Why does this rocket have no delta-v?

[SkyLimit]

I have 4 FL-R10 RCS fuel tanks on top of each other and then 1 48-7S "Spark" Liquid Fuel Engine. And so its telling me that it has no delta-v even though all the parts are the same size?

Sorry that I cant send a pic of my rocket cause I dont know how to

[HvP]

Hello @SkyLimit and welcome to KSP and the forums.

The reason you have no delta-v in that craft is because you're using the wrong fuel type for your engine. Most rocket engines, like the Spark, require a "Rocket Fuel" tank that carries liquid fuel and oxidizer - not mono-propellant fuel. RCS engines and thrusters use the mono-propellant in the type of RCS tank that you used on your craft. Mono-propellant cannot power standard rocket engines.

There are four different fuel tanks in the game, which you can learn about in the wiki HERE.

Liquid fuel only tanks carry jet fuel for planes and also power the nuclear LV-N "Nerv".

Rocket fuel tanks carry the fuel and oxidizer mixture that powers standard rocket engines. They also power a special type of RCS thruster called "vernor" engines which are more powerful than the standard RCS thrusters.

RCS tanks carry mono-propellant for RCS thrusters and the "Puff" mono-propellant engine.

Xenon tanks will only work with the "Dawn" electric ion engine.

Technically, solid rocket motors come pre-packed with their own solid rocket fuel already, so there is no separate tank for them.

When you right-click the engine or fuel tank in the parts list the information panel will have text that identifies the type of fuel that it needs/provides.

Edited July 11, 2021 by HvP

[Maria Sirona]

HvP gave an amazing explanation, but i must say, size has nothing to do with spacecraft having Delta-V or not. You can have a 3,75 meter tank with a terrier slapped under it and it produces thrust- size matters not.