Mastodon

[SLS.stan]

Should I use the mastodon engine for 2.5 m stacks or is it not worth it? I was making a 2.5 mun rocket and chose the mainsail over it. Did I make a mistake 

[DeadJohn]

If the Mastodon is enough thrust for your rocket, then using the Mainsail is less efficient. If the Mastodon doesn't have enough thrust for efficient liftoff, though, the Mainsail becomes a better choice. Lots of rockets can use either engine and work well enough.

What's the TWR (thrust to weight ratio) of your rocket? Check the VAB readouts to compare TWR and DV when you switch between the Mainsail vs Mastodon. If my TWR on liftoff will be less than 1.3 (*) I look for a bigger engine or add small SRBs to help liftoff.

(*) I play a heavily modded game and 1.3 TWR works well for me. Stock planets and your play style might prefer a different TWR target.

[SLS.stan]

Thanks! I launched the rocket yesterday and it reached its target location 

 

[jimmymcgoochie]

The Mastodon engine is a little bit odd in KSP, it seems underpowered for its role as an F-1 analog and is inferior to the Mainsail in both total thrust and vacuum ISP, though it performs slightly better in atmosphere and is noticeably cheaper and a bit lighter too. The difference in thrust isn't that huge, so if your rocket can manage with a Mastodon rather than a Mainsail it's a good choice to save funds.