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Well, whats wrong with this rocket????


Space boy
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I have created this apollo style rocket with a rover to minmus and when I go just outside the prograde marker, it flips! and i dont completely go outside the marker, its like just turning enough to get a gravity turn but my rocket flips instead! i am using the new srbs................. Is it the new update thats doing anything??? My rocket has the center of aerodynamics below the coM.

https://imgur.com/a/GvIF1RS ---- Link to photo on  imgur

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You have a massive fairing right on the nose.  That'll create a lot of drag which you need to counter with fins and/or control surfaces behind the CoM.  Press F12 to see the aerodynamics overlay as the vehicle ascends and you should see a huge spike in drag on the fairing just before it flips.

Edited by Pecan
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1 hour ago, Space boy said:

when I go just outside the prograde marker, it flips! ... My rocket has the center of aerodynamics below the coM.

Ahh, but is your CoP below your CoM when you're pointing a degree or two off prograde? I'm betting it isn't, and I'm betting that that mammoth fairing (which looks far larger in diameter than required BTW) is the culprit.
As soon as you steer away from prograde, you're presenting (some of) the side of the fairing to the airstream, and since the side profile of the fairing is so much larger than the rest of the rocket... Hilarity  ensues.

It is usually just possible to fly something like this, but you have to keep it locked to prograde once drag becomes significant by starting your gravity turn uber-early and controlling the shape with throttle. Not easy, especially with an SRB first stage.  
Probably better to just redesign the fairing and/or add more tailfins. More fins are good fins. Listen to Pecan.

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15 hours ago, steve_v said:
15 hours ago, steve_v said:

Ahh, but is your CoP below your CoM when you're pointing a degree or two off prograde? I'm betting it isn't, and I'm betting that that mammoth fairing (which looks far larger in diameter than required BTW) is the culprit.
As soon as you steer away from prograde, you're presenting (some of) the side of the fairing to the airstream, and since the side profile of the fairing is so much larger than the rest of the rocket... Hilarity  ensues.

It is usually just possible to fly something like this, but you have to keep it locked to prograde once drag becomes significant by starting your gravity turn uber-early and controlling the shape with throttle. Not easy, especially with an SRB first stage.  
Probably better to just redesign the fairing and/or add more tailfins. More fins are good fins. Listen to Pecan.

How much should I reduce the size of the fairings?? I will try replacing srbs with liquid fuel. 

 

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1 hour ago, Space boy said:

How much should I reduce the size of the fairings?? I will try replacing srbs with liquid fuel.

As much as humanly kerbally possible. Fairings should always just fit around what you're carrying, and come to a point at the top (not sure if that is modeled but hey just in case)

You don't show your launchpad TWR but I bet it's high, like over 2. You really want to not start higher than 1.6 and I personally prefer 1.3. You shouldn't fix it by throttling down, you should fix it by not bringing so much boom to the party :)

Replacing the solids with fuel is a good idea, but make sure you don't go in the other direction and have too low a TWR. If it's below 1 you won't even launch at all.

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1 hour ago, 5thHorseman said:

As much as humanly kerbally possible. Fairings should always just fit around what you're carrying, and come to a point at the top (not sure if that is modeled but hey just in case)

You don't show your launchpad TWR but I bet it's high, like over 2. You really want to not start higher than 1.6 and I personally prefer 1.3. You shouldn't fix it by throttling down, you should fix it by not bringing so much boom to the party :)

Replacing the solids with fuel is a good idea, but make sure you don't go in the other direction and have too low a TWR. If it's below 1 you won't even launch at all.

I reduced the size of the fairings to only what needed to be covered, I also replaced srbs with reliants. So now I have core mainsail stage and side relliant boosters, but it's still flipping out! Why is that happening? 

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45 minutes ago, Space boy said:

I reduced the size of the fairings to only what needed to be covered, I also replaced srbs with reliants. So now I have core mainsail stage and side relliant boosters, but it's still flipping out! Why is that happening? 

Just increase your fins at the back end and move them as far back as you can. 

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59 minutes ago, Space boy said:

I reduced the size of the fairings to only what needed to be covered, I also replaced srbs with reliants. So now I have core mainsail stage and side relliant boosters, but it's still flipping out! Why is that happening? 

TWR sounds too high, as I said you want it 1.3-1.6 at launch.

Also the Mainsail is heavy so it pulls your COM back. Essentially you're trying to shoot an arrow with the feathers at the front, twice as fast as arrows normally fly. Of course it's gonna flip around.

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More screenshots.

Are you still trying to use the SRBs and the mainsail separately like the staging in the old screenshots seems to imply? They both should run together with the SRBs acting as a support for the Mainsail powered stage for the first few kilometers until it burned up enough fuel to carry itself. A Mainsail always should have more than one big tank to actually get you somewhere.

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5 hours ago, Space boy said:

Now I changed the first stage to however you guys told me to do so.. But now, I don't have enough Delta v to get out of the atmosphere! And I can't seem to figure out a way to solve this problem. 

If you'd like to tell me the mass and height of your payload lander+command module as shown in picture 2 I will attempt to design a launch vehicle for it, just to illustrate how I'd go about it.  My first step, however, would be to reduce the fairing so that it only covers the lander and probably the command pod.  Specifically, I wouldn't extend the fairing to cover the fuel tanks and engine as you have, it just adds too much ... everything really (drag, mass, cost, difficulty - take your pick).

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1 hour ago, Pecan said:

I will attempt to design a launch vehicle for it

Hmmm, Just made a little mock-up 38-tonne payload where the command/habitation module looks like a reasonable copy of yours but the lander/rover's a lot simpler.  Then I put the fairing base between the command pod and the hitchhiker so the fairing is much, much smaller and less draggy.  It means carrying that base for the rest of the mission but there are (awkward and massy) ways around that if necessary.

As a first approximation I put a stack of 2 jumbo-64s and a mainsail under the payload then 2 similar side-stacks with nosecones and 4 basic fins each.  This gives just over 3.4km/s dV and a launch TWR of 1.55 but your payload is probably a bit heavier than mine so it might work out about right for you.  That's a lot of fuel and engine but it easily SSTOs the payload to a 75km, circularised orbit.  I had no steering/control issues at all.  To be honest, even a full-length fairing didn't make that much difference, so I'll await your payload details and try again with those.

 

Edited by Pecan
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If you ever watched an early Falcon 9 launch, you might have noticed that they got very excited about something called MaxQ and relaxed once they passed it.

Actually especially with SRB this might be part of your problem. As others pointed out rotating the big fairing sideways does increase drag a lot. Speeding up in thick air increases drag as well. So do turn early, but only few degrees (maybe 3-5) and watch dynamic pressure in aerodynamics stats window (Ctrl + Alt+ F12 > Physics > Aero > Display Aero GUI). If it starts to flip, throttle down a little in the next try until atmosphere gets thinner to keep dynamic pressure low. Or simply reduce TWR and add dV for the aero loss.

Draggy, instable stuff likes a more vertical launch trajectory than the optimal gravity turn of slim rockets, but you can launch almost anything into orbit :D

Edited by CBase
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IF your CoM is considerably above aero (and in the center "horizonatally looking") - then could be 1.8 ? - i have put MASSIVE fairings and TWR of up to ~3 etc (but sometimes you have to launch straight up obviously until atmosphere gets more sparse....) so i don't understand why that would be a problem.

(dunno why it would "flip out" (as opposed to "curve to be downward pointing") also, but would be dependant on word definition.)

if you have had no problems up until 1.8 then this would be suggestive also IMO - as maybe a 1.8 bug in some parts interaction because your rocket is complicated... (may be resolved with different parts or if you built it again ?).

(unless your TWR was way too low in the first place ?)

Edited by k00b
TWR ?
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16 hours ago, Harry Rhodan said:

More screenshots.

Are you still trying to use the SRBs and the mainsail separately like the staging in the old screenshots seems to imply? They both should run together with the SRBs acting as a support for the Mainsail powered stage for the first few kilometers until it burned up enough fuel to carry itself. A Mainsail always should have more than one big tank to actually get you somewhere.

I am using srbs with mainsails in one single stage. 

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What everyone else has said.

Your CoP might be behind the CoM but only on a pure prograde vector.

Stick some AV-R8s on the main stage as low as you can put them. If it's still flipping then we'll need more data like the altitude, speed and orientation of when it does so.

Also....

LAUNCH CLAMPS!

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21 hours ago, Space boy said:

https://imgur.com/tSXvtNL

what is wrong now???!!! I am using skippers and twr is 1.56 and the fairing is much much smaller than before but it still flips and it also is rotating.

If you have the 3.75m parts, use them for the stages outside the fairings.

Use Verniors with Skipper.

Add MOAR winglets at the rear.

Use T for SAS stability control during launch.

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you could also redesign the lander a bit so it doesn't require a 4+ meter wide fairing to contain it. you probably don't need 2 terrier engines for a minmus lander of that size. a few small radial engines would be enough, really. if the fairing isn't much wider than the 2.5m profile of the launcher rocket, some of your aerodynamic issues will already be resolved by that alone.

also i think you use too many stages and too many engines and not enough fuel tanks for an efficient launcher. a skipper engine with only the mid sized (3200) fuel tank is wasteful. skippers have enough thrust to push a 6400 (jumbo) 2.5 meter tank and payload. or you use a mainsail and 2-3 jumbo tanks and a couple of kickback boosters. the 1.8m booster rockets from the first image are way to powerful.

 

i always found it hard to aerodynamically balance unrealistic rockets. once you change it to something that resembles a real world launcher system, it will probably work just fine. i mean there's a reason why real rockets look like they look :)

 

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On 10/25/2019 at 8:28 PM, mk1980 said:

I always found it hard to aerodynamically balance unrealistic rockets. once you change it to something that resembles a real world launcher system, it will probably work just fine. i mean there's a reason why real rockets look like they look :)

Add more fins near the rear.

But yes, realistic rockets fly very well.

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