[Tutorial] A Guide to Basic Kerbal Rocket Design Through Rocket Science.

[Ziff]

I'm particularly interested in how T:W ratio affects the achievable altitude...that might take some testing as well.

If you need any help testing I would be happy to help out. So far I've found a T:W ratio of at least 2 to be most beneficial for launching, but not sure what you mean by how it affects achievable altitude.

[Vanamonde]

Wait. Are you telling me I did the math right? Seriously? I think I may faint. Score one for hazy memory and blind fumbling!

Geez, I was so excited about the math that I forgot half of what I wanted to say. In my figures, payload for a stage includes the actual parts the mission needs, but also everything that isn't fuel, such as the engines, SAS, etc. And then each subsequent stage just uses the full weight of the cumulative stages above plus its own engines, struts, fins, etc. as its total payload, Sound reasonable?

not sure what you mean by how it affects achievable altitude.

If I understand correctly, lower thrust would linger in atmo and the heavier gravity of lower altitudes longer, thereby making less efficient use of fuel, thereby running out at a lower peak.

Does the throttle setting affect an engine's Isp? In the real world I would think so, but I'm asking about the game's simulation.

Edited August 17, 2012 by Vanamonde

[quarthinos]

Wait. Are you telling me I did the math right? Seriously? I think I may faint. Score one for hazy memory and blind fumbling!

Geez, I was so excited about the math that I forgot half of what I wanted to say. In my figures, payload for a stage includes the actual parts the mission needs, but also everything that isn't fuel, such as the engines, SAS, etc. And then each subsequent stage just uses the full weight of the cumulative stages above plus its own engines, struts, fins, etc. as its total payload, Sound reasonable?

If I understand correctly, lower thrust would linger in atmo and the heavier gravity of lower altitudes longer, thereby making less efficient use of fuel, thereby running out at a lower peak.

Does the throttle setting affect an engine's Isp? In the real world I would think so, but I'm asking about the game's simulation.

Except for the altitude change, Isp isn't changed much by throttle setting in the real world, and doesn't change at all in KSP. Remember Isp is a measure of efficiency, so it's mostly dependant on propellant and the shape of the rocket nozzle. It varies with altitude on the non-spikes because a nozzle is optimized for a certain altitude in atmosphere (or for vacuum, of course). Aerospikes were made to give the same Isp regardless of altitude, which is the main way a given engine's Isp varies IRL.

Edited August 17, 2012 by quarthinos

[Major-Major]

In the current release, the fuel flow is bugged. Cut your power to 50%, and your thrust will decrease by half...but your burn time will increase by a factor of four. So you've effectively doubled your engine impulse, which is not how it's supposed to work. Try it--build a rocket with a very high T:W ratio, then launch at the minimum allowable throttle setting. If you can launch at, say, 25%, the delta-V you'll achieve will be 10-16 times higher than what it 'should' be.

So yeah, it's necessary to do everything at full throttle for now, at least as far as launching.

[Vanamonde]

I didn't realize the fuel bug was an issue that high up the throttle settings. I've been cheating without realizing it, and now have bad habits to break.

An aerospike is similar to a parabolic engine bell turned inside-out, right? It's the same curve, but it forms an inner core rather than an outer shell, if I understand correctly.

[quarthinos]

I think an aerospike is hyperbolic. It's engineered so that it's atmospheric pressure acting as the nozzle, rather than some fixed piece of metal. A quick look at wikipedia doesn't say what mathematical shape is used, although the article mentions at least three different ways to do it: A linear wedge (pictures), a torus, and a literal spike (both written). Reading wikipedia at bit more in-depth shows that in the late 90s NASA built a couple for the X-33. It's Isp was 339 at sea level and 436.5 in vacuum. Looking at a spec sheet for the original J-2 (the guts of which were reused for the X-33 prototypes), it got 200s at sea level and 421 in vacuum. So the aerospike was 70% more efficient at SL and 4% more efficient in vacuum. (Efficiency being amount of thrust generated per unit of mixed propellants).

And the throttle bug is pretty fierce. Either build with 100% solids, or keep the throttle all the way open all the time.

Edited August 17, 2012 by quarthinos

[Major-Major]

Using a 3-engine, 9-tank rocket, which can make it 300k straight up but doesn't have enough energy to come close to an actual orbit...I was able to escape Kerbin by keeping the throttle as low as possible. That's when I realized there must be a bug. I still have that little guy orbiting the sun, as a cautionary tale.

[Ziff]

There is a fix for the throttle fuel bug, I have been using it for awhile now with no adverse side effects so far. http://kerbalspaceprogram.com/forum/showthread.php/16439-0-16-0-Hacky-Fuel-Consumption-Bug-Fix?highlight=hacky+fuel

[2008dragon]

"u= Gravitational Parameter of Parent Body. (3530.461 km^3/s^2 for Kerbin).

r_1= The Radius of our first orbit. (100 km in this case).

r_2 = The Radius of our second orbit. (200 km in this case)."

(Quote was taking long for the long post )

Shouldn't the radii be 700km and 800km because of kerbin's 600km radius? When I use 100km and 200km in the equations it says I need 1700 m/s to circularize at 200km, but 700km + 800km gives me 150 m/s (Correct?)??

EDIT: Yea, it should be 700km/800km because later in the post it says " In a 100km orbit, our radius will be 700km."

Edited August 18, 2012 by 2008dragon

[Vanamonde]

Math and I weren't on speaking terms for a few days. I asked it how many tanks of fuel it would take to put my station in Mun orbit, and 2 hours later it had said 84 one time and -11 another time.

But we patched things up last night, and I flew my first methodically designed 3-man mission to Minmus and back. It was still quite inefficient: I think because a round-up in the calculations at the upper stages is magnified in each subsequent stage. However, it usually takes me several unsuccessful iterations to get as close as this one did on its first test, and it was kind of cool to watch a stage be exhausted and discarded just at the point in the flight where I intended it to happen. There are additional benefits to the methodical approach; it's surprising how much easier it is to steer your rockets when they're no bigger than they need to be for the job.

Which is kind of a roundabout way to say thanks for the work that went into this thread, and math is a good thing.

[Enture]

There are additional benefits to the methodical approach; it's surprising how much easier it is to steer your rockets when they're no bigger than they need to be for the job.

So true! I decided to put some real math and orbital physics in my rockets' conception yesterday (+actually being a physics student, it really can do no harm, can it?), and moments ago the first result of it, my Amandil-2X test flight, splashed down east of KSC: after having been brought to LKO by a rocket designed just to do this, and not more, this spacecraft consisting of a 3-crew pod and a single 1600L fuel tank + 2m 909-engine went all the way to a 5km-high circularized Munar orbit and back, with 100L of fuel to spare! Wouldn't I have done the math before that I wouldn't have believed it.

EDIT: in fact, for my ∆v-budget, I used the 900m/s for TMI mentionned in the first post, but then it was all approximations to figure out how much ∆v I might need to achieve the 5km Munar orbit. Turned out to be a good approximation, but I was wondering yesterday after reading Kosmo-not's interplanetary travel planning guide: is there a way to calculate "precisely" the ∆v required to go from a given Kerbin orbit to a given Mun orbit, or are there too many factors to consider?

Which is kind of a roundabout way to say thanks for the work that went into this thread, and math is a good thing.

Yep, thanks, and keep up the good work!

Edited August 23, 2012 by Enture

[AncientAstronaut]

Hey everyone. I'm going to try and get to all the questions here. Obviously this is a somewhat complicated subject for a lot of people and there are a lot of questions.

[AncientAstronaut]

[Edit] Hey AncientAstronaut, what's the best way to go about creating a delta-v map anyway?

Well, I do it the nooby way. I just fly the mission and log my remaining Delta-V after key points.

Orbit, TMI/TmI, Orbital Insertion, Landing, Ascent into Orbit, and then return.

But you can also do it based on Hohmann Transfer Delta-V calculations. That requires a lot more math and probably won't be as precise. You're better off flying the mission and keeping track of the Delta-V/

[Enture]

I've got yet another question, AncientAstronaut: this time concerning thrust: thrust is the force created when firing our rocket engines. Good. At liftoff (let's say on a body without atmosphere), it has to be greater than the weight of my rocket to actually leave the ground: Thrust>m(rocket)*g(body). BUT then I should never manage to lift off a 30t lander from the Mun using only 3 stock radial engines: the engines give me 80x3=240N thrust, while my munar weight remains 30000x0.166=4980N! And yet I can do it. (Same thing with all the rockets I applied the calculation to).

So am I missing something obvious, or are the KSP thrust values given in another unit, like kN or something, or...? Would be handy to be able to do the calculation, as we could then design really minimalistic landers without running the risk of stranding Kerbals everywhere...

[Cykyrios]

If I'm not mistaken, thrust is indeed given in kN, while mass is in tonnes. So you have a 3x80000=240000N thrust for your rocket. At full thrust, I guess that thing takes off pretty fast, doesn't it?

[Kosmo-not]

Mass: 30 tonnes

Munar weight: (30 tonnes)*(1.628 m/s²) = 48.84 kN

Thrust: 240 kN

TWR on Mun: 4.91

[Enture]

So those thrust indications were kN indeed, it explains part of my problem. But of course, if I use 0.166 instead of 1.628 for Munar gravity acceleration, I won't get very far (was mistaken by the 1.628m/s^2=0,166g thing)...

Well, thanks for the clarification! Back to designing my minimal Minmus lander now...

EDIT: @Cykyrios, yeah, this lander's ascent stage is *slightly* overpowered... It did save at least 9 Kerbals' lives though, when operating emergency landing aborts at extremely low altitudes.

Edited August 24, 2012 by Enture

[ilithyia]

Since this guide is really good, I took the liberty to copy it to the KSPwiki and adjust the formulas to the LaTeX math mode there. I hope you don't mind.

A Guide to Basic Kerbal Rocket Design Through Rocket Science

[tankcommando12]

i just used your rocket as a simple test to orbit kerbel. i then saw that i had enough fuel to orbit the mun and return. as i speeded up time i managed to pass the mun with only 10000 km from the surface its gravitaion changed my course and trowed me out of the planets system. i then saw that i have now managed to orbit the sun. i still got 20% fuel left from the final stage. Awesome rocket!!!

[MrHallESQ]

I cannot figure out the formula for the orbit transfer. If someone has time could you walk me through the formula with say a r1 of 150km and an r2 of 700km? thanks in advance

EDIT: figured it out, no worries

Edited September 3, 2012 by MrHallESQ

[MrBailey]

Just a heads up. The KSPwiki page with this info seems to be down...at least for me.

[JackAz]

Great job. This helped alot, thanks.

[Vanamonde]

I hope this very useful thread is going to be updated with delta-Vs for the new worlds. I don't trust my own ability to figure them out.

[Vanamonde]

I've been getting good results with a certain stage layout, but I don't know how to calculate its delta-V.

In the pic, the inner stack and outer stack engines are activated at the same time, but the outer stack is not only consuming fuel, but pumping fuel into the inner stack at the same time. When the outer stack runs dry I eject it, leaving the inner stack running. In effect, the outer stack is acting as a droptank for the inner stack, but it's a drop tank with an engine that supports its own weight and more. Meanwhile, I'm getting full use of the inner stack engines at the same time, but it still has full tanks when the previous stage is ejected. So there are two stages, but they overlap rather than acting sequentially.

Can I simply I treat it as one calcuation as the whole assembly, then re-do the figures as a separate stage of just the inner stack, and add the two?

[bsalis]

Lateral staging is no different to horizontal staging in terms of delta-V. Simply full:dry mass per stage as usual. Using the properly weighted ISP of course.