Help my rocket (Areodrag)

[Giygus]

I am trying to build my first lander to get to Duna and this is what I got:

Spoiler

My issue is that empty space for the small sized engine under the fuel tank.   I am assuming this will cause a good deal of Aero-drag, but upgrading from a Terrier to a Poodle costs me like 600 Dv that I don't want to lose if I don't have to.

Any ideas?

I ask because I am sure this will come up again in the future

[James Kerman]

I don't know about the drag but I'm pretty sure that rocket is going to wobble during ascent.  Inside the rockomax stack separator are 4 'tabs' that extend inward.  I usually attach 4 struts to the fuel tank above starting from the tabs.

[Nich]

You can use the 2.5 m faring and connect it to the tank

[Giygus]

3 minutes ago, Nich said:

You can use the 2.5 m faring and connect it to the tank

Tried, I can't get the Faring to close.

I can't say I have much experience with them, but I think they need to touch completely around itself, like a rocket nose cone, in order to be set as completed in the VAB.

Maybe I am wrong?

[OhioBob]

You can reduce drag quite a bit by using 2.5x1.25m tapered adapters to form smooth size transitions.  In your case, placing an adapter on the bottom of the top stage won't work because it messes up your lander.  But you can certainly place a tapered adapter on the top of the bottom stage.  You can also reduce drag and mass by using a 1.25m decoupler instead of a 2.5m.  I also agree that struts across the stages will make the connection a lot stronger.
 

Edited April 20, 2016 by OhioBob

[Nich]

Are the langing gear or docking ports getting in the way of closing? I know you can close a fairing on a fuel tank, ore tanks or isrus don't work

Edited April 19, 2016 by Nich

[Giygus]

18 minutes ago, Nich said:

Are the langing gear or docking ports getting in the way of closing? I know you can close a fairing on a fuel tank, ore tanks or isrus don't work

I can get the fairing to work if I make it very large, and pretty much encompass the entire top of the ship starting just under the decoupler.  That's not exactly a bad thing really, but the added weight is 1+ton, and I am unsure if that kills w/e efficiency I gain from reducing drag.

[Harry Rhodan]

7 hours ago, Giygus said:

I am assuming this will cause a good deal of Aero-drag,

Are you just assuming or does it really cause any problems?

Looking at your current staging that rocket will barely lift of on the SRBs while your liquid stage is completely overpowered and could definitely carry much more fuel that you will definitely need if you are going for a powered landing. There's also at least one asymmetrically placed goo container ate the top that might be a much bigger problem during ascend. And you should check, if the Terrier has enough omph to land on duna, because as an upper stage engine it performs pretty poorly in an atmosphere.

[Plusck]

7 hours ago, Giygus said:

Tried, I can't get the Faring to close.

I can't say I have much experience with them, but I think they need to touch completely around itself, like a rocket nose cone, in order to be set as completed in the VAB.

Maybe I am wrong?

You should be able to get it to close around near the top of the 2.5m tank, if you move the RCS thrusters down a touch.

Alternatively, use the very short 2.5 to 1.25m adapter, then 1.25 decoupler, then Terrier, then the short adapter again. And add struts. Sure, the longer adapter is better for drag, but there is a far bigger difference between "no adapter" and "short adapter" than between "short adapter" and "long adapter" - from my tests you're better off having two short adapters top and bottom than a single long adapter at the top (like you have here) if total length is an issue.

Having looked over the thread again I see that this is more or less exactly what OhioBob is saying, so +1 to that.

Or as a completely different alternative, how about using only 1.25m parts? It'll be less stable in one axis for landing, admittedly, but you could make the lander core a T800 tank, with one on each side as drop-tanks (with lander legs) then just return the core to orbit (since it looks like that's what you're planning to do here).

And since you have a service bay, why not move the science instruments inside? It'll be a tight squeeze but that'll help both aero and CoM-offset.

And is that a Mainsail on the bottom? If so, definitely swap for a Skipper - better I_sp in vacuum (and most of the time it'll be in near vaccum since it's virtually an SSTO) and lighter and you don't need all that thrust

[OhioBob]

13 hours ago, Giygus said:

I can get the fairing to work if I make it very large, and pretty much encompass the entire top of the ship starting just under the decoupler.  That's not exactly a bad thing really, but the added weight is 1+ton, and I am unsure if that kills w/e efficiency I gain from reducing drag.

Duna's air is thin enough that you might not have to worry about streamlining your lander.  (The density of Duna air at the 0km elevation is like being at an altitude of about 14 km m on Kerbin.)  Therefore, you could probably eliminate the nosecone and tapered adapter, which will save you some mass, and then just enclose the whole thing in a fairing for your initial launch.  (I also recommend switching to a 1.25m decoupler.)  Not streamlining the will cost you some drag losses when you launch from Duna, but in that thin air the losses should be tolerable.  The extra drag losses will likely be offset by not having to carry as much mass during your ejection and orbit insertion burns.  you might actually come out ahead.

[Vanamonde]

It will cause drag, but the effects are over-emphasized by people who want their rockets to look realistic. (No offense, really not meaning to insult anybody or start an argument!) The other day I tested an old design in new aero, and despite its INTENSELY un-aerodynamic configuration, it flew to orbit just fine. 

[Nich]

My faring closed just fine.  try removing the landing gear first

[Giygus]

Thank you for all the help! 

This particular rocket design was just a test, which is why I choose to only show the area I was not able to come up with a solution for on my own.  I will post a picture later to show how the fairing, with the added adapters, looks like, to see if you think I could do anything more. 

TL;DR:

Spoiler

The rest of the design was just a proof of concept to see what I could do to get the Dv where I needed it.  The lift end of the rocket will get the once over when I actually figure out what I want to lift.  Mostly, I am debating on weather or not it would be easier to refuel a smaller rocket, and exactly how many different times I could/should refuel.  I am sure getting a smaller rocket off the ground, and refueling later, would be way easier.  But the added complexity of sending out a refuel ship (or 2 or 3), and actually linking them up multiple times could take a lot of the fun away from the mission.

 

[OhioBob]

3 hours ago, Vanamonde said:

It will cause drag, but the effects are over-emphasized by people who want their rockets to look realistic. (No offense, really not meaning to insult anybody or start an argument!)

I agree.  A very non-aerodynamic payload probably doesn't cost more than about 100-200 m/s over a very streamlined payload.  What I find to be the biggest problem with draggy payloads is aerodynamic stability.  Increasing drag at the front end of the rocket will often cause the CP to move ahead of the CM, causing the rocket to flip.