SSTO design question

[Secuas]

So I decided to build up SSTO's again the other day I came up with this design:

It can deploy an 18 ton payload in 100x100 stable orbit. Considering the size of the payload bay it's quite big. (6 rapiers, 3 whiplash)

After trying another design I made a smaller one with even fewer engines. It can do the exact same as the first one despite it being smaller and having less engines.

Second desgin:


My question is how was this possible, I'm not quite sure why this works. Same payload less engines. (6 rapiers)

Edited October 21, 2017 by Kaname
engine details

[JK_Kerbineer]

1 hour ago, Kaname said:

My question is how was this possible, I'm not quite sure why this works. Same payload less engines. (6 rapiers)

1

The more engines you have, the less total efficiency you have throughout the rocket, so you go a shorter distance. Also, if you have more thrust, there is more drag on your SSTO. Not to mention the extra weight and having, even more drag from your engines sticking out. That's why 3-ton rocket is able to just as much as a 50-ton rocket if you design it right. Sadly, "MOAR BOOSTERS!" does not always work (though it sure is entertaining). 

[Spricigo]

36 minutes ago, JK_Kerbineer said:

 Also, if you have more thrust, there is more drag on your SSTO. 

Actually no. Drag depends on velocity, exposed area and fluid density.

 

@Kaname there is no mystery, the stronger and  bigger craft carry more to orbit. Unfortunately that 'extra' is the dead weigth you had on it.

 

[JK_Kerbineer]

11 minutes ago, Spricigo said:

Actually no. Drag depends on velocity, exposed area and fluid density.

 

@Kaname there is no mystery, the stronger and  bigger craft carry more to orbit. Unfortunately that 'extra' is the dead weigth you had on it.

 

3

Sorry, I meant that if you have a higher constant thrust, it would give you a higher velocity. Not that a higher thrust contributes directly to more drag,

[Spricigo]

First: why do you slashed fluid density? It is one of the factors. (You certainly had been told, and maybe even tell it yourself, that at higher altitudes there is less air and thus less drag.)

Second: Thrust is really one factor but so many others makes a bit problematic to konw what the "end result will be.

Edited October 21, 2017 by Spricigo

[Secuas]

So I actually just overengineered the first craft for the payload it is supposed to carry?

[JK_Kerbineer]

8 hours ago, Spricigo said:

First: why do you slashed fluid density? It is one of the factors. (You certainly had been told, and maybe even tell it yourself, that at higher altitudes there is less air and thus less drag.)

 

1

I know it certainly is a factor, but the question was how could one plane do the same as another one that was bigger. The fluid density would be constant so it wasn't really relevant to the question asked.

3 hours ago, Kaname said:

So I actually just overengineered the first craft for the payload it is supposed to carry?

With the amount of RAPIER's you strapped on that first one, I'd say yes...

[Laie]

My guess is that you saved a lot of mass throughout. On your first vessel I see a proper mk3 cockpit and a large monoprop tank, both of which are missing on the second. That's 8t right there.

More generally, jets not only provide thrust but also substantial mass, which becomes deadweight once they run out of air -- approximately the second half of the trip. Also, every jet requires a 1.25 attachment node which needs to be topped off in some way, so indirectly they are the cause for a lot of drag.

[bewing]

Shock cones have much less drag than the ramp intakes. Your new tailfin design has much less drag. The MK3 crew cabin has a lot of drag that is missing on the second design. That MK3 crew cabin has a node on the front -- I can't see what you put on it, but it's probably got a lot of drag. Whatever you did to mount the 6 rapiers in the first design caused a lot of drag. The first design had many extra air intakes, which added mass and drag. It had extra fuel tanks which added mass and drag. And the first design has drogue chutes, which add mass and drag. The reduced drag is the primary reason why your second design works better.

There are certainly several ways that you could simplify it more, and reduce the mass and drag more.

 

[Secuas]

Thanks for the insight guys! I redesigned the first one to carry one orange tank, It can reach an 150x150 orbit so it can rendezvous with my station at 100 km orbit. with only 10 Dv left. Any tips on improving this without making it larger? (the statistics in the picture are with orange tank in the carge bay included)

 

Edited October 21, 2017 by Kaname

[Palaceviking]

6 hours ago, bewing said:

Shock cones have much less drag than the ramp intakes. Your new tailfin design has much less drag. The MK3 crew cabin has a lot of drag that is missing on the second design. That MK3 crew cabin has a node on the front -- I can't see what you put on it, but it's probably got a lot of drag. Whatever you did to mount the 6 rapiers in the first design caused a lot of drag. The first design had many extra air intakes, which added mass and drag. It had extra fuel tanks which added mass and drag. And the first design has drogue chutes, which add mass and drag. The reduced drag is the primary reason why your second design works better.

There are certainly several ways that you could simplify it more, and reduce the mass and drag more.

 

Did you say mass and drag?

2 hours ago, Kaname said:

Thanks for the insight guys! I redesigned the first one to carry one orange tank, It can reach an 150x150 orbit so it can rendezvous with my station at 100 km orbit. with only 10 Dv left. Any tips on improving this without making it larger? (the statistics in the picture are with orange tank in the carge bay included)

 

How about adding a couple ant/spider engines to the tank and letting it circularize itself? Smaller masses are easier to move. 

2 hours ago, Kaname said:

Something 

Edited October 21, 2017 by Palaceviking

[Secuas]

1 hour ago, Palaceviking said:

 

How about adding a couple ant/spider engines to the tank and letting it circularize itself? Smaller masses are easier to move. 

 

That would defeat the point of my ssto, it's supposed to deliver it to the station and fly back.

[Aegolius13]

3 hours ago, Kaname said:

Thanks for the insight guys! I redesigned the first one to carry one orange tank, It can reach an 150x150 orbit so it can rendezvous with my station at 100 km orbit. with only 10 Dv left. Any tips on improving this without making it larger? (the statistics in the picture are with orange tank in the carge bay included)
 

Nice design!  No mean feat to get an orange tank to orbit.  A couple thoughts:

-That 2.5 nosecone is fairly draggy (or at least used to be; not 100% sure still the case) .  You could go with some adapters to taper your front down, and put a 1.25m advanced nose cone,  small nose cone, shock cone, or even a fairing you never pop in front.  

-Do you really need those radial air intakes?  If you're borderline, you might be able to eliminate if you add an extra shock cone per previous point.  

-You might consider the Big S delta wing.  It has the same lift-weight ratio as the modular wings, but comes with free LF storage.  It also has good temp tolerance, and tends to be less prone to breakage than the modular wings.  If you need a little more lift, you could look at the wings strakes too, use a bigger canard, or just slap a few modular wings on the back of the Big S.

-Are those multiple linear RCS ports?  You could try switching to Vernors.  That might save drag or at least simplify your fuel situation.  Or ideally, try to make it with no RCS at all.

-You might have a little more roll and yaw control authority than needed, though it is a big plane so maybe not.  You can also get the most of your pitch authority by putting it as far from the COM as possible.  That probably means putting big canards as far forward as they'll go.

-I have never really used it, but vaguely remember hearing some hate on the Space Shuttle-style engine mount for having high drag and mass.  If it does seem draggy, you could taper down to 2.5m and use one of the multi-couplers to mount your engines. 

-Those Mk.2 fuselages you have on the side also have a bad rap for being draggy.  You could look at doing two 1.25m stacks on each side in order to keep the same number of engines.  

[Spricigo]

1 hour ago, Kaname said:

 

That would defeat the point of my ssto, it's supposed to deliver it to the station and fly back.

I'm not saying you should adopt the idea, but it offer a way to solve your problem. 

You craft may not be able to 1.get in orbit; 2.rendezvous with your station; 3.delivery the payload; 4.land back in Kerbin. But it may be able to 1; deploy a cheap tug that does 2 and 3; 4.

 

 

[bewing]

8 hours ago, Kaname said:

Thanks for the insight guys! I redesigned the first one to carry one orange tank, It can reach an 150x150 orbit so it can rendezvous with my station at 100 km orbit. with only 10 Dv left. Any tips on improving this without making it larger? (the statistics in the picture are with orange tank in the carge bay included)

 

OK, so if I'm gonna "improve" your design, it would be like this.

1. Get rid of the nacelles, if at all possible. Especially nacelles built out of MK2 parts. MK2 parts have super-high drag.

2. Reduce the number of engines. Engines are heavy.

3. Reduce to one air intake. One air intake can handle several engines.

4. Use BigS wing parts, because they store LF for free.

5. Get rid of the RCS stuff. You can fly without it -- it just takes a little more time and care to do. And all that RCS stuff adds mass and drag.

6 hours ago, Palaceviking said:

Did you say mass and drag?

Y'know, I think I did!

 

Craft file: https://pastebin.com/raw/7bLeNt8q

 

[Secuas]

After implementing some of your advice I came up with this:
It's gotta at least look a little nice

https://imgur.com/a/grE4V

(Embedding it doesn't seem to be working)

 

Edited October 22, 2017 by Kaname

[AeroGav]

Adding engines actually harms performance by increasing dry mass (rapiers are 2 tons each, whiplash 1.8 ton) and cutting your delta V,  if you already have a large margin of thrust over drag and therefore have minimal gravity/drag losses.

The second design is lower drag too .

Also,   all air breathing engines have cap on the max velocity they can achieve,  beyond a certain point , thrust gets less the faster you go.  By mach 6, RAPIER thrust falls to zero.      So long as you got enough RAPIER to reach 1400 air breathing, you've probably got plenty.

[bewing]

You still have too many air intakes with those useless precoolers in your nacelles.

And "look a little nice"?? What is this thing of which you speak? (OK, yours does look nicer! I don't bother much with aesthetics. )

But congrats! Looks like you have a working doohickey!

 

[Secuas]

Thanks for the help everyone! One note though, are those pre-coolers actually useless? They still provide fuel and intake air.

[bewing]

2 hours ago, Kaname said:

Thanks for the help everyone! One note though, are those pre-coolers actually useless? They still provide fuel and intake air.

The 3 shock cones provide way more air than your engines need through your entire flight profile. And the precoolers only provide a tiny bit of fuel, for the cost of a relatively large dry mass.

[Spricigo]

2 hours ago, Kaname said:

Thanks for the help everyone! One note though, are those pre-coolers actually useless? They still provide fuel and intake air.

Not useless but redundant.  Anything you can remove and still do you mission without issues should be removed.

So, do you need the intake air from pre-coolers? Do you need the fuel form pre-coolers? (Even if you need, that's the best way to havevit?)

3 hours ago, bewing said:

And "look a little nice"?? What is this thing of which you speak? (OK, yours does look nicer! I don't bother much with aesthetics. )

For what is worth: I like how your craft looks. Not 'nice' but 'functional'.