[ggreer]

Several reasons. It's filmed in daylight, so any flame or exhaust will be less visible. The rocket engine is much smaller than anything that would go on an orbital booster, so there's less exhaust than what'd you see for an orbital launch. Also it's looks like it's a hydrolox rocket (using liquid hydrogen and oxygen as fuel), which has the least visible flame. The combustion product is almost entirely water vapor. Methalox (methane + liquid oxygen) is the next cleanest, which emits water, CO2, and a little bit of soot. Kerolox (RP-1 and oxygen) is the most common propellant used today, and it emits a significant amount of soot.

Solid boosters put out the most visible exhaust, as burning APCP[1] emits solid particles of metal oxides. Also some rockets (mostly Russian, Chinese, and Indian) use unsymmetrical dimethylhydrazine + dinitrogen tetroxide, which emits a reddish-orange exhaust. Both compounds are toxic, as is the exhaust.

1. https://en.wikipedia.org/wiki/Ammonium_perchlorate_composite...

[perihelions]

I doubt it's hydrogen, because the color looks off (blue, rather than pink), and because it'd be a poor fit for a small R&D project. They're not optimizing for performance-at-all-costs on this.

Ethanol/oxygen is my guess. Blue, and also very little soot.

[lupusreal]

Probably methalox I think. It's the trendy prop mix most reusable programs are settling on because it doesn't coke up engines like kerosene and is easier to model in computers, and doesn't cause metallurgical problems like hydrogen while being much more dense. Alcohol isn't impossible but seems unlikely to me because that's not what you'd want for the full scale rocket they're presumably working towards.

[ggreer]

Hydrogen engines aren't always pink. The exhaust color depends on the ratio of oxidizer to fuel. The Space Shuttle's main engines were hydrolox, but their exhaust had almost no pink/red.

It's hard to say for sure, but I lean towards Honda's rocket using hydrogen. Honda has experience with it. They use hydrogen in their fuel cell vehicles, and their press release from 2021 mentions using hydrogen for rockets.[1]

I'm pretty sure both fuel and oxidizer are cryogenic, because when the rocket lands it vents from several areas (most likely separate tanks). That would rule out ethanol or methanol as the fuel.

I don't see any secondary exhaust from a gas generator, and staged combustion would be something to brag about (and much higher thrust), so my guess is that it's an expander cycle. Expander cycle engines require a fuel that boils easily, so it would have to be fueled by propane, methane, or hydrogen. I don't think it's propane, as the only propane/lox rocket I've seen has orange exhaust.[2] If Honda poached some engineers from Mitsubishi, I could see them going with a hydrolox expander cycle, as that's what the H family of rockets use.

The only thing that doesn't line up with hydrogen is the low thrust given the propellant consumption. Based on the claimed wet/dry mass (1,312kg/900kg), they used at most 412kg of propellant. Flight duration was 56.6 seconds, so that's an average of 7.28kg of propellant per second. If the stated wet/dry mass is correct and the rocket used up all of its fuel, then the rocket's thrust was around 13kN at the start and around 7kN near the end. Let's say it averaged 10kN. Force equals mass flow rate times exhaust velocity. So 10kN divided by 7.28kg/sec is 1.374 km/s. Divide by standard Earth gravity and you get 140 seconds, which is pathetic for a rocket. It could be that they only used a small fraction of the available propellant, or they had a poor nozzle design, or the engine was throttled very low and was therefore less efficient. If we assume the test flight only used 40% of the available propellant, then we'd get a flow rate of 2.9kg/sec and a specific impulse of 352 seconds. But that sort of assumption can be used to come up with any Isp.

Still, I think it's using either hydrogen or methane as fuel. Nothing else fits with the video.

1. https://global.honda/en/newsroom/news/2021/c210930beng.html

2. Here's a video of Isar Aerospace's Spectrum rocket, which uses propane/lox: https://x.com/isaraerospace/status/1906418985173758236

[perihelions]

> "my guess is that it's an expander cycle"

It could simply be pressure-fed. No turbopump at all—just a helium tank.

You have a preference for assuming sophistication, but this is a one-off inexpensive test article with trivial performance needs. My guess is that they'd made the simplest engineering choices possible at every turn.