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Decided to make an experiment and compile the data for something this basic, yet very important. So, here is it! Craft: stock "Jumping Flea" (Flea booster with MK1), straight launch, variable TWR (adjusted in VAB from data given by KER): TWR 1.2, highest real altitude: 4100 m flameout: 3100m TWR 1.5, highest real altitude: 6200 m flameout: 4300m TWR 1.7, highest real altitude: 7200 m flameout: 4600m TWR 2.0, highest real altitude: 8000 m flameout: 4700m TWR 2.3, highest real altitude: 8250 m flameout: 4700m TWR 2.5, highest real altitude: 8320 m flameout: 4600m TWR 2.7, highest real altitude: 8350 m flameout: 4500m TWR 3.0, highest real altitude: 8350 m flameout: 4200m TWR 3.3, highest real altitude: 8350 m flameout: 4000m TWR 7.6 (maximum), highest real altitude: 8100 m flameout: 2100m Explaination: this means that: at TWR lower than 2, your craft will be burning fuel fighting the gravity, and at TWR higher than 2.5, your craft will be burning fuel fighting atmospheric drag. Enjoy! ## Update 2016/09/30 Quick check with Swivel+FL-T400+2xz1k batts+RC001s+Adv.NoseconeA (2,156 DeltaV) confirms the data: TWR: 1.7, highest real altitude: 87,600 m flameout: 31,000m TWR: 2.2, highest real altitude: 117,500 m flameout: 31,000m TWR 2.2: ## Update 2016/09/30 - 2 In response to criticism, I decided to make a small "usable" 7 tone "payload" (Payload A). The payload has everything to make orbit, keep in orbit and descend down. To repeat, this thread about ideal TWR for first stage / launching stage ### Payload A, first test Payload A config: Protective Nose Cone Mk7 + LCR01 RGU + Srvc.Bay 2.5m(3x Z1k batt, 2x SP-L solar panels, 2x MK2-R radial chutes) + Advanced Reaction Wheels, Large + X200-8 Fuel + LV-909 "Terrier"+Rockomax Decoupler. --- Payload A weight: 7,000 kg This payload will be launched using only one stage - using two ascend methods, using two different TWRs. The methods: 1) direct climb for statistics: SAS on, max throttle, ignition. No Stage 2 disconnect. 2) manual gravity turn: - 0 degree until 2,5km; - course to 15 degree(75 on navball) at 2,5km; - course to 35 degree (55 on navball) on pass 15km; - course to 55 degree (35 on navball) at 25km; - course to 75 degree (15 on navball) at 35 km; (this step was skipped on 2.2 TWR due to atmospheric heat **) - deactivate if apoapsis reaches 80km, wait until T-45s (accordingly, this step was much longer on 2.2 TWR) - burn at horizon line under prograde till end of stage: output stage 1 data - disconnect stage 1 and finalize burn using stage 2: output payload A in orbit fuel left will be also noted: out of fuel/flameout for 1st stage (altitude at; horizontal and vertical speeds), amount of fuel left on payload after circularization at 80/80km. #### Payload A (stage 2) - uplifter A (stage 1) uplifter A config: (Payload A +) Jumbo 64 tank + RE-M3 Mainsail (+3x launch stability enhancer, start only) 3,218 projected stage 1 DeltaV; 6,370 projected total DeltaV ) [ craft file link ] Results: ##### Direct climb data TWR 1.25, highest real altitude: 268,000 m flameout: 72,000m TWR 2.20, highest real altitude: 615,000 m flameout: 69,000m ##### Manual gravity turn data info: TWR 2.20 appears to compress air higher (hidden aerodynamic center) - thus rocket becomes less stable and either smoother transition between nodes (used here) or additional lifting surfaces required. TWR 1.25: apoapsis at flameout: 82,000m periapsis at flameout: -390,000m Stage 2 80km/80km orbit fuel left: fuel - 247/360; LOX - 302/440 TWR 2.20: apoapsis at flameout: 86,000m periapsis at flameout: -22,000m Stage 2 80km/80km (90/90km actually) orbit fuel left: fuel - 340/360; LOX - 416/440 ** apparently there is a room for improvement! "Engine idling on suborbit ascend equals wasted fuel". This case is free to be tested! Probable method: throttle down after leaving atmosphere (~28km). Reason: it looks like with TWR 2.2 atmosphere heat prevents efficient acceleration, requiring higher angle of attack. But according to Oberth effect, lower attitude accelerations are more efficient, thus its better to take it slower below.