orbital applications by making up for some of hydrogen’s drawbacks.

Hydrolox is well suited for near-future rockets that can’t or won’t use nuclear thermal, or for inter-orbital transfer vehicles where Hydrolox has a dry mass advantage over heavier nuclear options. It tends to be associated with total burn durations measured in minutes for Hohmann least-ΔV trajectories. It is poorly suited to long-duration spacecraft that need performance above 500 seconds and mission durations measured in years.

Design and Function

Hydrogen-oxygen engines are, like most chemical engines, fairly simple. You get some hydrogen, you get some oxygen, you pump them into a chamber, you introduce a spark or other ignition source, and steam comes out the back glowing with a very pretty clear flame, sometimes with varying minor coloration. They are essentially universally fuel-rich (since you want more hydrogen in the exhaust anyway) and hydrogen’s such a great coolant that there’s minimal temptation to use oxygen for regenerative cooling. Ablative or radiatively cooled nozzles exist, but regenerative cooling is pretty standard. The big decision is powering your pumps. The J-2 used a gas generator, having a secondary burner to power the turbopumps. The J-2S cut that, by tapping off a portion of the main chamber gas flow, and avoiding a separate combustion chamber. You can get more complex, like the multi-stage booster pumps the SSME used to get to nearly 3,000 psi (more than 200 atmospheres!). Alternately, you can go dead simple--the expander cycle cleverly manages to harvest all the pumping energy it needs for low-pressure vacuum engine operation from the hydrogen used to cool the chamber and nozzle bell, meaning the turbine inlet gas is in the realm of kitchen appliance, not even a car turbopump, and the rotor can be made of aluminum! Literally very cool.

Performance / Capabilities / Applications

The main benefit of Hydrolox is its performance: even a moderate pressure engine with a vacuum nozzle can break 440 seconds of specific impulse. No other engine type in present-day service can reach those levels of performance. At the same time, hydrogen is a fantastic coolant, more than an order of magnitude better than any other fuel, meaning it’s possible to build some incredibly clever engine cycles without problems like turbine temperature or propellant “coking” of hydrocarbons. As an example of the cleverness this allows, the turbine inlet temperature on the expander cycle RL-10 is below the boiling point of water, more like a kitchen appliance than the complex high-temperature turbomachinery needed in other fuels. Similar endurance and robustness was seen with the J-2 engines for Saturn. The Space Shuttle Decision by T A Heppenheirner quotes Rocketdyne's Paul Castenholz, one of the engineers managing J-2 development. "We never wore out an engine of the J-2 type. We could run it repeatedly; there was no erosion of the chamber, no damage to the turbine blades. If you looked at a J-2 after a hot firing, you would not see any difference from before that firing. The injectors always looked new; there was no erosion or corrosion on the injectors.” As Heppenheimer reports, this was inspired by examples like a single test engine which ran for 103 starts and 6.5 hours without overhaul. Higher performance Hydrolox engines like the Space Shuttle Main Engine (SSME) have a more complex reputation, though more recent tests of the AR-22 variant demonstrated 10 firings in 10 days with no major overhaul. With good performance from even relatively simple engines, it’s no wonder that rocket pioneers like Werner Von Braun were swayed to the Cult of Hydrolox from initial skepticism. For more than half a century, it’s been the propellant of choice for upper and sustainer stages for most American launch vehicles from Atlas to Delta to the Space Shuttle. For that reason, if you want performance with near-term engines, hydrolox has a lot to recommend it.

The downside of Hydrolox is the blasted hydrogen. Its performance comes from hydrogen’s low molecular mass--about the smallest anything can get and be legitimately called a molecule--but that also means it has to be incredibly cold to be liquid, at about 20 C colder than liquid oxygen. That’s troublesome for long duration storage, making “boiloff” the word of the day. Heat will get to your hydrogen any way it can, and before long, you won’t have any left. Even once kept liquid, the density is incredibly poor. That means heavier and physically larger tanks for the same performance. Even with hydrogen’s improved specific impulse, the Hydrolox Delta IV lower stage had to be five meters in diameter to lift the same payloads as the 3.8m Atlas V, since Hydrolox has about a third the mixed density of Kerolox.

Worked Example(s)

Planned to go here:

• RL-10 expander
• J-2S gas-gen
• SSME
• Maybe M-1 massive gas gen?

Additional References

GRAB SHUTTLE DECISION LINKS