[Spaceships] Engine size and performance

[Fred Brackin]

Originally Posted by the-red-scare

In Spaceships where mass flow rate can vary by a factor of 300,000, seems like it might come up…

Maybe with chemical rockets and maybe not. However, those are the design with the highest mass flow rate. All other engine types with higher Delta-V values use less reaction mass at higher velocities. Some of them don't even have "nozzles".

So even if you can show somehow that it is not possible to build a launch vehicle 1000x the size of a Saturn -V because it wouldn't have enough room on its' back end for rocket nozzles some more advanced system of propulsion would have a smaller (and likely much smaller) problem in that area.

[Anaraxes]

Originally Posted by the-red-scare

In Spaceships where mass flow rate can vary by a factor of 300,000, seems like it might come up…

Increasing the area by a factor of 300,000 means increasing the linear dimensions by a factor of 550.

The Saturn V had five F-1 engines on the first stage, which was 33 feet wide. The entry point into the nozzles for those engines was about 1.7 feet across. (The wide end of the nozzle was a bit over 12 feet wide, but all the propellant mass had to make it through the little hole, too.)

Expand that by a factor of 550, and you wind up with a nozzle entry 930 feet across. This is a bit less than 4x the beam of a Ford-class carrier (365,000 tons, 1100 ft long) or oil supertanker (ULCC, 564,000 DWT, 1400 ft long). So, those ships are SM+13 to SM+14. Bump them to SM+15, 3x the dimensions, and they'd be just about big enough to hold our oversized SM+15 F1 rocket engine. (Not that there's any reason you'd have to have a spaceship shaped like they are -- you could easily make them wider and shorter without the drawbacks that has for an ocean vessel. But just for the sake of mental imagery...)

The BOTE numbers don't seem to indicate that the size in SS would be wildly out of whack. There's a square-cube relationship there with area-volume, but over the size range of interest, it doesn't look like it's running away quite yet.

As for thrust, 550 times the mass flow should mean 550x the thrust. So, 825M pounds of thrust. But the Saturn V weighed 3000 metric tons, so our comparison ships are only 120-190x as massive, or <600x the mass when scaled to SM+15. Even with only 1 RBF-1* engine, that's still about half a G of acceleration. Which seems to fall into line with the Saturn V, recalling that it had five engines in the first stage.

--
* Really Big F-1

[Anthony]

Actual mass flow rate is very unlikely to be an issue for surface area, though it can cause some design issues for pumps. The problem is that you need enough area for your exhaust gases to expand properly. Thus, the 12' wide end of the nozzle on the Saturn-V mentioned below really did matter, and even a fairly modest increase in size would run a risk of running out of space on the bottom of the rocket (unless you came up with some other form of technological advance).

Non-chemical rockets will generally have problems with the surface area requirements for heat management and power generation before the actual rocket size becomes a problem. Solar and magnetic sails (and solar cells) don't scale up well at all.

[the-red-scare]

Originally Posted by Anthony

The idea that you can use the same designs with only minimal modifications over a size range of +5 to +15 (100,000 times) lacks real world justifications, a lot of systems have both minimum and maximum practical sizes or don't scale in a simple way, but Spaceships was always intended as quick and dirty, not an accurate simulation of anything.

I’m all about finding the breaking points to make things interesting. It’s really easy to make a lot of Spaceships vessels that are all basically identical except for, say, size. Or you make opposing cruisers for two space nations at war and realize they’re pretty much the same. I want the little nitpicky bits that give different ships character.

I’ve already got a way to account for volume and surface area (so hydrogen-filled fusion ships are much bigger and have weaker armor speed over more area than nuclear pellet-filled ships, for example). I have more detailed radiator rules with Heat Points and so on. Lots of house rules like that. So figuring out limitations to things like rocket size is all of a piece with that effort.

[Varyon]

Going along with the "shorter but wider" trend, I'd say that for reaction drives there's probably a limit beyond which a ship can no longer be streamlined and still mount 6 full-sized drives, then this would count down. After that you'd have a range where you can mount 6 full-sized drives, and eventually hit the point where you basically have to streamline the ship the "wrong way" (streamlined from the side) to be able to mount 6 full-sized drives, which would similarly count down until it's literally impossible to fit a full-sized drive. Note "wrong way" streamlining mostly just means armor doesn't protect as well, although you can probably designate one foe (or even a group if they're clustered together) each round that suffers a -1 to hit you from the side, as you angle to present your narrowest profile in that direction.

Going off Anaraxes' values for the RF-1, which in Spaceships would have 3G (chemical rocket) for a single system, and the Saturn V (or rather its first stage, but I'll just refer to this as "Saturn V" here) apparently had ~1.5 systems (4.5G) - so each RF-1 is a full-sized system for an SM+8 vessel (Saturn V was ~3000 tons, so SM+9, and each rocket was 0.3 systems, going off Spaceships performance of Chemical Rockets; note this is largely just rough estimation, but that's probably good enough here). Approximating the Saturn V as a cylinder with a 33 ft diameter, and saying the smallest the nozzle could be to achieve the correct thrust would be a circle with a 1.7 ft diameter, we see the Saturn V had a cross-sectional area of ~855.3 ft2 while each nozzle had a cross-sectional area of ~2.27 ft2. You need 3 of them to make up a full-sized system, for ~6.81 ft2, or ~8% of the cross-sectional area of a streamlined vessel; we'll round up to 10%. That's at SM+9. If we further assume that the Saturn V was only just above the unstreamlined vs streamlined cutoff (unlikely, but let's go with it), that means if it had more than 10 systems worth of Chemical Rockets, it wouldn't be able to be streamlined (as it can't have more than 100% of its cross-sectional area as nozzle). At SM+10, diameter increases by 1.5x, roughly doubling cross-sectional area. Our old SM+9 rockets take up 5% of the ship's cross-sectional area, but we need 3x as many for the same thrust, so they actually take up 15% per full-sized system - we can only fit up to around 7 and still be streamlined. At SM+11, this becomes up to 5, then at SM+12 it becomes 3, at SM+13 it becomes 2, at SM+14 it becomes 1.5, at SM+15 it becomes 1, and so forth, following the Size and Speed/Range Table (SSR). You've then got a pretty large range where you can mount 6 full-sized systems on an unstreamlined drive (you need the same relative dimensions - swapping length and width - as a Saturn V for it to count as side-streamlined).

That's apparently with the reaction drive that requires the largest nozzle size, and also with us being a bit harsh (like saying Saturn V is the closest to unstreamlined you can get and still count as streamlined). I feel that unless you're dealing with really big ships, this is probably something you can safely ignore. If you go into enough depth to calculate what nozzle size is needed for a given reaction drive, you can compare this to a minimally-streamlined reference vessel and see how many such nozzles would "fit" if you went to 100% capacity. Thereafter, each +1 to SM is -1 SSR (so x0.7, x0.5, x0.3, etc) to the number of nozzles that can fit.