Why did my rocket not launch???

[Loren Pechtel]

I'm trying my first orbital mission.  Sitting on the pad I'm seeing a TWR of 1.59 (from MechJeb), throttle to full, launch and it sits there.  As the engines burn the TWR goes above two--but it still doesn't budge.

As an experiment I put on 6 boosters around the core of 4 I originally was using.  This time it sat on the pad for 13 seconds then lifted off--but not crawling into the sky like I would expect if it had just reached enough thrust to lift.  Note that this design lifted at a lower TWR than the previous one still sat on the pad.

It also goes totally wonky at about 7000 meters.  I tried adding more fins, no change.  SAS on, I'm not even trying to do a turn.

3 tries, the same behavior each time. 

 

This game has a learning curve about like real rocket science!

[FullMetalMachinist]

The best thing you could do in this situation is to post either a screenshot of your rocket (or a few, like one in the VAB with relevant markers shown & mechjeb info open, one on the launch pad, one in flight, etc.) or post a link to the .craft file so other people can try it out for themselves and give you relevant advice.  Otherwise you'll get a bunch of vague, generalized advice that may or may not help, and will probably take a lot longer and a lot more trial and error on your part to finally get a solution.

[Snark]

+1 to Machinist's suggestion. There are a whole lot of things in KSP that could potentially go wrong, and a picture's worth a whole heap of words.

 

[Foxster]

You don't happen to have selected a body other than "Kerbin" in the MJ panel that displays TWR?

[KerikBalm]

and you released any launch clamps?

[Streetwind]

There's also still that old bug where sometimes rockets get stuck to the tier 2 launchpad (but not the other tiers).

Try using launch stability enhancers to lift your rocket slightly off the pad.

[Corw]

Could be vacuum TWR (e. g. trying to lift off with nukes...)

[Wemb]

My best guess would be that, if it is lifting, but extremely slowly, and with only thrusters attached, that the TWR is being displayed for a different body. E.g. Minmus has a gravity of 0.05g - so the TWR will be 20x times that of a rocket on the Kerbin launch pad. Ensure whatever tool you're reading is set to Kerbin. Otherwise, have you set the thrusters to full power? (probably not relevant if you're seeing this issue with SRBs too) - and, yes, a picture helps.

And what do you mean by 'it goes wonky'?

Wemb

[Sir_Robert]

Yea we're going to need more information about your exact ship and procedure before we can help you

[Starwaster]

5 hours ago, Streetwind said:

There's also still that old bug where sometimes rockets get stuck to the tier 2 launchpad (but not the other tiers).

Try using launch stability enhancers to lift your rocket slightly off the pad.

^^^^ this ^^^^

If your engine bells are too close to the pad or embedded in it, they won't provide thrust.

[Chaos_Klaus]

note that engines have reduced thrust when at sea level atmospheric pressure. Don't know about MJ, but KER has a button to switch between vacuum and atmospheric TWR readings.

[Sir_Robert]

8 minutes ago, Chaos_Klaus said:

note that engines have reduced thrust when at sea level atmospheric pressure. Don't know about MJ, but KER has a button to switch between vacuum and atmospheric TWR readings.

Be default, MJ shows TWR and Sea level TWR (which is the relavent when launching, and updates based on your current situation)

[jedensuscg]

33 minutes ago, Sir_Robert said:

Be default, MJ shows TWR and Sea level TWR (which is the relavent when launching, and updates based on your current situation)

Ya by default Mechjeb shows just TWR (does not specify) but it is sea level TWR, it also shows max TWR which is the TWR right before engine shutdown accounting for the loss of fuel.  

I would guess the author might have selected a different planet, however, if he is seeing the TWR on the pad, he is probably a victim of the T2 launch bug, because the real time TWR readout during launches is for your current location (to my knowledge) and the editor is the only place to change the location and only effects the readouts while in the editor for planning purposes.

Again SCREEN SHOTS!

Edited February 16, 2016 by jedensuscg

[Loren Pechtel]

12 hours ago, Streetwind said:

There's also still that old bug where sometimes rockets get stuck to the tier 2 launchpad (but not the other tiers).

Try using launch stability enhancers to lift your rocket slightly off the pad.

 

Yup, that was it.  I put some launch clamps on and it took off like it should.  Now to figure out why it wants to start spinning around at about 7,000m up.  SAS on, I didn't touch the controls after launch and yet this happens:

 

That was the second spin, I didn't manage to grab a snapshot at the right instant the first time around.  I added a second reaction wheel in case there wasn't enough authority in the first one but it did exactly the same thing.

[FullMetalMachinist]

Are you playing career?  If so, I would do my best to unlock the tech node that gives you the AV-R8 winglet. Replace all those basic fins with it (actually probably not a straight replace, I would get rid of all the Basic Fins and put one winglet on each side booster).  It's steerable, which will help greatly with stability issues.  Also, if you're using the LV-T30 "Reliant" I would switch them to the LV-T45 "Swivel", which has engine gimbal, which will also help.  You're kinda setting yourself up for failure though.  Those crew cabins are really light and draggy, and the back end of your rocket is really heavy with all those engines and fuel.  Any way you can redesign to give it a more "arrow" like shape (i.e. straight, with weight at the front, and draggy fins at the back)?

On a related note, what does your launch flight profile look like?  You mentioned that you flip even if you don't touch the controls.  That could mean that you're just going straight up. Which is not the best thing to do.  You can certainly get to orbit that way by going straight up to above the atmosphere then burning sideways, but it's really inefficient.  A better flight path looks like a loooooong, gentle, continuous turn, until you get to orbit.  Basically you start out straight up, but once you're going about 100 m/s or so, start a turn to the east by about 5 degrees.  Then you stop touching the controls and let gravity pull you down into a long, gentle turn.  At 10km you should be around 45 degree tilt, at 30km around50 or 60, and by 50km almost all the way over, pointed at the horizon.

Also you said that you flip at about 7km up.  That's potentially a really critical part of your flight.  Depending on your launch TWR (which you said was around 1.5), that could be about the time that you're breaking the sound barrier.  Once you do that drag goes WAY up, and if your rocket is a little unstable, things could very suddenly go bad.  I would suggest adding a bit more fuel to those side boosters to get your launch TWR down to something like 1.25.  That will keep you from going supersonic until you're above 10km where the atmosphere is much thinner.

Hope this helps, and let us know about any other questions you have.

Edited February 17, 2016 by FullMetalMachinist
Darn punctuation...

[Loren Pechtel]

26 minutes ago, FullMetalMachinist said:

Are you playing career?  If so, I would do my best to unlock the tech node that gives you the AV-R8 winglet. Replace all those basic fins with it (actually probably not a straight replace, I would get rid of all the Basic Fins and put one winglet on each side booster).  It's steerable, which will help greatly with stability issues.  Also, if you're using the LV-T30 "Reliant" I would switch them to the LV-T45 "Swivel", which has engine gimbal, which will also help.  You're kinda setting yourself up for failure though.  Those crew cabins are really light and draggy, and the back end of your rocket is really heavy with all those engines and fuel.  Any way you can redesign to give it a more "arrow" like shape (i.e. straight, with weight at the front, and draggy fins at the back)?

On a related note, what does your launch flight profile look like?  You mentioned that you flip even if you don't touch the controls.  That could mean that you're just going straight up. Which is not the best thing to do.  You can certainly get to orbit that way by going straight up to above the atmosphere then burning sideways, but it's really inefficient.  A better flight path looks like a loooooong, gentle, continuous turn, until you get to orbit.  Basically you start out straight up, but once you're going about 100 m/s or so, start a turn to the east by about 5 degrees.  Then you stop touching the controls and let gravity pull you down into a long, gentle turn.  At 10km you should be around 45 degree tilt, at 30km around50 or 60, and by 50km almost all the way over, pointed at the horizon.

Also you said that you flip at about 7km up.  That's potentially a really critical part of your flight.  Depending on your launch TWR (which you said was around 1.5), that could be about the time that you're breaking the sound barrier.  Once you do that drag goes WAY up, and if your rocket is a little unstable, things could very suddenly go bad.  I would suggest adding a bit more fuel to those side boosters to get your launch TWR down to something like 1.25.  That will keep you from going supersonic until you're above 10km where the atmosphere is much thinner.

Hope this helps, and let us know about any other questions you have.

 

I do have that winglet, I'll try that and the different engines.

The reason for going straight up was being sure it wasn't anything I was commanding--I know I need to start turning by then but I have managed to spin rockets out by turning so I was eliminating that as a factor.

I made the changes you suggested and got a **very** wild ride to orbit.  Starting at about 2,000m it practically shook itself apart (over-correction from the SAS, perhaps??  Or is my rocket too weak??) and when that calmed down it still shook as I started my turn.  Since I turned late I did a very wasteful profile but I had enough extra that I made it to orbit anyway.

And I'm kicking myself for not putting a material canister on this, I thought I had gotten that data from suborbital flights but now I see otherwise.

And then things went very wrong in the end.  I had quite a bit of fuel in the upper stage so I burned it on the way down to slow down.  Just as it was about to burn out the whole craft flipped to nose first.  I blew off the booster but I could never recover, I was coming in too fast for the chutes.

[pincushionman]

I'm going to go so far as to say you have too much booster and not enough center stack. You have six massive boosters and a center stack that looks to be only as massive as two boosters. And the boosters are stacked tanks.

As a result, the CoM of each booster will move back as fuel drains, and there isn't enough mass in the center stack to "overpower" that action - especially if you're running the center engine at the same time (as its own CoM will also be moving back). Are you running seven engines, or just the six boosters? If you're running the center too, you probably have enough TWR to try it using only the boosters and using the center engine after you drop them.

Picture in the VAB, then drain all your first-stage tanks and compare the positions of the vessel CoM. I'm guessing it starts too far aft and doesn't exactly get better as fuel drains.

Also, describe your turn. If you're trying to turn more than a couple degrees from prograde at 350-ish m/s (sound barrier), you're screwed.

[FullMetalMachinist]

3 minutes ago, Loren Pechtel said:

And then things went very wrong in the end.  I had quite a bit of fuel in the upper stage so I burned it on the way down to slow down.  Just as it was about to burn out the whole craft flipped to nose first.  I blew off the booster but I could never recover, I was coming in too fast for the chutes.

So at this point the thing that was hurting you on the way up, is hurting you in the same way on the way down.  Those two crew cabins are really light for their 1.25m cross section.  That means that on the way up, they're fighting to be at the back, and on the way down, after you've staged everything away, the mk 1 capsule is the heavy bit, and the crew cabins are the light bit that again wants to be at the back.  Which means you're heading in for reentry head first.  Which is not good.  One thing that you can try is to simply put a heat shield on the very top of your rocket.  So instead of the mk16 parachute on top, put a decoupler, then heat shield, then nose cone to cut drag on the way up.  You'll have to use radial parachutes instead of the mk16.  On the way in from orbit, your rocket will want to point prograde naturally, thus putting the heat shield out front (the nose cone might blow up, but who cares?).  After you've gotten past the hot part of reentry, decouple the heat shield, and try to angle your craft so that it has the long side toward the wind.  This will give body lift, and help slow you down enough to deploy your radial 'chutes.  

After you've gotten past this first bit of career, you can use fairings instead of the whole 'heat shield on top of your rocket' strategy.  Though, note that in stock 1.0.5 the fairings have a bug where their lift and drag forces are placed way in front of where they should be, and thus make your rocket very flip-happy. Claw's Stock Bug Fix Modules fixes this, among other things.

[Loren Pechtel]

12 minutes ago, pincushionman said:

I'm going to go so far as to say you have too much booster and not enough center stack. You have six massive boosters and a center stack that looks to be only as massive as two boosters. And the boosters are stacked tanks.

As a result, the CoM of each booster will move back as fuel drains, and there isn't enough mass in the center stack to "overpower" that action - especially if you're running the center engine at the same time (as its own CoM will also be moving back). Are you running seven engines, or just the six boosters? If you're running the center too, you probably have enough TWR to try it using only the boosters and using the center engine after you drop them.

Picture in the VAB, then drain all your first-stage tanks and compare the positions of the vessel CoM. I'm guessing it starts too far aft and doesn't exactly get better as fuel drains.

Also, describe your turn. If you're trying to turn more than a couple degrees from prograde at 350-ish m/s (sound barrier), you're screwed.

The outer boosters are to get me out of the atmosphere, the inner one is a Terrier--not worth lighting until I'm in space.  The COM is basically in the exact center of the outside boosters.

I probably do have too much, I didn't switch back to 4 outer boosters after trying adding more thrust fighting the stuck in the launchpad bug.  I don't understand about not enough center stack.  It's got almost 3000m/s of delta-v and did what it was supposed to do, even covering for my late turn.

11 minutes ago, FullMetalMachinist said:

So at this point the thing that was hurting you on the way up, is hurting you in the same way on the way down.  Those two crew cabins are really light for their 1.25m cross section.  That means that on the way up, they're fighting to be at the back, and on the way down, after you've staged everything away, the mk 1 capsule is the heavy bit, and the crew cabins are the light bit that again wants to be at the back.  Which means you're heading in for reentry head first.  Which is not good.  One thing that you can try is to simply put a heat shield on the very top of your rocket.  So instead of the mk16 parachute on top, put a decoupler, then heat shield, then nose cone to cut drag on the way up.  You'll have to use radial parachutes instead of the mk16.  On the way in from orbit, your rocket will want to point prograde naturally, thus putting the heat shield out front (the nose cone might blow up, but who cares?).  After you've gotten past the hot part of reentry, decouple the heat shield, and try to angle your craft so that it has the long side toward the wind.  This will give body lift, and help slow you down enough to deploy your radial 'chutes.  

After you've gotten past this first bit of career, you can use fairings instead of the whole 'heat shield on top of your rocket' strategy.  Though, note that in stock 1.0.5 the fairings have a bug where their lift and drag forces are placed way in front of where they should be, and thus make your rocket very flip-happy. Claw's Stock Bug Fix Modules fixes this, among other things.

 

The problem wasn't burning up--I got no temperature alarms.  The problem is nose first there's not enough drag--I was going Mach 1 when I ran out of altitude.

[FullMetalMachinist]

3 minutes ago, Loren Pechtel said:

The problem wasn't burning up--I got no temperature alarms.  The problem is nose first there's not enough drag--I was going Mach 1 when I ran out of altitude.

In that case your problem is either that your reentry angle is too steep, or your ship is not wanting to point any direction but prograde.  Meaning that you're either not spending enough time in the upper to mid levels of the atmosphere to bleed off some speed before you get really low, or you're "lawn darting" into the ground, yes?  Ideally you could just keep your engine pointed retrograde and let it slow you down, but I assume you've tried this and it didn't work?

To fix the first problem is relatively easy, just raise your periapsis that you reenter at (you are coming in from low Kerbin orbit, right?). 

The second problem is a little more tricky, and might necessitate a slight redesign of your rocket.  Even if you're not exploding on the way in, I would still suggest trying the 'head shield on top' design.  They are quite helpful at shedding speed as well as heat, and also can give you the chance to turn your rocket away from prograde, exposing the big belly of your craft, which gives you some body lift to counter your speed even more.  To that end, you could try adding the AV-R8 winglets to the bottom most part that you will be reentering with, to give you some more control authority than just the reaction wheels inside the command pod in order for you to, again, point away from prograde to take advantage of body lift to slow you down enough to deploy 'chutes.  Note that adding winglets to the front of your craft may make it more unstable during launch, so be careful. 

[Loren Pechtel]

9 minutes ago, FullMetalMachinist said:

In that case your problem is either that your reentry angle is too steep, or your ship is not wanting to point any direction but prograde.  Meaning that you're either not spending enough time in the upper to mid levels of the atmosphere to bleed off some speed before you get really low, or you're "lawn darting" into the ground, yes?  Ideally you could just keep your engine pointed retrograde and let it slow you down, but I assume you've tried this and it didn't work?

To fix the first problem is relatively easy, just raise your periapsis that you reenter at (you are coming in from low Kerbin orbit, right?). 

The second problem is a little more tricky, and might necessitate a slight redesign of your rocket.  Even if you're not exploding on the way in, I would still suggest trying the 'head shield on top' design.  They are quite helpful at shedding speed as well as heat, and also can give you the chance to turn your rocket away from prograde, exposing the big belly of your craft, which gives you some body lift to counter your speed even more.  To that end, you could try adding the AV-R8 winglets to the bottom most part that you will be reentering with, to give you some more control authority than just the reaction wheels inside the command pod in order for you to, again, point away from prograde to take advantage of body lift to slow you down enough to deploy 'chutes.  Note that adding winglets to the front of your craft may make it more unstable during launch, so be careful. 

That was exactly what I was trying--after my deorbit burn I had about 1000 m/s left and used it on the way down.  Just as it was running out of fuel I flipped and could never get back to tail first.

I put some struts between the boosters to see if that would help the wobbles.  If anything it was slightly worse.  Turning off SAS helps--but my first try that way caused it to flip like in the screenshot.

I managed to wobble my way up, although again with a bad turn.  I ended up with my apoapsis over 400km but the contract only says "orbit", it worked.  This time I jettisoned the engine at a higher altitude (and speed--I wasted even more fuel this time) and only had the capsule during the worst of the reentry.  I was able to keep it tail first.  It also helped that I came down over water instead of a mountain.  All in all I earned about 4x what the rocket cost, around 100 science and 200 prestige.  No comments from my tourists about getting more than they paid for.  (The tourists were only asking for suborbital.)

I wish I could haul tourists around without such a long reentry capsule.

[FullMetalMachinist]

40 minutes ago, Loren Pechtel said:

That was exactly what I was trying--after my deorbit burn I had about 1000 m/s left and used it on the way down.  Just as it was running out of fuel I flipped and could never get back to tail first.

I put some struts between the boosters to see if that would help the wobbles.  If anything it was slightly worse.  Turning off SAS helps--but my first try that way caused it to flip like in the screenshot.

I managed to wobble my way up, although again with a bad turn.  I ended up with my apoapsis over 400km but the contract only says "orbit", it worked.  This time I jettisoned the engine at a higher altitude (and speed--I wasted even more fuel this time) and only had the capsule during the worst of the reentry.  I was able to keep it tail first.  It also helped that I came down over water instead of a mountain.  All in all I earned about 4x what the rocket cost, around 100 science and 200 prestige.  No comments from my tourists about getting more than they paid for.  (The tourists were only asking for suborbital.)

I wish I could haul tourists around without such a long reentry capsule.

Sounds like you were able to more or less brute-force your way through that contract & orbital experience.  That's a big part of this game, trying different stuff to see what works and what doesn't.  I'm glad that you were able to end up with a favorable result.  And sorry that I couldn't give quite good enough advice to make it easier for you. If you have any other specific troubles, start a new thread, or keep adding to this one, and those of us on the forums will do our best to help you out.

[Loren Pechtel]

8 minutes ago, FullMetalMachinist said:

Sounds like you were able to more or less brute-force your way through that contract & orbital experience.  That's a big part of this game, trying different stuff to see what works and what doesn't.  I'm glad that you were able to end up with a favorable result.  And sorry that I couldn't give quite good enough advice to make it easier for you. If you have any other specific troubles, start a new thread, or keep adding to this one, and those of us on the forums will do our best to help you out.

What's the cure for a wobbly rocket like that one was?

[Snark]

Just to take a step back, here... rather than simply trying to fix the rocket, is it worth trying to fix the problem?  i.e. what's the purpose of this vehicle?

Reading between the lines (apologies if I've missed something along the way, the thread has gotten pretty long at this point), I gather that what you're after is a vehicle that can schlep four tourists to orbit and back home, and that's pretty much it, yes?  Maybe some science along the way, as a low-hanging fruit, but mainly what the ship is about is "get four tourists to LKO and then home again", yes?

So, my suggestion:  rather than trying to fix a particular design, start with the payload and design up from there.

Your center core is a great idea.  Command pod, couple of crew cabins.  Decoupler so that it's the only thing coming back, good.  I would suggest putting a pair of AV-R8 winglets left and right on the rear of that vehicle-- it will give you steering ability on reentry.  If you're only going to LKO, you can get by without a heat shield, as long as you have good steering ability so you can make maximum use of body lift.  You could even stick a couple of fixed fins on the middle of the craft, around the CoM.

So that's your reentry vehicle.  What next?

Below the decoupler, you put your upper orbital-insertion stage.  Suppose this is two of the 2-ton LFO tanks with a Terrier.  (I'm guessing you have to stack 'em because you haven't unlocked the tall skinny 4-ton tank yet.)  So at this point you've got a craft that's probably somewhere around 8.5 tons, carrying 4 tons of fuel.  With a Terrier, that's over 2100 m/s of dV, which is 2/3 of the needed dV to get to orbit, right there.  So all you need to do is give that thing a boost of a bit over 1000 km/s and you're good to go.

Let's say, for example, that you put 3 more of the 2-ton tanks under it, with a Swivel.  That'll raise total mass up to around 17 tons, of which 6 are fuel in that stage, so you'll get on the order of 1300 m/s dV.  When you build that, disable the top two tanks so that the bottom tank will drain first.  Your craft is really going to have some aero stability issues with that light, winged thing up at the front, so give this stage four of the AV-R8 winglets, put just as far to the rear as you can manage.

Theoretically that's enough dV, but it's pretty low TWR and we'd like to have some safety margin.  So now comes the MOAR BOOSTERS part of the design.  Strap on four radial Thumpers around that bottom stage.  With decouplers, nose cones, and maybe some more AV-R8's at the back of the boosters, that brings your total ship mass up to something like 48 tons.  If you want launchpad TWR of 1.5, you'd set the thrust limiters on the Thumpers to around 70%.

That ought to get you to orbit just fine, with a much lower part count.  Take off on the SRBs; by the time they burn out, you're already pitched over at 45 degrees or more, and the relatively low TWR of the Swivel stage won't matter as much.  Just remember to enable the tanks on that stage as the bottom one drains, so you can keep continuous power.

(Actually, you could give the SRBs a bit more legs by setting up the four radial boosters in two pairs.  Instead of setting all four of them  to 70%, you can set one pair to 85% and the other pair to 55%, so that the hi-power pair ditches runs out of fuel and ditches first.  Sort of a poor man's asparagus.)

Anyway, just a thought. 

[FullMetalMachinist]

Just now, Loren Pechtel said:

What's the cure for a wobbly rocket like that one was?

Well, one option is the Kerbal Joint Reinforcement mod.  It takes the stock joint reinforcement that happened in 0.23.5 and pushes it even further, making all your joints between parts even more rigid.  Now, there are some very well respected users on the forum that would say that KJR is more of a crutch, and that if you need it to function, then it's really a sign that your basic rocket design needs improvement.  I'm not refuting that opinion, just pointing out that differences between players exist.

Another option is careful, strategic use of connector struts.  The main thing to keep in mind when using struts is that they add a lot of drag, so use as few as you can get away with while still making your rocket more stable.

The last option is perhaps the least attractive.  Basically redesign your rocket from top to bottom, keeping in mind the whole time that it should look and behave like a real life rocket.  This is fairly difficult, and at times counter-intuitive.  If you have a design that you're mostly happy with, but is still wobbly, you can upload the .craft file somewhere (dropbox, google drive, kerbalx.com) and let myself or other players try it out, and modify it if necessary, to give you some ideas on what works and what doesn't.  

Other than that, useful screenshots and very descriptive problems usually get the most useful advice. If all else fails, keep trying. This game is not for the faint of heart, and can seem a bit daunting to begin with.  But it is so worth the effort when you do something awesome for the first time.  I'll never forget the accomplishment I felt when I first landed on the Mun.  So just keep at it, you'll get better with practice, and there are plenty of helpful players on the forum when you have problems.