TWR, Dv, ISP? Baffled

[Red Shirt]

Background - I was half watching something on SCI this morning. There was a shot of a missile fired off a submarine. What really caught my attention was thinking, look its a FL-T800. So that got me wondering can I get a single FL-T800 to orbit. I couldn't. So I added a small tank (FL-T200). Everything to my limited knowledge says it should make it but I can't do it. Tried various nosecones and ascent patterns. What am I missing?

 

Edit: I should add I have an Octo2, small reaction wheel, and battery all tucked in under the nosecone.

Edited January 10, 2018 by Red Shirt

[adsii1970]

Could it be the angle? Are you using a gravity assisted turn?

[Red Shirt]

18 minutes ago, adsii1970 said:

Could it be the angle? Are you using a gravity assisted turn?

I've tried several different methods. Turning slightly nearly immediately after launch thru old school go high and turn. Fall short by abt 300 dv

[adsii1970]

3 minutes ago, Red Shirt said:

I've tried several different methods. Turning slightly nearly immediately after launch thru old school go high and turn. Fall short by abt 300 dv

Your numbers look good. There's nothing I see that should prevent you from achieving a LKO...

 

[Red Shirt]

Just now, adsii1970 said:

Your numbers look good. There's nothing I see that should prevent you from achieving a LKO...

That's what I thought. Even without adding the small tank I thought it should make it to orbit. 

More info: The engine is the Valiant V-15 from the Squad release of the Porkjet parts. It has been a much used engine in my missions. 

[adsii1970]

Just now, Red Shirt said:

That's what I thought. Even without adding the small tank I thought it should make it to orbit. 

More info: The engine is the Valiant V-15 from the Squad release of the Porkjet parts. It has been a much used engine in my missions. 

I recognized it. It is a workhorse engine for me when I'm launching small payloads. There has to be something we are missing. Safe to assume you did some parts clipping then, right?

[linuxgurugamer]

Try using Gravity Turn, it yhe most efficient autopilot

[Fearless Son]

59 minutes ago, Red Shirt said:

Edit: I should add I have an Octo2, small reaction wheel, and battery all tucked in under the nosecone.

That might be part of it.  I cannot speak to the drag properties of the reaction wheel and battery, but I know that some aerodynamic structural parts do not occlude parts they clip over unless those aerodynamic parts are designed to accommodate interior payloads (at which point it checks to see that the payload is covered up and that the center of mass of the covered objects is inside the payload container.)

My advice would be to swap out the nosecone with a small fairing, clipping the base of the fairing in a little bit if that helps.  Put the reaction wheel and battery inside there and it will consider them "inside" an aeroshell and therefor not subject to atmospheric drag forces.

Edited January 10, 2018 by Fearless Son

[Foxster]

1 hour ago, Red Shirt said:

Edit: I should add I have an Octo2, small reaction wheel, and battery all tucked in under the nosecone.

That won't be helping. The nosecone won't be reducing the drag of those items and they will rob the craft of dV. 

[Snark]

 

1 hour ago, Red Shirt said:

Edit: I should add I have an Octo2, small reaction wheel, and battery all tucked in under the nosecone.

I suspect this may be a big part of it.  Clipping is completely irrelevant to aerodynamics.  It's cosmetic only.

If what you've done here is just to stick a nosecone on top of an OKTO2 on top of a small reaction wheel on top of a battery on top of the fuel tank... try putting that together in the VAB without doing any clipping (i.e. just attach things to the nodes, the way they want to be) and take a good look at it.  That is what the aerodynamic code is seeing.  If that's what you've done, then aerodynamically speaking, what your craft looks like is:

• a nice pointy front end (good!)
• the sudden flat backside of the nosecone (ouch!)
• a few 0.625m cylindrical parts (good!)
• a sudden stair-step radius increase from 0.625m to 1.25m, with the flat front of the fuel tank exposed un-shielded to the oncoming airflow (ouchity ouch ouch owie!)

@Fearless Son's suggestion of a fairing is probably a really good idea.

Another alternate approach would be to put a 1.25m-to-0.625m adapter on top of the fuel tank, then the battery / wheel / probe core on top of that, then a 0.625m probe core on top of that.  But my guess is that the fairing would probably work better.

Poor aerodynamics can really clobber you, especially for a small ship.  It's not just that you save dV from reduced drag (though you do)-- it's also that with a sleeker design, you can accelerate quite a bit faster while you're still low down, which in turn means you can follow a more aggressive gravity turn, which in turn gives you a more efficient trajectory to orbit with fewer gravity losses.

[helaeon]

How about putting your control parts inside a fairing so they really are inside the nose cone? If you really want to be cute about it you could eject the fairing when you get high enough and get some delta-v back due to reducing mass.

[Red Shirt]

And that's why I love this community. Dropped the small tank and went back to just the FL-T800. Used a fairing to cover the control parts. Launched straight to orbit with over 200 dv to spare. 

Who would have thought there would be that much drag simply stepping down to 0.625 between the nose cone and the tank. I had no idea.

[Snark]

...Oh, and moving this to Gameplay Questions.  Missed that before. 

[foamyesque]

On 1/10/2018 at 5:21 PM, Red Shirt said:

And that's why I love this community. Dropped the small tank and went back to just the FL-T800. Used a fairing to cover the control parts. Launched straight to orbit with over 200 dv to spare. 

Who would have thought there would be that much drag simply stepping down to 0.625 between the nose cone and the tank. I had no idea.

 

Especially on small rockets, drag is murder. I've gone to orbit on a pair of OSCAR-Bs and trust me it was not pleasant :v

Edited January 12, 2018 by foamyesque

[Red Shirt]

On 1/11/2018 at 7:21 PM, foamyesque said:

Especially on small rockets, drag is murder. I've gone to orbit on a pair of OSCAR-Bs and trust me it was not pleasant :v

I have been trying to get to orbit on two Oscar B's ever since reading this. Space was easy. Orbit not so much. A moment ago I finally clicked your link to see how you did it. Ingenious. Now I'm off to attempt to duplicate and maybe shave a little weight.  

[foamyesque]

39 minutes ago, Red Shirt said:

I have been trying to get to orbit on two Oscar B's ever since reading this. Space was easy. Orbit not so much. A moment ago I finally clicked your link to see how you did it. Ingenious. Now I'm off to attempt to duplicate and maybe shave a little weight.  

 

Good luck!