I think some lights are going on somewhere

[Starchaser]

I got a contract to put a probe into orbit around Minmus. I loaded up my 'Minmus Probe" ship.

Here it is.

And then I thought...This probe doesn't need a science jr. I need something there to mount batteries, antenna, solar panels, and the thermometer to, the 1 fuel tank will get really crowded. So I replaced it with a structural fuselage, half the weight, much cheaper. Then, upon more reflection, I replaced that with another tank. a hair more weight, still less cost, more delta-v. Then I thought, the nosecone and rockomax adapter can be replaced by a fairing, a bit more cost, less weight. Then I kinda frowned at the TWR of the last 2 stages and replaced the LV909 and it's fuel tanks with a monopropellant engine and the terrier with a poodle. Both exchanges were to engines with less Isp, but the weight reduction actually increased my delta V. 

Anyway, this is my new probe, with better ranged antenna and significantly less cost and weight. and more delta-v

 

[The Aziz]

And much worse aerodynamic profile. Take a look how irl fairings look like and make use of it.

[James Kerman]

Good work with the improvements, @Starchaser. 

I sometimes use mushroom fairings in KSP however, as The Aziz mentioned, this will subject the top of the rocket to more drag and make it less stable in atmospheric flight.  The solution is to add more fins (drag) to the bottom.  In KSP gravity losses outweigh aerodynamic losses so the faster you get out of the atmosphere (without blowing up your vessel) the more efficient you are.

I build my first stage to almost get me to orbit (3200ms DV TWR 1.8) and finish circularising with payload engines.  Once you are in orbit TWR does not matter unless you intend to land somewhere but in order not to die of boredom during burns I use around TWR 0.4.  This is where ISP is king and the terrier is an excellent choice if you don't have access to nukes or ions.

[Starchaser]

FIrst off, just...no. Maybe take a closer look at that first pic, and note all the struts, batteries, antennae and other things flopping in the breeze. The second is much better aerodynamically, and that's confirmed by flying the two one after another. I could have elongated the nose more, but I stubbified it so the fairing was cheaper. I could have added another fairing to cover the smaller engine, but again, I was looking at cost savings.

If you're complaining about it flaring out, well, the antennae clip through if I made it flat. The edge created by the smaller engine is a much bigger vortex generator.

25 minutes ago, James Kerman said:

I sometimes use mushroom fairings in KSP however, as The Aziz mentioned, this will subject the top of the rocket to more drag and make it less stable in atmospheric flight.  The solution is to add more fins (drag) to the bottom.  In KSP gravity losses outweigh aerodynamic losses so the faster you get out of the atmosphere (without blowing up your vessel) the more efficient you are.

As noted, the fairing was necessarily mushroomed out to not clip the paired RA-2's inside it.

The 4 AV-R8's aren't sufficient? The things are 95% control surface.

Edit. I launched them again, one after another.

Top probe 3587 dV to 100km circularized orbit

Bottom probe 3349 dV

It's fine.

 

Edited November 5, 2017 by Starchaser
new info

[Zeiss Ikon]

Or, in other words, you saved enough dV on orbit to make a full landing and relaunch at Minmus.  Twice.  And the vessel cost less.

You win.

[Rocket In My Pocket]

Shouldn't this be in Mission Reports or The Spacecraft Exchange?

(Also, on topic; I would'ave just launched the rocket in the first picture and been done with it but that's just me.)

[Archgeek]

If you need a low-mass, low-radius mounting solution that's not a service bay, I can strongly recommend an emptied mk-0 fuel tank.  Lightest thing per unit length in the game, with sub-meter radius (.75, .5?  I don't remember).  I like to use 'em for ion ship spines.

[Aegolius13]

I think many players (including myself) have a tendency to overvalue ISP and undervalue mass when it comes to engine selection.   Getting hard numbers from KER is pretty helpful to overcome this. 

You'd probably get less drag if you put the fairing right above the decoupler.  Changing from 2.5 to 1.25 then back to 2.5 adds a lot of drag.  But that would also mean you'd have to carry around the fairing base until you ditched the Terrier stage,  so there's a trade-off.

Why a monoprop upper stage,  instead of a Spark or something?  Mono has no particular advantage,  other than that it also works with RCS thrusters,  which you should not need on a mission like this. 

Edited November 9, 2017 by Aegolius13
Not sure what I was thinking - you could get rid of the fairing sooner if you put it lower.

[Rocket In My Pocket]

1 hour ago, Aegolius13 said:

Why a monoprop upper stage,  instead of a Spark or something?  Mono has no particular advantage,  other than that it also works with RCS thrusters,  which you should not need on a mission like this. 

Even worse than that!

It's the most inefficient fuel in the game. (Terrible ISP) I think the fuel tanks themselves are even less mass efficient last time I checked?

Anyways some rocket fuel and a Spark would be way better as @Aegolius13 suggests.

Edited November 8, 2017 by Rocket In My Pocket

[GoSlash27]

Starchaser,

 You have noticed something important: Higher Isp won't necessarily pay off against the weight penalty if the DV required for the mission isn't high enough.

 Next step, see if you can strip out all the dead weight. Carrying excess fuel that you never use is dead weight. Generally, when you eliminate the unnecessary fuel, you can use a lighter engine, which strips out even more dead weight, and even more fuel. Likewise, using a bigger engine than necessary is dead weight. On paper, higher t/w saves DV, but in practice, lower t/w can save mass and money.

 Finally, be sure to account for aerodynamics during the atmospheric boost stage as KSP models it. A design may "look" clean by real world standards, but be exceptionally draggy in KSP.

 I know from experience that you can deliver this same payload to Minmus orbit with a *much* smaller and cheaper vehicle.

For example:

I have used this model to not only collect science from Minmus orbit, but actually return it to Kerbin.

Good luck!
-Slashy

Edited November 8, 2017 by GoSlash27

[Guest]

Nice work. Monoprop engines are a perfectly viable choice; I've often used RCS thrusters for course corrections and the first time I did a crewed Laythe return I used Thuds and monoprop engines.

[Ty Tan Tu]

As long as we are talking about efficient design and drag, does anyone else add a bit of extra drag (like a communication antenna) on the east side of the craft so that you natural lean into a gravity assist?  

[Starchaser]

I am at work at the moment, so I can't answer with hard numbers, but I can give the general thought behind doing what I did and why. Yes, the monoprop engine has a worse specific impulse than the standard engines. I had a terrier in that slot before. However that engine weighs almost nothing in comparison. The tank is slightly heavier than the FL tank that was there, but in carrying a 1 component fuel instead of a 2 component fuel system, I am carrying much more fuel (almost twice) in burn time, and that engine/tank combo gave me at least 30% more dV for less cost. The other liquid fuel/oxidizer engines I had access to either were similar to the terrier in weight issues, or had too little thrust for my liking. I am not very far along in the tech tree because I like to hoard my points until I am pushed into a node or I find something I want to try

[qzgy]

25 minutes ago, Ty Tan Tu said:

As long as we are talking about efficient design and drag, does anyone else add a bit of extra drag (like a communication antenna) on the east side of the craft so that you natural lean into a gravity assist?  

No.... Not me....

[[insert_name_here]]

I mean, nearly 25,000 funds for a Minmus mission is pretty good considering my Mun/Minmus lander costs over 50,000 funds, and somehow a different rocket I designed to carry what is basically a slightly heavier payload than the lander into Munar/Minmus orbit costs twice as much (100,000 funds)...

Edited November 9, 2017 by [insert_name_here]

[Starchaser]

8 hours ago, Aegolius13 said:

Why a monoprop upper stage,  instead of a Spark or something?  Mono has no particular advantage,  other than that it also works with RCS thrusters,  which you should not need on a mission like this.

As promised, here's the answer with hard numbers and pictures

Since you specify the spark, I'll use it as an option

Simple vessel, just a mk-1 capsule.

First option, Pod, fl-t100 fuel tank, terrier

 

Cost 1140, weight 1.903t

Option 2 Pod, FL-T100 fuel tank, Spark

 

Cost 990.  Weight 1.503t

Option 3 Pod,  RCS-RL25 RCS Fuel Tank, LV-95 Orbital Manuevering engine

 

Cost 1550, weight 2.1t

 

Ok, so I was wrong with the cost and weight. I was ripping cost and weight off my OP rocket, so I got confused. However, it's more than twice the dV than the terrier, and almost 2/3 more dV than the spark

So let me take it one step further and add an FL-T800 and a swivel, and a decoupler that matches the engine in order to test how the extra weight of the top stage affects the whole rocket. Forgive me, but I'll just post the final dV instead of another set of pics.

Terrier 3226 dV

Spark 3653dV

LV-95  4243 dV

The difference is all because you essentially have more than twice as much fuel in the single component system, than in the 2 component liquid fuel/oxidizer system. That's why, Aegolius13, I picked the monoprop.

I've redesigned most of my probes to use this tank/engine combo now too.

[Aegolius13]

29 minutes ago, Starchaser said:

The difference is all because you essentially have more than twice as much fuel in the single component system, than in the 2 component liquid fuel/oxidizer system. That's why, Aegolius13, I picked the monoprop.

I've redesigned most of my probes to use this tank/engine combo now too.

Thanks for the info.  That OMS engine (from Near Future, I think?) is a lot more efficient than the horrible stock Puff, which is what I thought you were talking  about.  

Nonetheless, I think you might find that that a liquid fuel engine will still beat that thing out in total performance.  The Spark edges the OMS in both specific impulse and mass, which means mathematically it will always produce more delta-v with a given propellant mass.  The main reason you're seeing higher delta-v with monopropellant is that the mono tank holds a higher mass of fuel (around twice as much, I think) compared to that size 100 LFO tank.  You could probably get a still-better result in terms of delta-v vs mass by using, say, the Spark, and 200 rather size LFO tank.

All that said, the performance differences between the LV-95 and the Spark are pretty minor, so it probably comes down to personal preference.  And the LV-95 has some advantages if you plan to use monopropellant RCS as well.  The smaller volume of the mono tank might also be advantageous at times, though that's usually not too big of a deal.  

[GoSlash27]

2 hours ago, Starchaser said:

Ok, so I was wrong with the cost and weight. I was ripping cost and weight off my OP rocket, so I got confused. However, it's more than twice the dV than the terrier, and almost 2/3 more dV than the spark

Again... reducing cost and weight is beneficial to your space program, but "moar DV" is not. If you already have enough DV to do the job, then adding DV that never gets used is just dead weight and needless expense.

 Best,
-Slashy

[Starchaser]

3 hours ago, GoSlash27 said:

Again... reducing cost and weight is beneficial to your space program, but "moar DV" is not. If you already have enough DV to do the job, then adding DV that never gets used is just dead weight and needless expense.

You missed the point of my putting a first stage (fl-t800, swivel) in my example post. If the mission was to make a fly-by of the mun, for example, my monoprop rocket can do it as is. The other two rockets won't make it. In my OP rockets, making the change to a monoprop top stage raised my total dV on the mission enough to remove the 4 SRBs in the first stage. thus, reducing total cost and weight. Which IS the point. 

In fact the total dV is high enough I could possibly use a smaller first stage tank.

[Starchaser]

5 hours ago, Aegolius13 said:

Nonetheless, I think you might find that that a liquid fuel engine will still beat that thing out in total performance.  The Spark edges the OMS in both specific impulse and mass, which means mathematically it will always produce more delta-v with a given propellant mass.

Mass of FL-T100 0.5625 t

Mass of RCS-RL25 1.15 t 

ratio of mass(rcs):mass(fl) 2.04444444

ratio of thrust provided 2112:1032 = 2.0465

the difference in thrust per unit mass is less than 1% The weight of the LV-95 is 10% more than the weight of the spark (0.11t vs 0.1t) so the actual difference in thrust per unit mass of just the fuel itself is probably closer to 3 or 4% with the monoprop being higher, based on these engines. Different ISP will, of course, give you different results.

 

I am not sure why people are trying to shoot holes in my choice. It makes sense in every numerical analysis I've shown.

 

look at it this way. The Spark provides 20kN of thrust for 78.5 seconds with the fuel provided for a total thrust of 1570 kN

Th LV-95 produces 14 kN of thrust for 229.5 seconds with the fuel provided gives a total thrust of 3213. If you divide that by the weight ratio of the tanks (i.e.) providing the LV-95 with essentially the same amount of fuel as the spark has and you get 1571.6 kN. By this measure, the combinations are essentially identical.

Edited November 9, 2017 by Starchaser
I've forgotten why

[GoSlash27]

53 minutes ago, Starchaser said:

You missed the point of my putting a first stage (fl-t800, swivel) in my example post. If the mission was to make a fly-by of the mun, for example, my monoprop rocket can do it as is. The other two rockets won't make it.

Starchaser,

 If your mission requires 4km/sec DV, you should be staging.

29 minutes ago, Starchaser said:

In my OP rockets, making the change to a monoprop top stage raised my total dV on the mission enough to remove the 4 SRBs in the first stage. thus, reducing total cost and weight. Which IS the point. 

 

Right... and I responded by showing you a rocket that accomplishes the same mission at 1/2 the cost and 1/4 the mass. So clearly there's more room for improvement.

I hope you don't take any of it personally; it's not intended that way. I'm just congratulating you on finding a way to improve your rockets and telling you how you can improve them even more.

Best,
-Slashy

[Rocket In My Pocket]

2 hours ago, Starchaser said:

If the mission was to make a fly-by of the mun, for example, my monoprop rocket can do it as is. The other two rockets won't make it. In my OP rockets, making the change to a monoprop top stage raised my total dV on the mission enough to remove the 4 SRBs in the first stage. thus, reducing total cost and weight. Which IS the point. 

You are using a monoprop engine added by a mod so it kind of makes the point moot.

By that logic I could Hyperedit myself there for less DV and be "more efficient." (Not saying don't use mods, just that going back and forth over this isn't reasonable when everyone is using different sets of rules.)

The stock RCS monoprop engines aren't capable of anywhere near that level of performance. This simply wouldn't work sans mods. For the same weight of fuel you get 2078m/s. vs. 1699m/s

Spoiler

Stock comparison:

Monoprop

Vs.

Liq/Ox

 

Edited November 9, 2017 by Rocket In My Pocket

[Zeiss Ikon]

One question -- when did stock get a central monopropellant engine?  I just opened the one I have on this computer (1.2.2) and don't see anything like that.  The only monoprop engine I have that answers throttle (instead of RCS controls) is the Puff.  I do have 1.3.0 on my laptop, but it's a little precarious trying to use it at my desk (limited space) -- and I don't recall any added engines in that version.

[Starchaser]

14 hours ago, GoSlash27 said:

Right... and I responded by showing you a rocket that accomplishes the same mission at 1/2 the cost and 1/4 the mass. So clearly there's more room for improvement.

I hope you don't take any of it personally; it's not intended that way. I'm just congratulating you on finding a way to improve your rockets and telling you how you can improve them even more.

On the first point. I'm not challenging you by any means. However I went with a 2.5m base because my payload requires a mushroom fairing even over that.  I was concerned about the aerodynamics of mushrooming out a fairing to about 3m on a 1.25m rocket. I suppose instead of asking if it's possible, I should just try it though.

As for talking it personally, up to a point I was not. I enjoy debate and math. But one response seems to boil down to "you're practically cheating"...it's starting to feel personal.

[[insert_name_here]]

7 hours ago, Zeiss Ikon said:

One question -- when did stock get a central monopropellant engine?  I just opened the one I have on this computer (1.2.2) and don't see anything like that.  The only monoprop engine I have that answers throttle (instead of RCS controls) is the Puff.  I do have 1.3.0 on my laptop, but it's a little precarious trying to use it at my desk (limited space) -- and I don't recall any added engines in that version.

It's a mod.

17 hours ago, Rocket In My Pocket said:

You are using a monoprop engine added by a mod so it kind of makes the point moot.