RT-5 "Flea" makes a great cheap upper stage for light payloads

[rdem]

if you decrease the thrust to a very low level, it lasts very long and is easy to control. with a rt-10 (with thrust limit of 50) lower stage and a rt-5 upper stage (with t limit of 11) i managed to get a small probe with a tiny amount of fuel and an "ant" engine to a 75km sub orbital trajectory with near 2000 m/s speed. two solid boosters cost less than 1000 funds.

[regex]

It's a great kick stage for tiny probes too. You can send a half-ton probe to Duna with a Flea from an 80km orbit, just adjust with RCS or something on the way.

[rdem]

also i realized that when you use smaller decoupler between two srbs cause it's lighter, actually it creates way more drag than regular sized decoupler and limits the rocket.

[Fearless Son]

21 minutes ago, rdem said:

also i realized that when you use smaller decoupler between two srbs cause it's lighter, actually it creates way more drag than regular sized decoupler and limits the rocket.

On that topic, I wish that a size one decoupler would produce a size one shroud around the Flea.  It looks kind of ridiculous when I have two size one fuel tanks connected by a size one decoupler with a shroud narrow around the Flea.  

[Streetwind]

This is actually my plan for the Flea in a career save with custom PartUpgrades.

It starts off as this silly little low-efficiency proto-SRB with absurd TWR. But I'll give it a researchable part upgrade that ups its vac Isp and lowers its asl Isp, perhaps with a bit of thrust reduction. That way it will change roles completely - moving the part out of a niche in which it has been eclipsed, into a niche where it is unique. A true solid upper stage, just like several IRL rockets use 'em.

Edited October 18, 2016 by Streetwind

[moogoob]

2 hours ago, regex said:

It's a great kick stage for tiny probes too. You can send a half-ton probe to Duna with a Flea from an 80km orbit, just adjust with RCS or something on the way.

I've done this at least once! Quite a few RL probes are sent on their way this way, New Horizons in particular comes to mind.

[problemecium]

I still don't like using it merely because it looks stupid, and thus i continue to hope it gets redone to be a Size 0 booster.

[septemberWaves]

1 minute ago, parameciumkid said:

I still don't like using it merely because it looks stupid

It makes a nice upper stage if you stick it beneath the payload inside a fairing. Other than that though, it doesn't look too great.