2.5m rockets

[Delay]

I've been using 1.25m rocket parts since I began in KSP, and they work fantastically. But due to efficiency and other factors, I'm now more or less forced to come up with 2.5m rocket designs. For example, I wouldn't need a probe core anymore, just a pilot next sitting next to the tourist/ other crew member. I would save mass and I'd also able to control the craft even without connection to Kerbin.

Unfortunately, not a single design I tried gets me into LKO consistantly, sometimes noteven that. And it's becoming a problem; I have an asteroid to catch in just 28 Kerbin-days. So here's my question:

Are there tips that help me create a 2.5m (Later even 3.75) rocket that gets into orbit consistantly? It should also be easily expandable to perform more complex tasks.

2.5m parts I have: Entire "Rockomax X" fuel tank series, including the Jumbo 64,

Skipper engine

Poodle engine

(Small)Delta Wing

[DD_bwest]

How familiar are you to the terms TWR  and DeltaV?  (basically how fast you can accelerate, and how much velocity change you can get out of your fuel)

the 2.5m parts work the exact some way the smaller parts do, they just have different weight and thrust values.   do you have a picture of the rocket your trying to get in orbit?

[Ohm is Futile]

Pictures of your designs would help figure out what problems you might be having. Details of how you fail to get into orbit may also help, although I would assume you are running out of fuel.

7 minutes ago, Delay said:

I've been using 1.25m rocket parts since I began in KSP, and they work fantastically. But due to efficiency and other factors, I'm now more or less forced to come up with 2.5m rocket designs. For example, I wouldn't need a probe core anymore, just a pilot next sitting next to the tourist/ other crew member. I would save mass and I'd also able to control the craft even without connection to Kerbin.

The 2.5 meter capsule (Mk I-2 Command Pod) is not so efficient, actually. It's more than 4x heavier than Mk I pods, heavier than the habitation module with 4 seats and heavier than sticking a Mk I crew cabin under a Mk I Command Pod. So no, it doesn't really save mass, although it does have its perks and is convenient if only because it is both a command module and has more than one seat.

 

10 minutes ago, Delay said:

2.5m parts I have: Entire "Rockomax X" fuel tank series, including the Jumbo 64,

Skipper engine

Poodle engine

(Small)Delta Wing

One problem you might be having here is your thrust to weight ratio. Looking at your engines, while skippers and poodles are good engines, they do not really provide a great deal of thrust depending on the mass of your payload and you may run into problems when trying to balance fuel and thrust when designing an ascent stage with only those engines available in the 2.5 category.

With this limited amount of information, my suggestion would be, funnily enough, to add more boosters.

[Delay]

1 minute ago, DD_bwest said:

How familiar are you to the terms TWR  and DeltaV?

I am very familiar with the terms. I do dV calculations all the time, really. I typically use a TWR of 1.5.

 

3 minutes ago, DD_bwest said:

do you have a picture of the rocket your trying to get in orbit?

Unfortunately I don't have a picture at this time. I'll try to add it as soon as possible. Until then, this has to be enough: The second stage is powered by a Poodle fed by a Rockomax X200-8 360lf fuel tank. The bottom of the two stages consists of a Skipper fed by a Rockomax X200-32. The boosters on the side are made up of an FL-T 400 and a FL-T200 which power a Swivel. It weighs around 70-77tons, TWR is 1.47.

[DD_bwest]

4 minutes ago, Delay said:

I am very familiar with the terms. I do dV calculations all the time, really. I typically use a TWR of 1.5.

 

Unfortunately I don't have a picture at this time. I'll try to add it as soon as possible. Until then, this has to be enough: The second stage is powered by a Poodle fed by a Rockomax X200-8 360lf fuel tank. The bottom of the two stages consists of a Skipper fed by a Rockomax X200-32. The boosters on the side are made up of an FL-T 400 and a FL-T200 which power a Swivel. It weighs around 70-77tons, TWR is 1.47.

whats your payload?   ive been putting it together in my vab and with just a command pod and mine is considerably lighter than 70 tons.   it has a pretty high twr, but just barely enough delta v.

[Benjamin Kerman]

16 minutes ago, Ohm is Futile said:

With this limited amount of information, my suggestion would be, funnily enough, to add more boosters.

Yes, in this case, try using asparagus staging, with jumbos aranged in a circle with fuel lines. 

[Delay]

Here's a list of the unmentioned parts:

Mk16-XL Parachute,

Heat Shield (2.5m), (Full Ablator)

Rockomax Brand Decoupler

RV-105 RCS Thruster Block (4x)

Service Bay (2.5m), (beneath is stored inside)

Probodobodyne HECS,

Small Inline Reaction Wheel,

ROUND-8 Toroidal Fuel Tank,

Msytery Goo Containment Units (2x)

PresMat Barometer

2HOT Thermometer

Delta Wing (4x)

[DD_bwest]

you just dont have enough delta V.  im still getting a higher TWR than what you say though.    your 2nd stage could easily get more, i would double the fuel for it.  and add 50% more fuel to the lower main stage, and add 50% more fuel to your boosters.   then maybe add an extra set of boosters to the lowest stage if your TWR falls to low, but im pretty sure you wont have that problem.   Adding fuel lines to crossfeed the fuel from your boosters to the main tank will also get you a bit more out of it.

[Ohm is Futile]

12 minutes ago, Delay said:

Here's a list of the unmentioned parts:

Mk16-XL Parachute, OK

Heat Shield (2.5m), (Full Ablator) ~ No need full ablator normally for flights within Kerbin's SOI, especially if you are careful with your re-entry. Half of it is plenty. Even a heat shield without ablator should survive if you are careful enough.

Rockomax Brand Decoupler OK

RV-105 RCS Thruster Block (4x) ~ Does this ever dock? If not, don't bother with RCS. Which also means you can remove any monopropellant from a command module/tank.

Service Bay (2.5m), (beneath is stored inside) OK

Probodobodyne HECS, How many seats do you have/need? Is it bad if the vehicle goes without SAS after entering orbit?

Small Inline Reaction Wheel, Given you mentioned tourists, I assume you have a command pod or something? Do you really need the extra torque?

ROUND-8 Toroidal Fuel Tank, Why? Does this feed an engine?

Msytery Goo Containment Units (2x) OK

PresMat Barometer OK

2HOT Thermometer OK

Delta Wing (4x) Wings are double-edged swords, especially for rockets. Yes, they can make ascent to orbit more stable, but they can stabilize you in a very inefficient climb, too... and they generate drag. Consider removing wings.

 

Edited January 24, 2017 by Ohm is Futile

[Harry Rhodan]

20 minutes ago, Delay said:

Here's a list of the unmentioned parts:

Don't you think a screenshot would be the better solution?

 

Don't I think reading the whole thread before posting already answered questions would be the better solution?

Edited January 24, 2017 by Harry Rhodan

[Delay]

4 minutes ago, Ohm is Futile said:

Probodobodyne HECS, How many seats do you have/need? Is it bad if the vehicle goes without SAS after entering orbit?

I need all three seats in the Mk1-2. Is bad if the vehicle goe without SAS? Well, SAS makes a big difference. Especially when going for Minmus, perfect example for small SOI (good "aim" needed), so precise execution is important.

8 minutes ago, Ohm is Futile said:

Does this ever dock? If not, don't bother with RCS.

No, it doesn't dock. I wanted to use it as a secondary "last solution" propellant.

10 minutes ago, Ohm is Futile said:

Wings are double-edged swords, especially for rockets. Yes, they can make ascent to orbit more stable, but they can stabilize you in a very inefficient climb, too...

Without wings, the rocket flips uncontrollably after going supersonic. So i kinda need them.

 

10 minutes ago, Ohm is Futile said:

Given you mentioned tourists, I assume you have a command pod or something?

Yes I do. But the rocket is pretty slow when it comes to controll inputs.

12 minutes ago, Ohm is Futile said:

Why? Does this feed an engine?

Yes, it feeds the Poodle. The service bay is seperated in the same stage (so parachute is actually triggered before seperation).

 

13 minutes ago, Ohm is Futile said:

No need full ablator normally for flights within Kerbin's SOI,

I didn't expect them to be this robust. I'll lower the ablator

[DD_bwest]

adding fuel to that second stage will help you a ton.  its underutilized with that much fuel

[Ohm is Futile]

15 minutes ago, Delay said:

Without wings, the rocket flips uncontrollably after going supersonic. So i kinda need them.

This is rather concerning. You may indeed have more TWR than you think/need. A TWR too high will cause you to fight the atmosphere more than you need to and lose you precious m/s of dV. It can also cause stability issues since it will compound aerodynamic/drag issues. You may want to adjust the thrust limiter to get a TWR closer to 1.3.

The other problem this may indicate is an inadequate ascent trajectory. You *need* to keep your rocket pointed inside the prograde marker during ascent or the chances of it tumbling uncontrollably increase exponentially if you don't. If an agressive ascent profile feels too dangerous, just tip the rocket a little bit after 1km altitude and follow the prograde marker from there until 30km altitude, then turn the rocket according to how high your apoapsis is/needs to be to avoid wasting more fuel. Aerodynamic effects are minimal past 30km (unless you're really hard on the controls - also I haven't used wings on rockets in a while so I assume a no-wing rocket on ascent when suggesting this).

Edited January 24, 2017 by Ohm is Futile

[Tyko]

21 minutes ago, Ohm is Futile said:

This is rather concerning. You may indeed have more TWR than you think/need. A TWR too high will cause you to fight the atmosphere more than you need to and lose you precious m/s of dV. It can also cause stability issues since it will compound aerodynamic/drag issues. You may want to adjust the thrust limiter to get a TWR closer to 1.3.

The other problem this may indicate is an inadequate ascent trajectory. You *need* to keep your rocket pointed inside the prograde marker during ascent or the chances of it tumbling uncontrollably increase exponentially if you don't. If an agressive ascent profile feels too dangerous, just tip the rocket a little bit after 1km altitude and follow the prograde marker from there until 30km altitude, then turn the rocket according to how high your apoapsis is/needs to be to avoid wasting more fuel. Aerodynamic effects are minimal past 30km (unless you're really hard on the controls - also I haven't used wings on rockets in a while so I assume a no-wing rocket on ascent when suggesting this).

good points, but I don't think a 1.5TWR is the problem. That's my default take off TWR. If I ever have probs staying on prograde it's usually aerodynamics related. Totally agree on second point of keeping to prograde through lower atmosphere. The other thing to check is that your Center of Mass isn't off.

(By the way...watching your YouTube vids and they're really well done...congrats )

[Harry Rhodan]

Launch TWR is about 1.54 with SRBs set to 75% thrust. Wiggles a bit during ascent but gets to orbit with about 500m/s left.

[Ohm is Futile]

13 minutes ago, Tyko said:

The other thing to check is that your Center of Mass isn't off.

Oh yeah, that and/or uneven number of parts not being counterbalanced, ie.: putting all your lightweight science instruments on one side may not seem to do much to your CoM but it can cause large amount of drag and spin you out nonetheless.

 

12 minutes ago, Harry Rhodan said:

[removed the picture]

Launch TWR is about 1.54 with SRBs set to 75% thrust. Wiggles a bit during ascent but gets to orbit with about 500m/s left.

Looks good, I usually launch with much lower TWR values (usually about 1.25), but as long as it doesn't waste too much fuel fighting the atmosphere and gets up there without doing crazy figures, I'll give it a thumbs up. It sure looks like a design that could fix the OP's issues.

 

20 minutes ago, Tyko said:

(By the way...watching your YouTube vids and they're really well done...congrats )

(Thanks, I try very hard, I'm a bit of a perfectionist so I am not entirely happy with them (the audio bugs me the most), but I think I did a decent job so far.)

[DerekL1963]

@Delay What a lot of people don't realize is that you can use Tail Connector A (which is pretty low in the tech tree) to cluster engines.  I use this method almost exclusively for building 2.5 and 3.75 first stages.  (Don't forget to use the center node though.)

This vehicle (for example) uses a mix of LV-T30's (for performance) and LV-T45's (for control) on the first stage, a Skipper on the second stage, and a Poodle on the CSM.

[Superfluous J]

2 hours ago, Delay said:

No, it doesn't dock. I wanted to use it as a secondary "last solution" propellant.

Don't do this. It's like filling your trunk with moonshine in case you run out of gasoline. Just bring more gasoline. It's more efficient per unit of mass and you already have the engines. In fact, I've stopped using monoprop all together and now just use Verniers.

My first thought of your ship is it's got too much engine for the fuel and payload. I'd double the fuel in the first stage (swap the small tank for an orange one, or use 3 of the half orange tanks if you want the rocket all white) and maybe swap the hammers for BACCs.

And I also wouldn't throttle down the solids. Their benefit is they're cheap and have good TWR, but they're also heavy and have poor I_sp. Every second they're on your ship it's less efficient than it can be, so you want to get their benefit and toss them ASAP. If you're going too fast on launch, use less solids or bring more LFO.

Edited January 25, 2017 by 5thHorseman

[FullMetalMachinist]

13 minutes ago, 5thHorseman said:

Don't do this. It's like filling your trunk with moonshine in case you run out of gasoline. Just bring more gasoline.

Don't forget that you also have to bring the RCS thrusters. So it's like bringing moonshine and a 300lb guy named Bubba who drinks the moonshine and pushes the car. 

[Firemetal]

Unlock the rest. Orange tank, mainsail, ect. Skipper and poodle for lower gravity places.

Fire

[Sharpy]

Forget Skipper. It's too strong and inefficient for low-gravity worlds, and too weak for Kerbin.

Orange tank or two, and Twin Boar or Mainsail go SSTO with a good 15 ton payload. Poodle is great for travel in space and landing on low-gravity bodies until you unlock nukes (and later once you want to go cheap, or with large unwieldy payload that won't neatly fit in 1.25m fairing).

 

[GoSlash27]

Delay,

 You mentioned that you are very comfortable doing DV and t/w calculations. Perhaps it's time for you to start doing them backwards...

 Designing rockets in 2.5m scale is exactly the same as designing them in 1.25m. In fact, it's a little more forgiving because 2.5m rockets have less drag per tonne.

 FWIW, I love the skipper. It's a great all- around engine and very economical as well. It makes a good upper stage for heavy payloads and a good lower stage for medium payloads. It almost begs for a nerf.

In your case, you're looking at a pretty light payload; one that can be handled easily with 1.25m parts.

http://wikisend.com/download/181788/Cheep15Final.craft

This little guy can get 15 tonnes to the circularization point, which is way more than you need.

Best,
-Slashy

 

 

 

[GoSlash27]

Delay,

 Here's an example of a launcher for your mission:

It has 2,000 m/sec DV on orbit and the full ablator. It's designed to be cheap, so total cost at launch is $26,000.

Best,
-Slashy

Edited January 25, 2017 by GoSlash27

[Delay]

This rocket (with 3 boosters and 25% less clamps) can get into orbit with around 1800m/s left. It weighs slighty more than 70 tons and has a TWR of ~1.3 (Also note the extremely good name of the vehicle).

Further imrpovements should get this rocket further, so I guess the question is solved.

[Spricigo]

4 hours ago, GoSlash27 said:

http://wikisend.com/download/181788/Cheep15Final.craft

 

 

 

 I approve this product (not that it means much..or anything at all)