Mun Orbit and Back - Fuel question w/ video

[mTecNic]

Hey guys!

I've been lurking in here for a few weeks and now decided its time i need your help. **Disclaimer** i have mechjeb installed only for data. **Disclaimer**

This game is incredible and I've been able to make orbit about 8 times now. This last time I recorded a video because i was hopeful and turns out it was for a good reason. I've left myself with fuel in orbit!

Does anyone mind watching the video and letting me know if you think i have enough fuel for a munar orbit and back to kerbin ? Also if someone wants to critique my launch vessel or help so i can end with more fuel I'm all ears!

Thanks a million!

mTecNic

Edited March 5, 2015 by mTecNic

[eddiew]

You seem to be on a highly inclined orbit, which unfortunately will make the delta-v requirements extremely high. Mun is on an equatorial path around Kerbin, and you've got a lot of polar velocity you need to remove (think of it as having to undo about 1000m/s going north and add 1000m/s going east).

From an equatorial orbit, you'll need about 850 to reach Mun's SOI, and 250 to circularise around it. ~300 would get you back from there, leaving a grand total of about 1400 required. But I think with that inclination, you'd be talking double that, perhaps more.

For future missions, try to stay as much in the plane as possible to help minimise your fuel requirements (Also your rocket really is overkill for a Mun orbit, if you don't get knocked off course.)

[Harry Rhodan]

All those SRBs give your rocket a massive amount of torque and you SAS can't handle that and that's why you can't get a decent orbit heading to the east. With a TWR of more than 5 you also lose most of the delta v from the SRBs to drag. Try to aim for something around 1.5 or 2.

[Reactordrone]

Tweak down the thrust of the SRBs in the VAB and you won't be fighting drag as much. You should also be starting your turn much earlier as you were only at 45° by about 63km instead of 12km. By 40km you should be pretty horizontal. Once in low orbit you'll need just under 900m/s to get to the Mun's orbit and another 270m/s to capture followed by another 270m/s to return to an aerocapture at Kerbin. With that ship you'll definitely be able to do a flyby and with better piloting you might be able to get it into Mun orbit and back but you'll beed a bit more if you want to try for a landing..

[Mallock]

Holy boosters.

You ended up with more than enough delta-v to make it to the Mun and back ... if your orbit wasn't so inclined. I can't build a demo rocket to serve as an visual aid right now, but dial back those boosters to only 4 in your first stage. For the rest of the rocket use liquid engines and the starter engine (whatever it's number is). Give yourself 4500 delta-v in the VAB according to mechjeb. Note that your first couple of stages you should use the "atmosphere" delta-v values from MJ and your last one will use the "vacuum" values. Don't just total up the vacuum amounts. When you launch go straight up to about 10k meters and then start tipping over slowly to a heading of 90 deg (due east).

You're also wasting delta-v by circularizing too early. Don't start burning horizontal right away when you leave the atmosphere (70k), wait until you're closer to your apoapsis. With an early rocket like this and assuming a TWR of 1.2-2 you can start burning when you're only 5-10 seconds away from your apoapsis. That will leave your orbit nice and circular at 100k instead of elliptical from 100k-160k like the video shows. After all that you'll need an additional 1400 delta-v as eddie says just to orbit the Mun and return. Minmus would just be another 100 delta-v if I remember right, but it brings extra challenges since it's inclined.

[Xannari Ferrows]

Do you want an equatorial orbit? You don't need one. Try an encounter first, and an inclination change once you're in orbit. You'll save so much of that Delta-V.

You can make it to the Mun, but only with real fuel efficiency, and no landing, of course.

[Pecan]

MJ tells you the deltaV you have and you know what you need ... as eddiew said, it'd be a tight call but it might be doable. Best to aim for an Apollo-style free-return trajectory and settle for a fly-by rather than orbit, I'd say.

For future missions:

Much less low-altitude thrust - the white atmospheric effects show you're going way too fast (any faster and they'd become red plasma effects, as in re-entry).

Lose the SRB second stage - they might be cheap but they're heavy and uncontrollable so only use for a first stage, if at all.

Start and finish your gravity turn much earlier - be flat by the time you've got an apoapsis around 60km, using more thrust to get the horizontal speed you need for orbit.

Read any tutorial on how to get to orbit - 'Induction To Construction' has the standard newbie-rules in Chapter 6, although it is intended as a guide to the screens and controls rather than flight.

Look in the drawing board (link also in my signature) for tutorials that are intended to teach how to orbit, and orbital manoeuvres.

On the other hand ... there's no wrong way to have fun. Congratulations on managing to get to orbit repeatedly and reliably. It's quite an achievement in itself, and doing it in a ship you designed is waaay better than using anyone else's :-)

[Kryxal]

People have already pointed out how poor the ascent was ... drop the booster thrust to a third of what it currently is via tweakables and you'll have a much nicer ascent, nudge the rocket over a few degrees around 2000m and set SAS to prograde mode once the marker catches up and your flight path should just happen a LOT better.

As for going to the Mun, you don't need to do a full-up plane change, just start your transfer burn at the AN or DN (and doing a partial plane-change then WILL make your orbit later a fair bit easier, always combine maneuvers if you can reasonably do so).

[Yasmy]

On the other hand ... there's no wrong way to have fun. Congratulations on managing to get to orbit repeatedly and reliably. It's quite an achievement in itself, and doing it in a ship you designed is waaay better than using anyone else's :-)

Seconded. Cheers!

[WARodst10175]

Just use Hyperedit. Works all of the time

[MarvinKitFox]

With this current launch of yours, with your current level of skill, I would not advise it.

A Mun flyby is quite doable, but there is very little margin for error.

Using the same basic design a bit more optimally, you could do it easily.

You are burning a *lot* of your launch thrust in punching a hole in the air. Slow down a bit. If you ever see visible speed lines while ascending, it means you are WAY over speed.

to do this:

Drop the thrust on those SRBs. You need less than half the thrust.

With less thrust on your launch, you will actually achieve a better resulting speed by the time your first stage burns out.

The reduced thrust will also allow your stability controls to keep a better grip on the launch. It looked like the slightly off-straight power from those SRB's was giving you a hard time.

[Navy2k]

Hi,

would have liked to help you but your video is blocked in germany due to your music choice. =(

(German copyright agency wants more money than youtube wants to pay)

I heard about it not being feasible from a highly inclined orbit to reach Mun. But you can try to just change the Apoapsis to munar orbit height and wait until mun captures you, possible even as a free return trajectory.

Or with a TWR of 5, as i heard you have, you could always bring more fuel with you in the last stage

Greetings

Ben

[ATSinged]

What everyone is saying about your early thrust level and ascent profile is spot on. Less thrust and an earlier turn will get you far better results.

As far as the Mun, a free return mission carrying a couple of goos and a materials lab can be a boatload of science, particularly if you add EVA and crew reports, it's great for kick starting a career. I never quite hit the pure free return trajectory but my last one required only 2 very short correction burns. If you go that route once or twice, you'll learn a lot about mission planning and the science to actually land and orbit will be yours in no time.

[Zephram Kerman]

After twelve replies, what more could be said? Well if you're willing to do a little research, check out the Goddard Dilemma. Everyone's suggestion to slow down your ascent are based on Robert Goddard's real life research. It turns out that an optimized ascent is speed-limited to terminal velocity.