How would you stat a rotating detonation engine?

[Say, it isn't that bad!]

Since I didn't see this as an option when searching Spaceships. Although I understand the idea, I'm not sure how to explain it, so here's an explanatory video:

https://www.youtube.com/watch?v=rG_Eh0J_4_s

It seems sound, with the only challenges being engineering challenges. That being said, it is definitely a complex engineering challenge that requires a lot of precision; but not one that would seem to require any new science or "super-tech". To note, I am simply an interested layman who watches videos by people like Scott Manley, so informed opinions are definitely welcome. ;)

The optimistic efficiency increase estimate seems to be +25%; so I'm estimating down to +20% efficiency. Doing a square root on that to get an acceleration increase gives 9.5%, which I round to 10% in the following write-up:

Rotating Detonation Engine (TL9) [Rear]

Any non-nuclear rocket engine may be made a Rotating Detonation Engine (RDE). When applying this option, you may choose to either increase the maximum delta-v of the engine by x1.2, or the acceleration of the engine by x1.1. An RDE may not be a *variable-thrust engine. If the RDE option is applied to a variable-thrust engine, the engine becomes fixed thrust, and you must choose the thrust provided by the engine when it is created. The cost of these engines is increased by x2**; in addition, an RDE may explode on a related critical failure due to the shockwave becoming uncontrolled.

* Given that the shockwaves require precise timing as they travel around the chamber, and have to move at a constant speed, variable thrust does not seem at all compatible. However, at least most variable thrust engines can be set to a constant speed; which eliminates this incompatibility by way of making the variable thrust engine a fixed-thrust engine.
** This is an uninformed guess.

[Anthony]

I would call it flavor text of a particular type of TL 9 rocketry.

[Say, it isn't that bad!]

Originally Posted by Anthony

I would call it flavor text of a particular type of TL 9 rocketry.

To which TL9 rocket engine in the Spaceships line would you assign this?

[Anthony]

Originally Posted by Say, it isn't that bad!

To which TL9 rocket engine in the Spaceships line would you assign this?

To every one it's an effective option, if any. Spaceships doesn't address mechanical details of rockets, so if there's a way you can make it better, assume it's just used.

[Say, it isn't that bad!]

Originally Posted by Anthony

To every one it's an effective option, if any. Spaceships doesn't address mechanical details of rockets, so if there's a way you can make it better, assume it's just used.

I would certainly do so for any simple improvement that could be easily added; however, I would not call a score of rotating supersonic detonations per second, "a simple improvement that could be easily added".

And, we do have a textual answer for the difference between a TL7 chemical rocket engine and a TL9 chemical rocket engine; "Uses high-energy density materials such as metallic hydrogen or metastable helium".

Although I am not a rocket scientist, I do not think that being a rocket scientist would make me less hesitant to casually combine the words "metallic hydrogen" and "supersonic detonations under partial confinement".

[Anthony]

Originally Posted by Say, it isn't that bad!

I would certainly do so for any simple improvement that could be easily added; however, I would not call a score of rotating supersonic detonations per second, "a simple improvement that could be easily added".

It's mechanically complex but from a perspective of the spaceship design sequence it's just a rocket with a different internal design.

[Say, it isn't that bad!]

Originally Posted by Anthony

It's mechanically complex but from a perspective of the spaceship design sequence it's just a rocket with a different internal design.

I feel like our points are passing each other a ways off. ;)

To try to bring them together, it's true that you could swap the current rocket combustion system with a rotating detonation engine, once the mechanical problems are worked out. However, so far as I know, a rotating detonation engine:

1) Is not the only possible path forward for the internals of a rocket engine (it's also not something I see as necessary, at least for TL8 to TL9).
2) Has different mechanical stress and failure-points, as well as a greater possibility of the engine (as opposed to the fuel tank) encountering "rapid unscheduled disassembly" - and the initial explosion would be a detonation (super-sonic), as opposed to a deflagration (sub-sonic).
3) Has a distinct operating difference, in that while most? "regular" rocket engines can be "throttled down" simply by injecting less fuel-per-second, an RDE cannot be throttled down, only turned off or on.

Thus, while I now see your point about simplifying and abstracting the mechanical differences of TL9 rocket engines, threads that offer new options are rarely about simplification or abstraction, and I do feel this offers an interesting variant on a more traditional deflagaration engine, or even a simple pulse detonation engine. ;)

[Fred Brackin]

Originally Posted by Say, it isn't that bad!

S
Any non-nuclear rocket engine may be made a Rotating Detonation Engine (RDE). When applying this option, you may choose to either increase the maximum delta-v of the engine by x1.2, or the acceleration of the engine by x1.1. An RDE may not be a *variable-thrust engine. If the RDE option is applied to a variable-thrust engine, the engine becomes fixed thrust, and you must choose the thrust provided by the engine when it is created. s.

If I understand what you are saying then all reaction engines in Spaceships are "variable thrust" (a term not used in Spaceships or anywhere else I know of). Even Nuclear Pulse engines can vary the rate of fire for detonating thrust bombs or pellets. I'm not entirely sure what the rules for a "fixed thrust" engine should be. "Unsuitable for landing or other controlled maneuvers" might eb one possibility.

I believe what Anthony is trying to convey is that if this concept worked and provided the Delta-V increase you antiicipate it would displace all other chemical rocket engine designs and become the standard. Delta-V is _everything_ at least compared to all other traits. That's why it has the place in Spaceships it does..

I didn't watch the linked to video. YouTube videos are (IMHO) a horrible way to gain information. I did Google the term instead and here's a Wikipedia link with links to more sources.

https://en.wikipedia.org/wiki/Rotati...onation_engine

[doctorevilbrain]

What would be the point of using a rotating detonation engine? From what I've read, the rotation doesn't seem to make any difference.

[Say, it isn't that bad!]

Originally Posted by Fred Brackin

If I understand what you are saying then all reaction engines in Spaceships are "variable thrust" (a term not used in Spaceships or anywhere else I know of). Even Nuclear Pulse engines can vary the rate of fire for detonating thrust bombs or pellets. I'm not entirely sure what the rules for a "fixed thrust" engine should be. "Unsuitable for landing or other controlled maneuvers" might eb one possibility.

Here's a random US government article I found on google in five seconds by searching "variable thrust engines".
https://www.science.gov/topicpages/v...+thrust+engine

That being said, of course there's very little talk about variable- versus fixed-thrust engines; every single rocket engine we use is variable-thrust, and fixed-thrust was left behind with solid fuel rockets. In somewhat the same way, almost nobody used the term "gas-powered cars" until electric cars became widespread, or if they were advocating alternatives like bicycles.

The tl;dr of all this is that regular rocket engine thrust can be varied, and a rotating detonation engine's thrust cannot be varied; except "off" or "on"; and the meaning of the term I used is self-evident from the text I used it in. Although the implications naturally take some thought.

Originally Posted by Fred Brackin

I believe what Anthony is trying to convey is that if this concept worked and provided the Delta-V increase you antiicipate it would displace all other chemical rocket engine designs and become the standard. Delta-V is _everything_ at least compared to all other traits. That's why it has the place in Spaceships it does..

This is where I realize that you've never heard the phrase "throttle down for max-Q".

Originally Posted by Fred Brackin

I didn't watch the linked to video. YouTube videos are (IMHO) a horrible way to gain information. I did Google the term instead and here's a Wikipedia link with links to more sources.

https://en.wikipedia.org/wiki/Rotati...onation_engine

And this is where I realize you've never heard the phrase "throttle down for max-Q" because you've never watched SpaceX launch rockets, because those SpaceX videos are on youtube.

"max-Q" is the period of maximum dynamic pressure, where the momentum of the rocket is such that the rocket literally cannot take any additional pressure without significant structural risk. At this point, the rocket engines will be throttled down to only maintain the rocket's velocity, until the atmosphere becomes sufficiently thinner.

If a rocket used a fixed-thrust, its acceleration would need to be lower overall, so as to avoid this point of max-Q. Which would reduce any benefit that might be gained from an RDE. In addition, not being able to throttle down a rocket, would eliminate basically every single fancy flying trick SpaceX is using to have re-usable first-stage engines. This +20% delta-V is not free, and comes with significant perceivable downsides.

Which does leave me wondering if that "+25% optimistic estimate" is in part due to the necessity of a lower acceleration when doing surface-to-orbit launches. It isn't as if we are currently doing anything else; and any plans otherwise are indefinite years in the future due to the cosmic ray problem.

Your classification of all informational videos on youtube as "terrible" is noted and ignored. I'm sure plenty of other people's parents also called the internet, "terrible".

By the way, I predate the internet.