Rocket planning based on final payload?

[egneil]

I've been messing around in KSP for a while now and I always have trouble designing the stages needed for atmospheric ascent off of Kerbin. I'm already familiar with the equations for finding the delta-v for a craft, but it seems to require me finishing my design before I can learn if it has enough fuel for the task at hand. So I'm wondering if there is an equation or set of equations for determining how much fuel I need for the first stage ascent of a craft for a given final stage payload.

If there's any additional information that I missed that would be helpful, let me know where I can find it and I'll post it for you.

Edited May 8, 2015 by egneil

[Venusgate]

If you are looking just for howto get into orbit with craft design, the Kerbal Engineering Redux mod will save you a ton of time.

If you are taking an academic approach to calculation, based on experience of other players, the deltaV to reach orbit through the atmosphere is about 3400 m/s with an efficient (but not perfect) launch profile.

Keep in mind, the amount of delta v required to go through the atmo is not perfectly calculatable for all crafts, since it changes based on air resistance based on cross sectional area of your rocket, as well as piloting technique in your launch profile.

[impyre]

I design "standardized" launch stages using a dummy weight to represent the payload. For a given mass in tons it doesn't matter what the payload is. Just make sure your payload isn't contributing fuel to your launch stage. Other than that, build then test, then tweak then test. Once you get it perfect, save it as a subassembly. I use names like "RLS, 20t" for my recoverable launch stage designed to lift 20 tons. I design all of mine to be able to lift rated payload to 250km circular orbit (where my station is parked). So payload can go over rated mass for lower orbits.

Once you have a whole set (I have a 5t, 10t, 20t, 30t, 50t, 80t, and 100t so far), you generally won't need to keep redesigning launch stages. Just look at the mass of whatever you're building in the VAB and pick the best launch stage to slap under it.

EDIT: do *not* include fairings in your standardized launch stage... if needed at all, include them in the payload... as the weight of the fairing will vary depending on size/shape of payload. This way you can still use your launch stages the same way every time, regardless of whether a fairing is needed or not.

Edited May 8, 2015 by impyre

[egneil]

I design "standardized" launch stages using a dummy weight to represent the payload. For a given mass in tons it doesn't matter what the payload is. Just make sure your payload isn't contributing fuel to your launch stage. Other than that, build then test, then tweak then test. Once you get it perfect, save it as a subassembly. I use names like "RLS, 20t" for my recoverable launch stage designed to lift 20 tons. I design all of mine to be able to lift rated payload to 250km circular orbit (where my station is parked). So payload can go over rated mass for lower orbits.

Once you have a whole set (I have a 5t, 10t, 20t, 30t, 50t, 80t, and 100t so far), you generally won't need to keep redesigning launch stages. Just look at the mass of whatever you're building in the VAB and pick the best launch stage to slap under it.

EDIT: do *not* include fairings in your standardized launch stage... if needed at all, include them in the payload... as the weight of the fairing will vary depending on size/shape of payload. This way you can still use your launch stages the same way every time, regardless of whether a fairing is needed or not.

Thanks for the advice, although I'm not sure how useful it'll be in my early career mode game. Still when I can get a stable launching platform it should help with my planning.

Mainly though, I'm just trying to get my head around how much fuel I need for a static payload. Thinking about it now I think my main question is if you had a 9000dv first stage how much should you expect it to drop when you put on a 20t payload?

[Garius]

To answer directly, it depends on how heavy your lifter is and how much fuel it has. It's not nice clean formula. Indirectly though, I don't think that's a good way to think of it. As the above poster said, you should design your lifter with a particular payload weight in mind. The DV of the lifter with 0 payload isn't a particularly useful number.

I generally aim for about 20% payload delivery, so my lifter will be about 4x as heavy, two stages. So eg a 5T lifter would weight 20T, for 25T total. This means I probably want 2 Reliants for liftoff. DV is usually optimized around roughly equal weight stages but it's pretty flat so I prefer more weight in the first stage to minimize TWR issues in second stage. So I'd probably do something like 12T/8T, with a single swivel on the second stage (this setup can be asparagus staged which gives the bonus of gimbal from takeoff).

This gives about 1400 m/s in the first stage and 2200 in the second stage, which makes orbit on a good ascent. Tossing some BACCs on gives a bit more leeway.

[impyre]

Well, a couple things.

1) Yeah, early career is tough. You have to redesign often as better parts become available, reaction wheels, elevons/wings, separators. But launchers generally only require a handful of parts, and once you unlock those you're pretty much set.

2) As Garius pointed out, TWR issues can be a huge problem... *especially* during launch. So how much dV drops during ascent is *not* linear. It depends on TWR, ascent profile, and payload mass.... *not just payload mass*. TWR can have a huge impact if it goes too low.

3) There's not really a good reason to be concerned with how much dV will be left after attaining orbit. Once in orbit, payload should be detached (IMO). This allows those stages' dV's to be calculated accurately for vacuum only operation (which is appropriate for most space operations, and much much simpler than concerning yourself with atmospheric dVs). Once the payload is detached, it's a simple matter. The remainder of the launch vehicle can be recovered if you're clever and compensate for that in your design process.

And lastly, 4) There's no quick answer since everything depends on how you build and how you fly. You just have to keep doing it until you get a feel for it... but be aware that trying to operate outside what might be considered "nominal" can have unpredictable (and often *fun*) results.

Sorry if I haven't helped out much. I'm just not sure there's a satisfactory answer I could give you that would be correct. Calculating vacuum dV and comparing to that may help you get a feel for it over several launches.

[ajburges]

Early career is rough for running a structured fleet. This is where I let my artist out (with KER riding shotgun for analysis)

I have a payload,

Slap an upper stage for .5-1 Km/s vacuum. (Unless the payload is a suitable upper stage) TWR is not terribly important, but keep it high enough to circularize in 2 min (.3 should be fine)

Engineer a lower stage (or set of them) for a total dV of about to LKO with the upper stage (3.5 Km/s these days). TWR is important for these.

Slap some boosters so the previous rocket gives me some change in dV to play with. If possible, tweak the for better TWR though their full burn. TWR of 4 near cut-off is fun, but can cause issues.

Strut that thing so it don't blow up.

[egneil]

Thanks for all the help. I guess I'm just trying to build my rockets too small, as they can barely break 70~75km altitude straight up. Mostly I've been trying to keep the cost of my rockets on par with the pay of the tourism contracts because they have such a static payment. (All sub-orbital flights give 13k per kerbal, and I think mun-orbital give around 22~23k per kerbal.) I'm not trying to limit my cost too much, but I would like to make a net profit from every launch.

[Northlight]

Calculating the size of a rocket involves much more math than calculating a rockets ÃŽâ€V.

The main equation for it looks like this:

m_prop = Mass of propellant (fuel)

m_pay = Mass of payload. Everything except fuel tanks.

f_i = How much of fuel tanks are not fuel. For tanks in KSP this is 0.11, execpt the T100 (0.13) and ROUND-8 (0.14)

And that is just for one stage. With a singe engine, or multiple of the same engine.

[adinfinitum]

If you're having trouble netting a profit from tourist contracts, try waiting until you have multiple tourists with the same destinations, or ones that overlap. For example, Dudley Kerman wants a sub-orbital flight, but Lazie Kerman wants a Kerbin orbit, both can be completed in the same flight. I've found that it's more cost effective to take multiple kerbals on a flight than launching separate ones.