[Stock] The Bare Minimun II

[Apotheosist]

Closette, much of my logic for designing rockets is based on things you and others have worked out in challenges such as the 'max altitude with this supplied spacecraft' challenge. Choosing to have just enough engines to have a T/W close to1.5, was based on that very graph and a remark I think you made on it about only losing 10% efficiency at T/W 1.5. Same with optimium velocities at certain altitudes. I\'ll take this opportunity to say I have learnt a great deal from your posts and others like you who apply maths to KSP to find out what works best. I now play KSP with a notepad, pencil and calculator beside me

Also, I have noticed that both the altered rockets you posted have 2 liquid fuel tanks less than my orignial rocket. Was this a deliberate alteration, or had you accidentally removed 2 tanks? If you had it might explain why you 'can never make it all the way home with the LFEs alone'.

[closette]

You are very kind - in fact most of the work on the atmosphere was done in collaboration with Kosmo-not, jebbe and others. Glad that some of it has been useful!

Unfortunately, an optimum 2D ascent to orbit profile remains elusive and may require the full apparatus of optimal control algorithms specific to each rocket, which is beyond my skills.

I removed the 2 tanks from my modified EZ Minimun IIs to see if it would help with launch TWR and acceleration (specifically, trying to reach 100 m/s by 1200 m and 250 m/s by 10 000 m), and I did try replacing them with half-tanks at some point in my testing. But since I could at least sometimes get back with RCS only, I kind of forgot about them.

For the EB version with SRBs I guess I could put the 2 full tanks back. The nice thing about your design is that one can add and subtract from the first stage easily, without breaking other connections, fuel lines etc.

[Newt0570]

I was wondering how I calculate a TWR with a given craft using KSP\'s unitless numbers. I know my liftoff stage has 2550 KSP units of thrust, and ~100 units of weight before liftoff. The craft gets to 100m/s almost before an altitude of 1000m. It\'s by no means a minimal craft, But I\'d like to know how to do the conversions and calculations for designing future craft. the craft I\'m mentioning is posted >here<. (Crossfeed v3).

[closette]

Thrust to Weight Ratio (TWR) is fairly easy to calculate as long as you are careful adding up the numbers given to you in the VAB for the parts\' masses. (Note for fuel tanks, use the 'Full' mass, and NOT the 'units of fuel').

Some people refer to the VAB part masses in 'tonnes' and thrusts given in 'kiloNewtons' although it\'s not clear if that is the case. At least the units are consistent where 1 thrust unit will accelerate 1 mass unit at 1 m/s².

The numbers you give for your craft seem a bit high to me for its size - each tank has a mass of 2.5 and an LTV-30 engine a mass of 2 tonnes and a thrust of 200 kN. But I could be wrong. Note that you should use TOTAL mass at launch - not just for the liftoff stage - and obviously only the first stage 'active' engines for thrust.

So assuming the numbers you give, the only thing you are missing is to convert the mass (tonnes) into weight (in kN), by multiplying by local gravity g = 9.8 m/s². So your craft\'s weight is then (100 tonnes x g) = 980 kN, and the launch TWR is just the ratio

2250 kN / 980 kN = 2.3.

That is pretty good for an efficient launch, and of course the TWR will increase as you lose fuel mass (and as Kerbin\'s gravity weakens with distance from its center, but not by much while you are still below a few hundred km altitude).

Hope this helps.