Aerodynamics problem on munar mission

[Snowstorm]

Hello everyone!  For a long time now, I've been trying to make it to the Mun.  I have created this ship as a prototype, and all seems well, but when I launch, I want to fix the aerodynamic and structural weakness on the rocket.  Any advice?  Here is a picture of the rocket: 

[Plusck]

Well, struts obviously. More struts!

 

Seriously though, if you have a weak joint like the two you have here, put a 3x or 4x symmetry set of struts between the two widest parts.

 

For the design, it's a bit difficult because all of the parts go together to make up the total abilities of the rocket. I

Is that an LV-N on top of the jumbo orange tanks?

For a single trip to the Mun (which has to be the case here, since it will be discarded) I wouldn't bother with it: it is heavy and in this case, your fuel tanks are not LF-only so its adding even more tank weight for nothing. Replace it with a Poodle and the stage will hold together much better for a fraction of the weight. Obviously you need more fuel for the same delta-v, but I'd be surprised if you didn't make a net weight saving here.

On the top stage, I would either put outboard engines OR a central one, not both which seems to be the case here. It's making a weak link for your final payload (struts help!) and adding height to your rocket. That is also a lot of fuel for your landing stage - you should be able to get away with less if the previous stage can get you to low Mun orbit.

For the bottom stage, I'd go asparagus-style and avoid stacking jumbo tanks. But then again, I always end up with squat rockets which isn't ideal either...

[Snowstorm]

10 minutes ago, Plusck said:

Well, struts obviously. More struts!

 

Seriously though, if you have a weak joint like the two you have here, put a 3x or 4x symmetry set of struts between the two widest parts.

 

For the design, it's a bit difficult because all of the parts go together to make up the total abilities of the rocket. I

Is that an LV-N on top of the jumbo orange tanks?

For a single trip to the Mun (which has to be the case here, since it will be discarded) I wouldn't bother with it: it is heavy and in this case, your fuel tanks are not LF-only so its adding even more tank weight for nothing. Replace it with a Poodle and the stage will hold together much better for a fraction of the weight. Obviously you need more fuel for the same delta-v, but I'd be surprised if you didn't make a net weight saving here.

On the top stage, I would either put outboard engines OR a central one, not both which seems to be the case here. It's making a weak link for your final payload (struts help!) and adding height to your rocket. That is also a lot of fuel for your landing stage - you should be able to get away with less if the previous stage can get you to low Mun orbit.

For the bottom stage, I'd go asparagus-style and avoid stacking jumbo tanks. But then again, I always end up with squat rockets which isn't ideal either...

Thanks for the advice.  I'll try it out tomorrow after school.

Also, to answer your question, it is the "nerv" atomic rocket motor on top of the jumbo tanks. 

Edited December 18, 2015 by Snowstorm

[Plusck]

Thanks for upvoting my post. :up:

Since nobody else has responded yet, I took another look at the pic.

First off, it's hugely ambitious. It took me a while before I was ready to try setting a whole science lab (and therefore all the engines and fuel to get it there) down on the Mun. I did a few Minmus labs first then scaled up the most efficient result to suit the Mun's gravity. And the booster stage for those builds was monstruous.

Stability on landing is going to be very diffficult. Like, horribly difficult. Most of my labs have the thinnest tank underneath and small radial tanks with engines around the sides. And I still managed to tip over one or two of my first Mun attempts.

And looking more closely at the four outboard engines there, they look like Puffs. They are decent for their size, but they use monopropellant, which means that you need two sorts of fuel to get your 140kN thrust. With very careful balancing I'm sure the weight comes out much the same, but you're staging them at a different time to the LfO engine (which looks like a Terrier) so I think there's a high chance you'll run out of monopropellant first, and most of your descent will be with only 60kN of thrust available. With the big fuel tanks, it'll take a very long time to burn through the fuel and you'll have a very poor TWR.

But again, as I said earlier, I tend to end up with squat rockets, so my suggestions are not necessarily the best !

[Wanderfound]

As suggested above: replace the Terrier and LV-N with Poodles and run some stabilising struts from halfway up the core to the landing legs. I'd also add some nosecones to your SRBs, and strut the SRBs to the core.

It also looks like there's nowhere near enough booster for the payload. The LV-N/Terrier/Poodle are good for orbital work, but they don't have the TWR for boosting to orbit. It looks like you're going to run out of high-TWR booster long before reaching orbit.

Also: if I'm reading it right, you've never been to the Mun before?

A lab is a very big and heavy payload. Go small to start with; walk before you run. Start with a minimal lander, then scale up once you get the hang of it.

Edited December 19, 2015 by Wanderfound

[lukeoftheaura]

try putting nose cones on the SRBs. It might help.

[Snark]

Several suggestions.

First, a general one for all rockets:  Try to keep a consistent stack size.  If you have a 2.5m stack, use all 2.5m components.  You can have a wide stack that narrows to a smaller stack above it (using a conical adapter for streamlining), but don't go narrower-and-then-wider if you can avoid it.  (Places that your rocket does this:  when you have those skinny 1.25m engines in between 2.5m fuel tanks.)

Don't have any flat faces that are facing forward.  Put nosecones on radial boosters.  If your stack size changes (e.g. from 2.5m down to 1.25m), use a conical adapter.  (Places that your rocket does this: your SRBs; the 1.25m engines sitting on top of 2.5m tanks; the cupola up top).

It's almost never a good idea to stack tall-skinny fuel tanks on top of each other.  Yes, you can do it, but it tends to lead to floppy rockets, and can cause CoM problems when the top tank drains first.  i.e. don't stack two orange tanks on top of each other.

For efficient multi-stage rockets:  Try to follow a smooth "power law" size progression in terms of mass and thrust.  That is, pick a ratio-- 2 or 3 works well-- and try to arrange it so that each stage is that many times smaller in mass and thrust than the last one.  For example, if your first stage has a Mainsail with a big orange tank (1379 kN thrust, 32 tons fuel), then a good second stage is a Skipper (650 kN) with a 16-ton tank, since that's about half.  Compare that to what you have-- you've got a Nerv sitting on top of a Mainsail, you're going down in thrust by a factor of more than 20!  You've got oddball ratios of both thrust and fuel mass all up and down the stack.

Nervs are generally not a good idea for Mun missions, they're overkill.  They're mainly useful for interplanetary missions, and should usually be 3rd or later stage.

Suggestions:

• Reduce the two stacked orange tanks down to one.
• I see that you're taking off with the Mainsail + boosters.  Add more radial boosters (don't forget to adjust thrust) so that you can take off on radial boosters alone, and trigger the Mainsail when the SRBs are done.
• Replace the Nerv with a Skipper.
• Replace the Terrier with a Poodle.
• Get rid of the cupola, it's heavy and not aerodynamic.  Put a conical adapter and then a Mk1 command pod.

That will still leave you with a very tall, tip-prone lander, but it's a start. 

 

Edited December 19, 2015 by Snark

[Snark]

Oh, and I forgot to add: welcome to the forums!

[Snowstorm]

On 12/19/2015 at 10:18 PM, Snark said:

Oh, and I forgot to add: welcome to the forums!

Thank you!  And thanks everyone for all the suggestions!  

[Wanderfound]

BTW, as a demo of a Mun rocket that works: