Comment by breput | Hacker News Reader

Comment by breput | Hacker News Reader

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Positives:

* Uses liquid methane and oxygen for fuel, which is far easier to work with than the current Vulcain hydrogen/oxygen fueled engine and burns cleaner than kerosene/oxygen engines, which is better for reusability.

* This is an engine that might work well on a small to medium reusable launcher, which ESA no longer has access to after their Vega rocket repeatedly failed and they lost access to Russia's Soyuz rockets[0].

* ESA is at least trying to do reusability after dismissing the idea for years.

Negatives:

* "100-tonne thrust class" is half the thrust of either SpaceX's Raptor or Blue Origin's BE-4 engines, which will be similarly reusable and using the same fuels.

* Additive layer manufacturing sounds cool but I'm not sure it has any real benefits over conventional engine manufacturing techniques. SpaceX is reportedly close to building one Raptor engine _per day_ without 3D printing.

* They are undecided on the fuel ("A version using liquid hydrogen-liquid oxygen is also being developed."). That is an entirely different engine and ground support infrastructure, but they are presumably doing it so it could be used directly in the Ariane 6 rocket?

* Arianespace/ESA is at least a decade[1] behind SpaceX in reusability and due to rocket failures and retirements, and the delays with Ariane 6, won't have _any_ flying rockets until 2024. So they are far behind and do not have a good track record with speedy development.

[0] https://spacenews.com/europe-grappling-with-space-access-cha...

[1] https://www.space.com/europe-no-reusable-rocket-until-2030s

The thrust of 100 tonne isn't really that important IMO. The Raptor has far less thrust than the F-1 engine used on the Apollo missions, but if you strap 33 Raptors to a rocket, they can beat five F-1s.

That's a good point, I'm guessing the bigger issue is that the chamber pressure is 1/3 of the Raptor 2's (100 bar vs 300 bar), which I assume means much lower ISP and therefore fuel efficiency, and therefore cargo lifting capacity after carrying additional necessary fuel.

What really matters at the end of the day for actual rocket building is thrust per area of nozzle size. Or how much thrust can you put under a rocket of some core size X.

Raptor is by far the best engine ever built according to that measure.

Prometheus doesn't seem that impressive but we don't have all the information on it as far as I can find.

> What really matters at the end of the day for actual rocket building is thrust per area of nozzle size.

What? I am a casual observer but I believe specific impulse is most important - how much thrust do you get per unit of fuel. The amount of fuel you have to carry has a huge impact on how much. Of course things like ion thrusters have high ISP but not enough thrust for launch - but once you get enough thrust to launch a rocket, you want a high ISP. High ISP means way less fuel required which means even with less thrust you will be okay. So thrust per nozzle area isn't strictly that important if the engine is so inefficient you need to carry twice as much fuel.

There are quite a few important numbers here.

Weight/thrust is important; thrust/area is important; ISP is important; thrust/dollar is important...

> thrust/dollar is important.

something more like that is probably the most important variable.

all the rest of them impress space nerds, the ones ending in per-dollar are the ones that impress accountants and finance people.

Accountants and finance people are not the almighty rulers of projects.

If you can not get enough thrust under your rocket sectional area, your capsule will simply not leave the ground and your astronauts won't make it to the Moon, independently on the accountants opinion on the matter.

All of those things are important.

If you have a high ISP engine that is low thrust, your rocket will either not get of the ground or if it does get off the ground it will suffer from a very high amount of gravity loses. High ISP on the engine doesn't matter when your rocket spends a long time just fighting gravity.

High thrust to weight ratio is important for a rocket. The higher your TWR the less gravity loses your rocket suffers.

There is a reason most rockets main liftoff comes from lowers ISP RP-1 engines or solids rocket. High ISP hydrogen engines just result in losses.

SpaceX Raptor actually deliberately reduced its ISP to increase its thrust.

ISP isn't everything, even in space, as you don't have infinite time to get places, transfer burns etc are more efficient when closer to being instantaneous (in terms of dV) and capture burns need to happen within a fixed timeframe.

Also I think what they meant is that the engine's thrust dictates the height above it that it can lift. So lots of larger low thrust engines = short and stubby rocket, which adds all sorts of limitations (construction, transportation, aerodynamics).

High Isp is actually a bad thing in a first stage. That stage is done almost immediately, so high Isp doesn't buy much, but low propellant density (which tends to go with high Isp) makes the stage larger and more expensive. This is why launchers tend to use hydrocarbons, not hydrogen, as fuel in the first stage. Low propellant density also makes the pumps larger and so reduces the thrust/mass ratio of the engines.

One example of additive layer manufacturing I remember reading about when it comes to engines (and I think Raptor does use it) is the ability to reduce weight and potential failure points by combing parts which would otherwise have to be separated with methods of connecting them adding to weight and failure points. IIRC the engine cooling solution also becomes much easier to engineer and manufacture with 3d printing.

When it comes to mass production, 3d printing isn't that problematic, as you can just have several printers.

Lots of space companies have different opinions on 3d printing. Relativity Space uses it a lot. Firefly doesn't as much. 3d printing is just one way of combining parts there are others.

> 3d printing isn't that problematic, as you can just have several printers.

The faster you want to manufacture the more printers you need. Its ok for most engine plant but for SpaceX Raptor manufacturing line, it would be challenging.

Based on https://wccftech.com/spacexs-3d-manufacturing-systems-suppli...

SpaceX does use 3d printing in Raptor in some form. Although it might just be for one off parts for prototyping before changing the production line.

Additive layer manufacturing has major benefits... unless you want to have a super-high production rate like Raptor. So it's a positive for them.

Like normal 3D printers, they are slow, but the benefits are, that you can change the design quickly and improve it - and then in the end you can invest into optimizing production with a different manufacturing process.

(But that is of course hard, to get the same results)

Pretty hard to reproduce the exact same design coming out of a 3D printer with other methods. Unless you put real effort into making sure the part is build-able with other methods you could run into real problems.

I considered putting it as both a positive and negative, but I consider it overall a negative until the technology is proven to be as strong and reliable as traditional manufacturing techniques - not to mention faster and less expensive.

I also wonder about durability of materials. Will the engine last shorter or longer due to being additive-layer-manufactured, all else being equal? Compared, say, to the Raptor?

Today there are lots of different metals that can be 3D printed. Some have very high capability. Are there any that compete with custom forged parts used in Raptor? I don't know, I don't think people outside of industry really have this information.

We know that SpaceX has its own material science team developing its own alloys specifically for their use case. This material doesn't seem to be designed for 3D printing (as far as we know).

What if SpaceX had instead invested that money in superior material for 3D printing instead?

Maybe somebody who is doing their PhD on material science in rocketry can comment.

I would also consider that engine likely has other places that would be more likely to put a limiting factor on durability so maybe it doesn't matter so much.

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