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Tipping, but not in a jar the way you'd want it


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The M700 vexes me.

[IMG]https://dl.dropboxusercontent.com/u/109944963/2015-11-17.png[/IMG]

No matter what I seem to do, no matter how big or how small I build, I can't seem to launch one. Even MechJeb2 can't do it - I get to about a thousand feet and

[IMG]https://dl.dropboxusercontent.com/u/109944963/2015-11-17b.png[/IMG]

I've tried with and without fairings, I've tried engines and tanks of various sizes and powers, I've tried riding the throttle and ignoring the throttle.

My CoM and CoT look good, so what gives? It has to be something herptastic I'm missing because I was able to launch this single piece
[IMG]https://dl.dropboxusercontent.com/u/109944963/2015-11-17x.png[/IMG]
to orbit by hand, but I can't get an m700 to space even with mechjeb... :(

Does it *have* to go in a cargo bay? Is there some magical effect it has on rockets to force you to do this?


My list of addons:

KSP: 1.0.5 (Win32) - Unity: 4.6.4f1 - OS: Windows 8.1 (6.3.10586) 64bit
Better Science Labs Continued - 0.1.6
Contract Configurator - 1.8.1
DMagic Orbital Science - 1.0.9.1
Editor Extensions - 2.12
EVA Enhancements - 1.0.1
EVA Transfer - 1.0.3
RasterPropMonitor - 0.24
Kerbal Engineer Redux - 1.0.18
KerboKatzUtilities - 1.2.11
ForScienceContinued! - 1.0.4
KSP-AVC Plugin - 1.1.5
SCANsat - 1.1.4.4
SpaceTux: Shared Assets - 0.3.9
SpaceY Lifters - 1.5
TextureReplacer - 2.4.11
+MechJeb 2 Edited by kfsone
in which I no-longer repeat myself
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Use the fins, Luke. Trust your (air)foilings.

EDIT-
Sticking a couple of control surfaces down toward your main rocket engine will give you more control in the lower part of the atmosphere. Like 10 times more than an SAS unit at the top.

My theory on your successful one: compared to the probe core, while your cupola is inefficiently placed for control purposes, the sheer power of the SAS unit in it probably made up for it. Edited by Venusgate
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Center of thrust doesn't matter much, it's where center of [I][U]drag[/U][/I] is that's the problem. You have a big, light, draggy front end, way out in front of your CoM.

You've got a seriously unstable rocket. Even sticking fins on might not help enough, if they're not far enough behind the CoM. You could help a little with that by sticking a Structural Fuselage in between the engine and the fuel tank and putting the fins down at the bottom of that.

Also, that looks like a Reliant you've got on there? Replace it with a Swivel. The engine gimbal will help you (especially if you move the engine down with a Structural Fuselage as I mentioned).

And if all else fails, you can take it slow for a while until you get up higher. It will mean a less efficient ascent and will be wasteful of fuel, but it will make stability issues easier.
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[quote name='Snark']Center of thrust doesn't matter much, it's where center of [I][U]drag[/U][/I] is that's the problem. You have a big, light, draggy front end, way out in front of your CoM.[/QUOTE]

Yep - so I started out by dropping an anchor chain on the far end. Not literally. Literally, I went out and flew my quadcopter until I parked it in a tree. Then I "dropped an anchor chain" on the end of a rocket. I have no idea what any of that means (apart from the tree bit which is genuine) but it sounds innuendoy.

[quote name='Snark']You've got a seriously unstable rocket. Even sticking fins on might not help enough, if they're not far enough behind the CoM. You could help a little with that by sticking a Structural Fuselage in between the engine and the fuel tank and putting the fins down at the bottom of that.[/QUOTE]

I tried 1, 2 and 3, to try and give me better balance, but still - 1-2km and *flip*

[quote name='Snark']Also, that looks like a Reliant you've got on there? Replace it with a Swivel. The engine gimbal will help you (especially if you move the engine down with a Structural Fuselage as I mentioned).[/QUOTE]

I'd tried the swivel (and the vector) which got me to 4k (and 5k, respectively). And then, flip.

[quote name='Snark']And if all else fails, you can take it slow for a while until you get up higher. It will mean a less efficient ascent and will be wasteful of fuel, but it will make stability issues easier.[/QUOTE]

I often ride the throttle anyway - remember we have better aerodynamic modelling now, and a few quick tests with a clean rocket will prove that riding it all the way at full throttle is *less* efficient than riding it below q (which is perhaps why mechjeb added this in the latest version).

Again, look at the station-under-construction I posted, the great big this-is-definitely-not-aerodynamic chunk with the 6-way node, I actually managed to tune to be aerodynamic and launch to orbit and docking by hand. But I [I]cannot [/I]get a damn M700 above 5k.

[COLOR="silver"][SIZE=1]- - - Updated - - -[/SIZE][/COLOR]

[quote name='Venusgate']Use the fins, Luke. Trust your (air)foilings.

EDIT-
Sticking a couple of control surfaces down toward your main rocket engine will give you more control in the lower part of the atmosphere. Like 10 times more than an SAS unit at the top.

My theory on your successful one: compared to the probe core, while your cupola is inefficiently placed for control purposes, the sheer power of the SAS unit in it probably made up for it.[/QUOTE]

Heh, the launcher for that was a hefty sob.

[img]https://dl.dropboxusercontent.com/u/109944963/2015-11-17D-for-derp.png[/img]

But I'm pretty sure it worked entirely because of the traffic cone on the top.

Anyway - interesting theory. I was going to say "but I don't have an SAS" - and there's the herp. The RC-01S has a built in herp. DERP!

Lets try without...
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The sheer power and gimbal of those engines made up for any drag problems. Drag causes and issue when it is stronger than the force pushing against it, that also means when it has more leverage. A big rocket like that will easily overcome drag forces and the gimbal on the KS-25 just helps that much more. The small rockets with less gimbal will be more likely to get shoved around by drag. Edited by Alshain
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[quote name='Venusgate']Use the fins, Luke. Trust your (air)foilings.

EDIT-
Sticking a couple of control surfaces down toward your main rocket engine will give you more control in the lower part of the atmosphere. Like 10 times more than an SAS unit at the top.

My theory on your successful one: compared to the probe core, while your cupola is inefficiently placed for control purposes, the sheer power of the SAS unit in it probably made up for it.[/QUOTE]

And that's how we get ants, Lana...

[img]https://dl.dropboxusercontent.com/u/109944963/ants.jpg[/img]

Replaced the RC-001S with the OKTO2, added a Z1k battery, and I was able to make orbit.

[url]https://dl.dropboxusercontent.com/u/109944963/CantEverGoBackToArizona.craft[/url]

[COLOR="silver"][SIZE=1]- - - Updated - - -[/SIZE][/COLOR]

Perversely, the fairing's weight offsets it's aerodynamic advantage, so I find it easier to get a 700 into space without a fairing. Indeed, if I build with one and jetison it during takeoff, the craft immediately becomes easier to control. I think the problem there is that the fairing is bigger than the rocket behind it. This can be countered with larger control surfaces but those also add weight which also helps so ...
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Your fairing did have a very flat top. The perverse part is that the way that fairing was built, it was [I]less[/I] aerodynamic than the payload it was supposed to shield :P

The m700 is pretty pointy to begin with, so to improve on that, you'll have to build a [I]very[/I] pointy fairing. Of course, as you figured out, it is also pointy enough to make do without a fairing.
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[quote name='Streetwind']Your fairing did have a very flat top. The perverse part is that the way that fairing was built, it was [I]less[/I] aerodynamic than the payload it was supposed to shield :P[/QUOTE]

I'm inclined to agree... fairings pull double duty as heat shields and drag reducers - the latter requires them to be pointy at the top. Like a... well, rocket :)

Also, of note, three fins isn't ultra-stable aerodynamically because you can't get them all to be flat/vertical at the same time. Effectively it gives you anhedral wings and some unwanted rolling forces - which quickly turn into sideslip forces when you overcompensate. With four fins, they're in balance. Three fins is better than no fins, but four is better than three :)
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And that is really your whole rocket? Even without closing the fairing it only has about 2800m/s in a single stage. That is not enough to get to orbit, especially if you try to go straight up too long like your last picture shows. The TWR of more than 3 will also make you hit supersonic speeds pretty early while using only one stubby stage with a comparably heavy engine will make your dangerously low CoM drop even more during ascent. Adding a wide fairing will only give the aerodynamic forces even more torque to push you around.

What you really need is a second stage to keep the CoM up and enough juice in the first to carry you high enough for a safe stage seperation.
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[quote name='Harry Rhodan']And that is really your whole rocket? Even without closing the fairing it only has about 2800m/s in a single stage. That is not enough to get to orbit, especially if you try to go straight up too long like your last picture shows. The TWR of more than 3 will also make you hit supersonic speeds pretty early while using only one stubby stage with a comparably heavy engine will make your dangerously low CoM drop even more during ascent. Adding a wide fairing will only give the aerodynamic forces even more torque to push you around.

What you really need is a second stage to keep the CoM up and enough juice in the first to carry you high enough for a safe stage seperation.[/QUOTE]

No, it is just the trivial example one I made for the thread after dozens of genuine attempts; a "demonstrator". I'm a software engineer and we tend to try and reduce problems to their simplest reproducible steps, I could probably have made this even simpler but I expected *this* question many replies sooner.

Ultimately it was the combination of weird aerodynamics + overwhelming reaction wheel that was the problem.

[COLOR=silver][SIZE=1]- - - Updated - - -[/SIZE][/COLOR]

[quote name='FancyMouse']Since it is a light rocket with a fairing, it is more likely to be affected by a [URL="http://forum.kerbalspaceprogram.com/threads/139219"]stock bug[/URL] that incorrectly sets CoL for fairing. Try Claw's stock bug fix see if it helps.[/QUOTE]

Oooh! Yep - that helped a lot, but I was also making the mistake of making my fairings pointy to save weight rather than making them nicely aerodynamic to save drag. I thought the effect would be more like the rounded shapes you often see:

[IMG]http://i.space.com/images/i/000/021/485/original/india-pslv-c21-rocket-100th-mission-nose-cone.jpg?1347312624[/IMG]

but it's not.
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[quote name='kfsone']No, it is just the trivial example one I made for the thread after dozens of genuine attempts; a "demonstrator". I'm a software engineer and we tend to try and reduce problems to their simplest reproducible steps, I could probably have made this even simpler but I expected *this* question many replies sooner.

Ultimately it was the combination of weird aerodynamics + overwhelming reaction wheel that was the problem.

[COLOR=silver][SIZE=1]- - - Updated - - -[/SIZE][/COLOR]



Oooh! Yep - that helped a lot, but I was also making the mistake of making my fairings pointy to save weight rather than making them nicely aerodynamic to save drag. I thought the effect would be more like the rounded shapes you often see:

[URL]http://i.space.com/images/i/000/021/485/original/india-pslv-c21-rocket-100th-mission-nose-cone.jpg?1347312624[/URL]

but it's not.[/QUOTE]

You can still make rounded fairings like that, and it will work, but yours was not round, it was downright flat.


[IMG]http://images.akamai.steamusercontent.com/ugc/403432173248416497/C68832A42F92FC385D54D02144A3C9F2C27CEC93/[/IMG]
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