The real size of the new engines

[Foxster]

Thought I'd record here the actual sizes of the new engines and their variants. This is important because it might look as though an engine matches a tank but in reality its mismatched and will lead to a lot of drag. I've put in brackets where visually the part size looks different to what it actually is drag-wise...

(1=1.25m, 1.5=1.875m, 2=2.5m)

Cheetah 1 and 1.5

Skiff 1.5 (visually 1) and 2

Kodiak 1 and 1.5 (one 1.5 variant visually 1)

Bobcat 1.5

Wolfhound 1.5 (visually 1) and 2

Mastodon 2  (one variant visually 1 and one variant visually 1.5)

Hopefully you can see that you can't rely visually on a variant matching a tank. To check, just attach a part below and look at the size of the shroud produced on the engine. 

 

[Aegolius13]

Interesting, and good call on the shroud. 

So the only conceivable advantage of the Mastodon (it would be the most powerful engine you can put on a 1.25m node, if you could go by visuals) doesn't actually pan out.  Great.

I guess this also means the Kodiak might be better than the Reliant on a 1.875 meter stack, so it will have less drag. 

[fourfa]

If you're looking at launching a standard vertical-stacked rocket from Kerbin, big enough that you're thinking about the Mastodon - the difference in engine drag between one node size and another is really not a big deal.  Often, discussion around rocket drag is about drag near the front of the rocket causing flips and instability.  Drag at the back will in fact add stability.  I wouldn't over-react to the information Foxster has reported here.

Now - for launching spaceplanes (which spend ages at high speed in the atmosphere), launching from Eve (whose extreme atmospheric density doesn't allow for suboptimal drag), modded scaled planets, RSS etc - these findings are extremely interesting and useful.

The advantage of being able to smash a lot of Mastodons on the back of 3.75 & 5m stacks (and consequently lift a lot of fuel with them) is much, much bigger than the small changes in drag.  Of course the same has always been true with the other big lift engines (Skipper, Mainsail, Mammoth etc) but the ugly mismatched or clipped tank butts just give people the willies, enough to prevent them from playing around with it apparently.

[Foxster]

Very true, fourfa.

It would bother the hell out of me to have a mismatched engine but might not others under some circumstances. It could lead to some head-scratching when a craft that should won't make orbit won't. 

Of course, it would be nice if it was more obvious so that we'd know to use an adapter, or a matched tank size. It's really obvious with the old engines, definitely not with the new. 

[Aegolius13]

20 hours ago, fourfa said:

The advantage of being able to smash a lot of Mastodons on the back of 3.75 & 5m stacks (and consequently lift a lot of fuel with them) is much, much bigger than the small changes in drag.  Of course the same has always been true with the other big lift engines (Skipper, Mainsail, Mammoth etc) but the ugly mismatched or clipped tank butts just give people the willies, enough to prevent them from playing around with it apparently.

True, but I still don't think the Mastodon has much going for it, even in the clustered launch engine scenario.  E.g., since the Vector model is smaller than even the compact Mastodon, you can fit more of them without the engine bells clipping into each other, so you can get a comparable total thrust for a given stack size.  And the better ISP means you'll probably end up using less fuel.  

 

[Capt. Hunt]

The Skiff does kinda bug me, isn't it supposed to be a 3.75 meter diameter, not a 2.5?