Delta-V and how it works?

[danagor]

Hi, i'm new to the game and i have been reading the wiki a little, seen some vids, do some (explotions!!!... ehem...) testing with my rockets, but i can't make it to the Mun, land and return to kerbin, i have done a lot of math to get TWR, Delta-V and all that jazz but i don't understand why i can't make it, i'll try to describe everything so you can help me oh and i'm playing on carrer mode it's sweet!!!

Stage I:

Mk16 Parachute, Pod Mk1, 2x Z-100 Bateries, FL-R25 RCS Tank, 2x Goo things, 4x RCS Thrusters, 4x LT-1 Landing Struts, 5x Pegasus Ladders, LV-909 Engine and a FL-T400 Fuel Tank

Delta-V= 1969.795

TWR = 1.025

Stage II:

Stack Decoupler, FL-T200 + FL-T400 Fuel tanks and a LV-T30 Engine

Delta-V= 1761.053

TWR= 2.964

Stage III:

Stack Decoupler, FL-T800 Fuel Tank and a LV-T45 Engine

Delta-V= 1837.362

TWR= 2.025

Stage IV:

4x Radial Decouplers, 4x AV-r8 Winglets, 4x Nose Cones and 4x Rockomax Boosters

Delta-V= 2587.647

TRW= 3.437

Total Delta-V= 8031.710

The Isp i used on the Stages II and III was and averge from Vacum / Sea level

Here a photo:

https://skydrive.live.com/redir?resid=5FB025AC43809FB3!439&authkey=!AIcnYHunk9Mfw-k&v=3

And that doesn't do the job... i just get to make a round orbit and some that's it... please help me i don;t understand what i'm doing wrong... thanks in advance

[vexx32]

Moved to Gameplay Questions and Tutorials.

[thiosk]

WELL.

What I'd like to see now is your ascent profile.

I have an intuitive feeling that you SHOULD be able to reach Mun with a craft like that.

I don't like the choice of SRBs rather than liquid tanks. What happens is that your thrust to weight ratio drops precipitously as soon as you drop the SRBs. You probably won't be above 10 km by then, and then the single engine doesn't have enough thrust to keep you accelerating as you get above the atmosphere. Before long, you're coming down, and no amount of thrust can keep you up.

While I have the impression that asparagus is falling out of favor for being a bit on the cheap side,

watch this tutorial.

[Specialist290]

First off, welcome to the forums!

(EDIT: There are days when I just can't read. Meh...)

Do you think you could describe your flight plan from Kerbin's surface into orbit? About when do you start to tilt it over to extend your orbit sideways (relative to Kerbin's surface), and how far up are you? Around what altitude do you drop each of your stages, how fast are you going when that happens, and what does your trajectory look like at those points? What are you left with when you do reach orbit? The info we can get from answering these qusetions might help us find ways we can improve your flight plan as well.

Edited November 8, 2013 by Specialist290

[numerobis]

That stage is the lander and return vehicle. It's got TWR 6 with respect to Mun; more than I'd prefer.

Spacecraft looks fine; you have excess thrust for the first minute, though, so you'll be losing more deltaV to drag than you might prefer. A pair of BACC around a T30 in the core might work better, since that will let you throttle appropriately.

But you didn't tell us your flight plan, which is more likely the problem.

Edited November 8, 2013 by numerobis

[Superfluous J]

Actually that stage 1 is fine, it's the top of the rocket.

Your problem is the solids. For a rocket this sized you're far better off with liquids. TWR of 3 is way too high to be efficient. You hit terminal velocity and start burning WAY too much fuel fighting atmosphere.

[Specialist290]

D'oh, y'all are right. Somehow I read it from bottom to top, instead of the other way around.

The bit about the flight profile still stands, though.

[numerobis]

Probably because stage 1 for all rocketry outside KSP is the launch stage, as opposed to the penultimate stage? I had to reread to get the order right myself.

[zxczxczbfg]

My recommendation: Use liquid boosters with fuel lines that extend from the boosters to the center stack, with the boosters and center stack on the same stage. That way, the fuel that gets used up by the center engine gets refilled from the outer tanks, and when the boosters run out they will be jettisoned, and your center stack will keep burning because it was kept full by the flow from the boosters. IN SHORT: liquid boosters with fuel lines flowing inwards.

[Vanamonde]

Your launch and Mun stages look reasonable enough, but I believe the intermediate stages will not burn long enough to do the job you want. Is this the latest version of player-made delta-V maps?

Close enough, anyway. Add up the numbers between where you are and where you want to go.

Also, for this kind of mission, the RCS tank and thrusters are weight you can do without.

If you would like an example career moon rocket you can download, the third one in this thread is similar in size to yours, and is made out of parts from tiers 0-3.

[capi3101]

OP: I'll have to check your math on your delta-V; for now I'll take your word for it. Presumably you've used atmospheric Isp for the bottom two stages and vacuum Isp for the top two, right?

So...flight profile. Straight up to 10k, then 45 degrees elevation on course 090 and then follow the navball as it comes down. When you're more than 55 seconds to apoapsis, burn along the horizon until your projected apoapsis is where you want it. kill your burn, wait until your craft is above 70k, then plan a maneuver at apoapsis that will circularize the periapsis. All the while keeping the craft's gee meter near the top of the green zone and throttling back as necessary (to keep the craft around that magic 2.2 TWR). Start your burn for circularization when the time to the maneuver node equals half the total time of the burn; minus a second or two is still fine.

Is your flight profile to orbit anywhere close to that?

[danagor]

Waoh so many answers you people are great thanks a lot for all the help here i go with some more info:

Boosters get me around 8-10k then i start my turn around 12k i aim for an orbit of 90-100 plan my circularization at the apoapse, i start to burn round 15-30 before eta, usually that stage dies after 50% of the burn i have to use my lander to finish the circularization, from there i just target the mun and i can make it there but not with enough fuel to land...

I did try with liquid boosters got to the mun and i'm able to land but i go throu all my fuel while landing, i guess i need a landing tutorial for the mun i guess i'm wasting fuel...

Thanks again for the help everyone

[Superfluous J]

Your ascent sounds fine, at least without seeing a video I can't give you any more help

Landing, you ideally want to bring your ship down to either a shallow crash on the surface, or periapsis right above where you want to land. F5 to save and then when you get close to your burn point, burn like mad and hope you don't crash. If you do, figure out why you crashed, hit F9 to reload your quicksave, and try again. That's a pretty fuel efficient burn.

What you do NOT want to do while landing is stop your orbit and fall straight down. That wastes a TON of fuel.

[rpayne88]

Do you have problems getting your apoapsis to the Mun's orbit, or do you just have trouble encountering the Mun?

Edit: didn't even notice the second page.

[danagor]

Your ascent sounds fine, at least without seeing a video I can't give you any more help

Landing, you ideally want to bring your ship down to either a shallow crash on the surface, or periapsis right above where you want to land. F5 to save and then when you get close to your burn point, burn like mad and hope you don't crash. If you do, figure out why you crashed, hit F9 to reload your quicksave, and try again. That's a pretty fuel efficient burn.

What you do NOT want to do while landing is stop your orbit and fall straight down. That wastes a TON of fuel.

I did that, i can land on the mun most of the time but i burn so much fuel so i don't crash that i don;t have enought to get back to kerbin... guess i'm not doing the landing right... but hey i landed i did it!!!

[capi3101]

Ah - okay...looking back at your lander design, I see that you've got a very basic craft (what I sometimes call a "Phallus 7", if you'll forgive me for being crude). Most folks start off with a design like that - a Mk16 Chute, a Mk1 Pod, an FL-T400, an LV-909, three or four lander legs and a ladder (with panels and batts later as they started to be required). With that design, your "bingo fuel" level is 80 Liquid Fuel Units remaining; once you're there, hit space or plan on not returning from the Mün.

Don't start a steady burn until you're around 5,000 meters. Better yet, go IVA and watch your radar altimeter gauge. When it starts to twitch, you're 2500 meters over the surface. Keep your speed around 100 until you're a thousand meters AGL, then start burning. 1 m/s per ten meters AGL is a good rough guesstimate as to where you want your speed, at least until you get a better feel for your craft. Be sure to use the radar altimeter - your stage screen altimeter is practically useless when it comes to landing (as you no doubt know because you say you've stuck the landing before).

Now, since you've got fuel lines, let me make a suggestion: ditch the FL-T400 in favor of an FL-T200 centerline with three more FL-T200s outboard, attached radially. Run fuel lines from those outboard tanks to the centerline tank and attach your lander legs to those outboard tanks (use two lander legs if you wish). There are a couple of net effects there:

1) You've effectively doubled the amount of fuel you had available to you (basically you've got the same amount now as an FL-T800, the long small tank)

2) You've widened and shortened your craft, which means it will be less prone to tip over on you when you land. As undulating as the terrain is on Mün that can be very important.

[danagor]

Awesome i did it, i went to the mun and back with some adjustments, i think i understand how things works a little better, my next step or question would be, how to get farther away from kerbin, say i want to land on Duna, and go back, how should i approach that? looking at the Delta-V map on page 1, i need 7490m/s to get there? and how much to go back? if so that means i can use the same rocket to get to duna?...

EDIT: The thing is that, the bigger i build my rockets the less efficient they are, i can't really do better than 8000m/s delta-v any advice?

Edited November 11, 2013 by danagor

[Specialist290]

For interplanetary travel, I'd highly recommend alexmoon's Launch Window Planner to help with both the trip out and the return trip. It's a slightly more accurate calculator than olex's interplanetary calculator, but you can use the explanations on that page to help you decipher what all the terms mean. Note that for planets with atmospheres like Duna and Kerbin, you can actually save a good bit of delta-v by using the atmosphere to brake into an orbit, a process you'll sometimes hear called "aerobraking."

Let us know if you run into any snags

[capi3101]

For interplanetary flight, you're going to want to be using the LV-N Atomic Rocket Engine; only reason why you wouldn't want to use it is if you're doing something for a challenge. It's an efficient engine (800 Isp) that produces a reasonable amount of thrust. Build your transfer stage like this:

Javascript is disabled. View full album

This way you avoid having to deal with those pesky LV-N shrouds that knock off pieces of your rocket when you activate the stupid things.

The delta-V map that Vanamonde posted in the first page of this thread is your friend; use that to plan, and add more than what it says because unless you're using flight assistance mods you're going to muck it up. Go with 125% of the values listed.

And plan your mission backwards. Let's say you want to do a Duna landing. So you need to design a lander that can go down and come back up again, minimum. Will it need also need to transfer back to Kerbin, or are you going to include a dedicated tug that does the transfer that the lander will dock to? You pick out the pieces of what you're going to need for the craft - command part, chutes for return, panels/batteries or RTGs, lights, lander legs, decouplers, engines, RCS and docking ports if you're going to dock it to the transfer stage - add sum up the mass of all that, everything but the fuel tanks. You've got a delta-V target for your craft, and you've got a "deadmass" of all the necessary equipment. You then work Tsiolkovsky backwards, adding that dead mass to both mass terms in the equation. You can make the assumption that M=9Md, because the way all but two of the liquid fuel tanks in KSP are designed (the exceptions being the Oscar B and the Round 8), the full-to-dry ratio is 9:1. Solve the equation for dry mass, then use that same relationship (M=9Md) to determine how much fuel you need to include for the delta-V you want that part of the mission to have. And then you pick out a combination of tanks that gives you roughly that amount. Just keep on doing that for each stage.

If and if that sounds like too much mass, wing it; many KSP players just do it and pray for the best. Sometimes they get lucky, other times they don't.

You might also want to check this out; its a discussion on the merits and proper use of asparagus staging. For now, it remains the most efficient way of lifting payloads in KSP.

And if all else fails, there's these.