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Shuttle using re-balanced Kickback and Vector


Tweeker

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I spent some time messing with the .cfg files for the Vector and Kickback, as a result of some discussion in the General forum. I was trying to re-balance the Vector closer to the other 1.25m engines, I thought about 300 Kn would be about right. but I needed to increase the Kickback to a more real-world ratio with respect to the Vector, as the shuttle gets about 70-80% of it's thrust from it's SRBs so I changed the Kickback to have 1750 Kn of thrust, and 7500 soild fuel.

     On the first launch attempt the shuttle nose over backwards, The SRBs where pushing off straight up on an off center load, and the Vectors didn't have enough thrust to compensate. It seemed my efforts were in vain. After some digging I found that the SLS booster gimbal 5 degrees, So I added a 5 degree gimbal, and tried again. The launch was a success, but it turned out the SRBs burned out too soon, and the top stage didn't have enough thrust, it slowed down after the SRB burned out at about 10 Km.

     I kept adjusting the numbers, it was a bit of a fine balance, more sold fuel meant the SRB lost thrust, more SRB thrust meant that the Shuttle would flip over, and adding more thrust to the Vector would make it unbalanced vs the exsiting 1.25m engines.   I ended up with the SRBs at 2400 Kn, and 15000 soild fuel, and a 5 degree gimbal. and The vector with 400 Kn of thrust.

Here it is is in action: 202965DD6718A2EDDCA4C7EAF536995D68E815EF

The booster are re-sized to 1.875M

98B6336C387D56093009A20B0F8BAE2790C1DC0B

Lift-Off !!!

It flies fairly easy, if it's properly strutted. Almost hands-off Fly straight up to 10 Km in stability assist. switch to prograde. At about 15Km switch from surface to orbital, Booster burns out at about 25 Km. F6C09704E55226161C77CA252E5A1120576F0C98

From there just keep following prograde. you will stay about 1 min, 20 sec from apoapsis. 28BD8E2F1662E771118BEEDD2A33E7B79CB8EE55

Eventually you will start pushing apoapsis out, when you are comfortable circularize,

95A5C9DC86340BF83DA66B3770414D8984D934B3 You end up in a decent LKO orbit, with about 700-800 m/s DV left, Probably less depending on payload. But not enough to go to GEO

 

 

B01862B8A46B1EB264362D793EA30D3DCCA04C88

 

I'd love to hear your thoughts on this, rebalancing of the Kickback and Vector.

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@Tweeker The real shuttle's SRB's, along with gimbaling, also had a propellant burn-through that would gradually ease off on the thrust as the solid fuel was expended.  By the time the SRB's were about to be jettisoned, they were producing significantly less thrust than at liftoff (Read somewhere it was something like a 30% reduction a minute after liftoff. I may be mis-remembering though).

The shuttle system is obviously a really complicated engineering challenge, ha ha :P

Edited by Raptor9
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Thrust balance at liftoff is also making making shuttles a lot harder than it should in KSP: SRBs are severely underpowered. While the RS-25 have about twice the thrust of the Vector, the SSSRBs had 12MN of thrust at liftoff (20 times the Kickbacks' !), so there's that. 

To make a somehow realistic copy of a space shuttle, you absolutely need to rebalance the Vector and SRBs (preferably make the SRBs a lot more powerful or give us bigger ones).

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8 hours ago, Raptor9 said:

@Tweeker The real shuttle's SRB's, along with gimbaling, also had a propellant burn-through that would gradually ease off on the thrust as the solid fuel was expended.  By the time the SRB's were about to be jettisoned, they were producing significantly less thrust than at liftoff (Read somewhere it was something like a 30% reduction a minute after liftoff. I may be mis-remembering though).

The shuttle system is obviously a really complicated engineering challenge, ha ha :P

I am told there is a mod got that:

https://github.com/Crzyrndm/VariableThrustLimiter

I haven't had a chance to try it yet, as work had flared up again.  Maybe next week I'll get a chance to give it a go.

 

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8 minutes ago, Tweeker said:

I am told there is a mod got that:

https://github.com/Crzyrndm/VariableThrustLimiter

I haven't had a chance to try it yet, as work had flared up again.  Maybe next week I'll get a chance to give it a go.

 

Yes, this mod is great, and not only for shuttles. You can get realistic thrust proportions (Ariane 5 is 90% SRB thrust at liftoff IIRC) without crushing your rocket under dozens of Gs before flameout.

You can basically set a start thrust limit and end thrust limit, and the thrust of the SRB will vary linearly with the fuel left.

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8 hours ago, Gaarst said:

Thrust balance at liftoff is also making making shuttles a lot harder than it should in KSP: SRBs are severely underpowered. While the RS-25 have about twice the thrust of the Vector, the SSSRBs had 12MN of thrust at liftoff (20 times the Kickbacks' !), so there's that. 

To make a somehow realistic copy of a space shuttle, you absolutely need to rebalance the Vector and SRBs (preferably make the SRBs a lot more powerful or give us bigger ones).

I have an Ares 1 and V that are 67% scale and use 2.5m SRB's (clipped) with several empty tanks on top to make up the height. It's not just a matter of power or size, it has to scale with Kerbin. The real SRB's are just way OP for a Kerbin sized universe. I have a Kerbin scaled Ares V type craft (not the above) for sending a DAV to Duna and it just has two stock kickbacks, if I added more power I probably would only need a circularisation engine, which is a bit silly to mostly orbit with SRB's.

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7 minutes ago, selfish_meme said:

I have an Ares 1 and V that are 67% scale and use 2.5m SRB's (clipped) with several empty tanks on top to make up the height. It's not just a matter of power or size, it has to scale with Kerbin. The real SRB's are just way OP for a Kerbin sized universe. I have a Kerbin scaled Ares V type craft (not the above) for sending a DAV to Duna and it just has two stock kickbacks, if I added more power I probably would only need a circularisation engine, which is a bit silly to mostly orbit with SRB's.

I agree, but it's not really a question of absolute power, more relative: the Vector should be 10x less powerful when compared to KIckbacks (or the opposite).
Big crafts have a lot of dV, too much for stock KSP; and the Vector and Mk3 parts are too close to real life to have sensible performance, same for rockets in general: if I make a realistic looking design with properly sized booster in RSS, the booster stage gives generally about 4km/s dV, which is orbit in stock game.

Even if SRBs are not scaled to real life (what even would you do on Kerbin with a 12MN booster ?), they should be powered up at least a little, or we should have bigger boosters. Less powerful than real SRBs, but powerful enough for KSP SRBs to play their actual role: provide the majority of thrust at liftoff, not just assist the liquid fueled core.

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28 minutes ago, Gaarst said:

I agree, but it's not really a question of absolute power, more relative: the Vector should be 10x less powerful when compared to KIckbacks (or the opposite).
Big crafts have a lot of dV, too much for stock KSP; and the Vector and Mk3 parts are too close to real life to have sensible performance, same for rockets in general: if I make a realistic looking design with properly sized booster in RSS, the booster stage gives generally about 4km/s dV, which is orbit in stock game.

Even if SRBs are not scaled to real life (what even would you do on Kerbin with a 12MN booster ?), they should be powered up at least a little, or we should have bigger boosters. Less powerful than real SRBs, but powerful enough for KSP SRBs to play their actual role: provide the majority of thrust at liftoff, not just assist the liquid fueled core.

I guess I agree with you, I have a shuttle of my own which only uses one vector because it is so OP compared to the SRB's and if I scaled it up it would get more vectors and more balance problems because there is no bigger SRB to go to than the one it already uses.

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I pushed some more numbers around today, with the intention of re-scaling the fuel tank and SRBs to more visually accurate sizes. Starting with the shuttle as a baseline, because it has the most parts, and is therefore the least desireable to resize.

      I found that the fuselage is ~ 5.2m wide. If we are using 3.75m parts the tells us it is 72% scale, maybe smaller because the shuttle parts aren't round. But in the ballpark anyway. 

      From the I find the SRBS are 71% of the shuttle's width, 3.71m/5.2m. The fuel tank is 1.61 times the diameter of the shuttle, 8.4m/5.2m. 

       Going from here the SRBs should be 2.7m,  3.75m for the stock shuttle time 72% .   I think I can call 2.5m close enough.   The main fuel tank should be 6.03m, 3.75 times 1.61.  I think I'l try 5m. This will make it 20% under size vs the shuttle, but only 12% vs the 2.5m SRBS

     When I get back home, and I can spend some time at my computer I'll give it a try and see how it looks.

 

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So basically, this thread boils down to "we want a 2.5m SRB", right? If so, I concur. Maybe add a bit of nerf in thrust to the Vector while you are at it.

 

Rune. A thrust curve for SRB would also be nice, instead of it being flat, but it's not a must-have.

 

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3 hours ago, Rune said:

Rune. A thrust curve for SRB would also be nice, instead of it being flat, but it's not a must-have.

Personally, I'd massively appreciate it if TWR stayed constant or something like that. Would prevent getting shoddy liftoff twr and still getting 15Gs at booster cutoff.

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3 hours ago, Redhaze said:

Personally, I'd massively appreciate it if TWR stayed constant or something like that. Would prevent getting shoddy liftoff twr and still getting 15Gs at booster cutoff.

That's an issue in RL rocketry, now that you mention it. Not that they can't tailor their SRBs to throttle down, unlike us, but that they can't really throttle down liquid engines that much (also unlike us). Merlin 1D is considered a very throttleable engine and it can only go to 70%, for example. Which is why the Saturn V shut off one F-1 before the others, for example, to keep the rocket from collapsing from excessive G-loading.

 

Rune. So maybe a linear thing to 50% thrust, at most, to keep things challenging.

Edited by Rune
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3 hours ago, Rune said:

That's an issue in RL rocketry, now that you mention it. Not that they can't tailor their SRBs to throttle down, unlike us, but that they can't really throttle down liquid engines that much (also unlike us). Merlin 1D is considered a very throttleable engine and it can only go to 70%, for example. Which is why the Saturn V shut off one F-1 before the others, for example, to keep the rocket from collapsing from excessive G-loading.

 

Rune. So maybe a linear thing to 50% thrust, at most, to keep things challenging.

Not anymore SpaceX and a few others now have throttle able booster engines

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1 hour ago, selfish_meme said:

Not anymore SpaceX and a few others now have throttle able booster engines

To a limit. SpaceX still has to shut down one or more engines to not squash their precious little recyclable booster when landing empty.

4 hours ago, Rune said:

That's an issue in RL rocketry, now that you mention it. Not that they can't tailor their SRBs to throttle down, unlike us, but that they can't really throttle down liquid engines that much (also unlike us). Merlin 1D is considered a very throttleable engine and it can only go to 70%, for example. Which is why the Saturn V shut off one F-1 before the others, for example, to keep the rocket from collapsing from excessive G-loading.

 

Rune. So maybe a linear thing to 50% thrust, at most, to keep things challenging.

Hmmmh, fair enough, though I'm not really inclined towards the whole "This is too easy so lets do it like they do IRL" thing of Realisim Overhaul et al.

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On 1/21/2016 at 7:58 PM, Rune said:

 Merlin 1D is considered a very throttleable engine and it can only go to 70%

I think it's been revised to 55% with the 1D+ as opposed to the 70% of the 1D

Engine Merlin 1D Full Thrust
Engine Type Gas Generator, Open-Cycle
Propellant Feed Turbopump
Merlin 1 D Thrust Sea Level: 756 kN – Vac: 825 kN
Engine Diameter ~1.0 m
Engine Dry Weight 470 kg
Burn Time 162s
Specific Impulse 282s (SL) 311s (Vac) (M1D Standard)
Chamber Pressure >97 bar (M1D Standard)
Expansion Ratio 16
Throttle Capability 55% to 100%

from 

http://spaceflight101.com/spacerockets/falcon-9-ft/

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1 hour ago, EladDv said:

I think it's been revised to 55% with the 1D+ as opposed to the 70% of the 1D

Engine Merlin 1D Full Thrust
Engine Type Gas Generator, Open-Cycle
Propellant Feed Turbopump
Merlin 1 D Thrust Sea Level: 756 kN – Vac: 825 kN
Engine Diameter ~1.0 m
Engine Dry Weight 470 kg
Burn Time 162s
Specific Impulse 282s (SL) 311s (Vac) (M1D Standard)
Chamber Pressure >97 bar (M1D Standard)
Expansion Ratio 16
Throttle Capability 55% to 100%

from 

http://spaceflight101.com/spacerockets/falcon-9-ft/

Well, glad to be corrected in such a way. The Merlin gets more amazing with each year, 55% is a heck of a lot! I wonder if that's what's allowing them slower hoverslams in these last attempts, getting them closer to TRW 1 at landing.

 

Rune. The Raptor has a lot of reputation to live up to.

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50 minutes ago, Rune said:

Well, glad to be corrected in such a way. The Merlin gets more amazing with each year, 55% is a heck of a lot! I wonder if that's what's allowing them slower hoverslams in these last attempts, getting them closer to TRW 1 at landing.

 

Rune. The Raptor has a lot of reputation to live up to.

i guess so, you could probably calculate that up to a certain range at least. i'd guess they can do around 1 TWR since you can see they are coming in way slower now.

can't wait for the raptor mate...

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