Three Minute Challenge

[DoctorEvo]

Allow me to rephrase what I think I meant to say. Having the thrust at the bottom means that the air resistance in-atmo is always trying to push the rocket over.

Not if the center of mass is forward to the center of pressure. Putting heavy components such as engines in the back of the rocket will of course shift the center-of-mass back, but it isn't the lengthwise-positioning of the thrust that is directly causing this issue.

if there is any flexibility in the stack, the top of the rocket will bend to one side or the other, and that will lead to a spin-out.

If your rocket is bending under compression such that your thrustline is thrown out of alignment, that's a structural issue (one which, mind you, will not go away if you fire the rocket in a vacuum instead). Yes, I suppose a rocket built to be under tension when firing will tolerate elasticity better, but it sure is a hokey way to build a rocket vs. simply stiffening the thing as needed.

If the thrust is at the top of the rocket, the drag from the rest of the rocket trailing behind will try to keep it on target. That's what I was talking about when I said this:If there is drag from below the boosters, it will help, somewhat, to stabilize the rocket.

Your reasoning is still fallacious. It is the center of gravity, NOT the location of thrust, that will help stabilize the rocket by being placed at the front.

If things aren't balanced, though, it does matter a great deal. Take a look at Sunday Punch's medium SRBs and how moving them along a radial coupler affects their flight characteristics-- or how you can make an L, and use that to build a spin-stabilised doom machine.

Even with regular SRBs you can find a pronounced effect on atmospheric flight characteristics on shorter rockets, by simply moving them up and down. This is because of the issue I mentioned before - if the center-of-gravity is lower than the center-of-pressure, your rocket will be aerodynamically unstable. Adjusting the height of side-mounted SRBs affects BOTH of these, and the direction in which they should be moved to improve stability may vary from rocket to rocket (heavy rockets lacking in aerodynamic surface will prefer to have the SRB's drag to be added down low, whereas light rockets with lots of surface will benefit from the mass of the SRBs being moved forward).

Another thing worth mentioning is that a rocket that is TOO aerodynamically stable will resist diverging from a nose-forward, ballistic orientation. If you are off-course, or if your intended course is not fully ballistic, you MAY not be able to steer back onto course once the stabilizing aerodynamic forces overcome your control forces. It is important to recognize this situation, and understand that it does not mean your rocket is unstable, but rather uncontrollable. After fiddling with Entroper's rocket a bit, it seems like this may be a large part of the problem - it diverges during the initial acceleration (possibly due to flex) and then STRONGLY resists turning in any direction but the one it is going. This is aggravated by the fact that, after the SRBs are ejected, the design becomes tail-heavy and UNstable due to the significant weight of the three LFE at the very bottom, and the light and draggy tricoupler, capsule, and the (now-emptying) upper LFTs at the top.

[semininja]

The center of gravity doesn't matter if the drag is at the bottom and the thrust is at the top. The only reason having the CG further forward would help is if you were traveling on velocity alone, without power (like a dart), because then all the momentum is in the front, and the drag behind pulls the rocket into alignment. However, the same principle works if it's thrust instead of momentum. As long as the drag is behind far enough, it doesn't matter where the weight is.

[DoctorEvo]

The center of gravity doesn't matter if the drag is at the bottom and the thrust is at the top. The only reason having the CG further forward would help is if you were traveling on velocity alone, without power (like a dart), because then all the momentum is in the front, and the drag behind pulls the rocket into alignment. However, the same principle works if it's thrust instead of momentum. As long as the drag is behind far enough, it doesn't matter where the weight is.

That is wrong. You are STILL wrong. Do I have to draw a frickin' picture?

If your rocket begins to spin, the engine will spin with it. The engine will NOT keep thrusting in the direction you were going - it will thrust in whatever direction you point it. Drag, on the other hand, always pulls opposite the direction you are moving (not necessarily opposite of where you're pointed), and inertia will of course ALWAYS tend to keep you moving in the direction you are already moving. Since inertia acts at the center of mass, it DOES help to push this point forward, and to push drag aft. The location of thrust along the thrustline is completely irrelevant, just as long as the total thrustline is inline with the rocket's axis. (Note that, for a rocket which is not aligned perfectly, having the thrust CLOSE to the center of mass, rather than forward or aft, will reduce the torque moment caused by this imprecision - but that's a whole different ballgame).

Seriously, bro. You're denying the findings of Robert-frikin'-Goddard, here. It's just rocket science, not brain surgery.

[Letum]

Dr.Evo is right.

The center of thrust is not a pivot point, nor does it change the pivot point because the direction of thrust follows the direction of the rocket as a whole.

If the rocket motors where a force that only pushed up, like having a string attached to the rocket, pulling it up, then they would be a pivot point, but rockets don't push up. They push in any direction they face.

If a rocket starts spinning anti-clockwise, it needs a force pushing it clockwise to stop the spin. Whether the rocket motor is at the top, bottom or middle, it isn't going to create or cause any force pushing clockwise.

Even if the rocket is at the top, above the center of gravity, it's not a pivot point, so it's not going to cause the rest of the rocket to hang below it like a pendulum.

[semininja]

Consider these; .craft files are included as well.

testing done in 0.9 X3

[semininja]

I'll redo these later with properly built rockets so that nothing changes but the point of thrust, and then we'll see what actually happens.

[zarakon]

Using one of the rockets i made for the 100km drag race challenge, I got to 218202 in 3 minutes. The last ~50 seconds I just coasted without fuel

Pod

LFT

LFE

Stack decoupler

Tricoupler

LFTx3

LFEx3

Radial decoupler x9

SRBx9

[DoctorEvo]

I'll redo these later with properly built rockets so that nothing changes but the point of thrust, and then we'll see what actually happens.

If you move the engines, you'll be moving the CG as well, causing a change in stability. In THIS sense, certain rockets CAN benefit from having the engines moved up to the front (as I already mentioned), but it is not because you are moving the thrust so much as moving the weight of the engines.

The only way to test this properly would be to put engines at the top AND bottom of a rocket, and then fiddle with the staging so that you can test it with ONLY the top engines or ONLY the bottom engines firing. Also this test would best be done quickly, before significant fuel burn affects the center-of-gravity.

[semininja]

If you move the engines, you'll be moving the CG as well, causing a change in stability. In THIS sense, certain rockets CAN benefit from having the engines moved up to the front (as I already mentioned), but it is not because you are moving the thrust so much as moving the weight of the engines.

The only way to test this properly would be to put engines at the top AND bottom of a rocket, and then fiddle with the staging so that you can test it with ONLY the top engines or ONLY the bottom engines firing. Also this test would best be done quickly, before significant fuel burn affects the center-of-gravity.

I agree entirely. I don't have the time to test this ATM, but I was planning on redoing this properly.

[ZMX]

222633M in 3 minutes.

[Entroper]

The problem I'm encountering has little to do with where the thrust is located, and everything to do with the atmospheric drag, because the instability doesn't appear until I reach high speeds in the lower atmosphere. It would stand to reason that a low center of gravity helps to reduce drag instability for the same reason that a high center of gravity reduces thrust instability. Correct me if I'm wrong?

[semininja]

As a matter of fact, I think a high center of gravity might help with drag instability as well (the dart effect?)

[Entroper]

As a matter of fact, I think a high center of gravity might help with drag instability as well (the dart effect?)

Hm, I see what you mean. An un-centered force would push the 'light end' of the rocket off course faster than the 'heavy end', causing the flip.

[DoctorEvo]

As a matter of fact, I think a high center of gravity might help with drag instability as well (the dart effect?)

Indeed. If you roll back a few pages, you'll see that I brought this up already.

Of course, in most cases shifting the CG may also shift the center of pressure, so if you shift the CoP FURTHER forward than you shift the CG, you will only destabilize the rocket. If your rocket is built entirely of LFTs and LFEs (i.e. high density, high ballistic coefficient), some SRBs would be better off placed near the bottom as their drag will be more influential than their weight. However, a rocket with several winglets would certainly benefit from forward-mounted SRBs.

[trydyingtolive]

I made this a few weeks ago. 247Km

[Oviraptor]

248km

Not a very large gain, I realize.

[trydyingtolive]

Only 2 more Km to go.

[Oviraptor]

So that 248km was using your design. I tried out my own design and managed to get 254 km.

I think I might even be able to tweak it a bit to get a little more out of it.

Edit: I've managed to take it up from 254.2km to 254.4km.

---

Edit2: Well, I just crushed those: 262.5km

[Bobsama]

This is a worthy challenge that must be resurrected! Especially the all-stock variation!

That\'s 292KM in 3 minutes. Peak speed was 4241.3 m/s and engines ran out around 2m50s.

The rocket is completely stock for 0.11.1.

[kerpollo]

My first attempt reached 279km into the heavens.

In true Kerbal fashion, attempts at cleverly improving the design resulted in decreased results, increased explosions.

This was the layout:

Stage 1: 6x SRB + 6x radial decoupler + 3x LFE + 3x LFT + tricoupler

(stack decoupler)

Stage 2: 1x LFE + 1x LFT + space sarcophagus capsule

Having realised how little there was to be gained from tampering with the second stage (except in sheer, fiery spectacle), I just went back to the time tested method of More Boosters. Here is my little tin of Kerbals reaching 294km:

Stage 1: 12x SRB + 12x radial decoupler + 3x LFE + 3x LFT + tricoupler

(stack decoupler)

Stage 2: 1x LFE + 1x LFT + kapsule

By now there\'s a lot to be gained from sheer piloting skills, since this design spins more than Medusa\'s tits when she goes clubbing. Then SRB separation causes extreme sideways action, which takes a while to correct. The ride is much smoother after that, but between the spinning and the drifting there\'s a lot of potential altitude loss.

[Omnivore]

UPDATE: Broke the 300km barrier with a Voidhawk V (IV with 8 more SRBs), hitting 302410m at 3:00 minutes.

My first attempts didn\'t do so well as I\'ve been using modified heavy lifter designs instead of barebones. However, finally, a Voidhawk IV model just squeaked into the lead hitting 294883m at 3:00 minutes. Yeehaw!

Not sure how much more I can get out of the design though, it uses an ASAS and has been stripped to a bare minimum of struts and winglets so manually piloting this collection of 14 LFTs, 14 LFEs, and 32 SRBs is quite out of the question!! Here it is at the 0:001 mark just after liftoff - I\'m just amazed the thing doesn\'t assplodey before 30 seconds.

The rocket is completely stock for 0.11.1.

[BitPoet]

351,000 at the 3 minute mark.

Take Omnivore\'s concept, rip off the SRBs on the outer ring. Make it a 6-way instead of a 4-way balance. Add a SAS to keep it going in the right direction, and a few struts to keep it in one piece, and you\'re off to the races.

[Salda007]

Lag made me miss the time by a second, but knock off the end velocity and the result is 443,300

[BitPoet]

Damn, just cracked 400, and find it\'s just been shattered.

welp, back to the drawing board...

[Omnivore]

Yeah I\'ve switched to a 219 SRB stack and can get to around 415 pretty regularly, only way I see to get more is to add a 7th stage and/or move the ASAS down a couple stages and manually pilot the higher stack. Either way I\'m up against the limits of my setup - even the VAB is laggy as heck with that monster loaded. I did notice one other possible optimization but heh don\'t expect to crack the 440\'s till I get a new comp.

Congrats Salda007, knocked me right back to the drawing board too.