MainSail vs Skipper vs 5 x LV-T45s: Fundamentals of Rocket Construction Question

[Oddible]

I've watched a bunch of tutorial vids but have some fundamental questions as to when / why to use different engine configurations (primarily I'm having trouble getting a 10Mg payload into high orbit without an absurdly enormous rocket). My question is, how do I know which config to use depending on my payload. I'm using Kerbal Engineer to look at stats.

TWR: I know I need > 1 to get off the ground in atmosphere

dV: Change in velocity achievable

ISP: Engine efficiency

Time: I think this is a bigger factor than I'm giving it credit for

So how should I be reading these to ensure my rocket can get a payload into high orbit Or is dV the only number that really matters? Here are the stats for the 3 engines listed with just an orange tank and an okto2.

[table=width: 600, class: grid]

[tr]

[td]

Skipper

TWR: 1.66

dV: 4725m/s

ISP: 300s

Time: 2:24s

Thrust: 650kN

dV w/10Mg: 3003m/s[/td]

[td]

5 x LV-T45

TWR: 2.34

dV: 4168m/s

ISP: 320s

Time: 1:40s

Thrust: 1000kN

dV w/10Mg: 2858m/s[/td]

[td]

MainSail

TWR: 3.64

dV: 3934m/s

ISP: 280s

Time: 58s

Thrust: 1500kN

dV w/10Mg: 2621m/2[/td]

[/tr]

[/table]

From all appearances, the T45s seem to strike a balance between the MainSail and the Skipper in terms of Thrust, burn time, TWR and dV, but have the advantage of a higher atmospheric ISP.

When I add load (I've added 10Mg just to look at numbers). The only thing that changes is dV. Looking at an online dV calculator / charts suggests that I need ~4500dV to get into a low Kerbin orbit. This suggests that a single orange tank with a MainSail can't even get into orbit (adding a 2nd tank gets me to 4700dV, just barely making low orbit).

So some questions:

1. Can I just add up the dV from my stages to know if I'm going to make it?
   
2. Are there other atmospheric factors to consider?
   
3. Why would anyone ever use anything but a Skipper on an orange tank? (and a follow-up, why ever use a MainSail?)
   
4. What are good targets for my stages? I assume getting to 70K is a priority then I want to stage out to an engine that has a more efficient ISP in vacuum.
   

If dV is the only number I need and the only number that changes, why am I looking at ISP and the other values at all?

Thanks, any illumination appreciated!

[footman04]

1. Yep. I did some experiments with vertical SSTO probe launchers and this was an important factor in the success of them.

2. Terminal velocity. You lose some d/v to air friction, but it hasn't been an issue for me yet.

3. I've always used Mainsails to get large loads to orbit. The Skipper has proven to be a very good upper stage engine.

4. It depends on how many stages you've got. Asparagus staging is important. On a quad booster setup, I eject the first two at 3-5k, the second two somewhere beyond 10k, when they run out. The center stage is usually sufficient to circularize most crafts.

[numerobis]

"Why would anyone ever use anything but a Skipper on an orange tank? (and a follow-up, why ever use a MainSail?)"

You could use 2 T30s and a T45 to get the same mass as a skipper (4t), almost the same thrust (660 kN rather than 650 kN), and higher Isp. Or, you could add more fuel to get the same TWR and actually be able to use all the thrust at your disposal from the 5 T45s or the mainsail.

I normally wouldn't use so many T45s; the T30 is lighter and has more thrust, so I only use as few T45s as I can get away with while still having enough control authority. One T45 and four T30s is lighter and has better thrust, so it can haul up more fuel for itself.

Edited July 26, 2013 by numerobis

[draeath]

Once you are out of the atmosphere it's your dV that really matters, though extra weight may make maneuvering more annoying.

However for your first stage (in atmosphere) you want more thrust even if the dV is lower, as it gets you up and out of drag and gravity quicker.

[Rastaman]

Thrust in important in space too - you need it for cleaner escape burns that utilize the Oberth effect.

[AmpsterMan]

Since I am at work and have limited time I will try to give you a short observation of mine.

The rocket equation used to calculate Delta-v is:

dV=9.81*Isp*ln(Mi/Mf)

dV is delta v

Isp is Specific impulse of that stage

Mi is mass of stage when fuel is full

Mf is mass of stage when empty.

Because the only vaiables in this equation are Mi and Mf I usually consider (Mi/Mf) as one variable "R"

So now the equation is

dV=9.81*Isp*ln®

Here it is easy to see two things. One, the ONLY way to increase Delta v for a stage is to increase Isp or to increase R. Isp is increased by using more efficient engines whereas R is increased by putting more fuel (Increasing Mi) or decreasing payload (Decrease Mf) please note that payload means everything above the stage you are calculating.

The second thing one notices is that doubling Isp doubles your delta v but doubling your fuel ,"R" does not. In fact, because Ln is a logarithmic function to the base "e" you experience diminishing returns especially after R=2.72

What does this all mean? That there is only a specific range where an engine should be used. If you like I could post something on TWRs later on when I have more time

[Oddible]

Thanks all... following-up...

the ONLY way to increase Delta v for a stage is to increase Isp or to increase R.

This would imply that you should always use the engine with the highest ISP for your environment (atmosphere or vacuum), therefore, that the 5xT45s are always the way to go out of the 3 I listed above. But, as you imply later in your post, TWR is a significant factor. The Skipper and the 5xT45s just don't have the thrust to lift the heavy loads, which I think is why so many people use MainSails, which are horribly inefficient. Then does it become a space and weight constraint? You can't fit 7.5 T45s onto the bottom of an orange tank so you can't get to the thrust of a MainSail with the efficiency of a T45.

Edited July 26, 2013 by Oddible

[MalfunctionM1Ke]

Very Good Explanation AmpsterMan but I think he wanted a handy Guideline instead of difficult equations.

[Bengsch85]

The reason adding fuel only works to a point in adding delta-v is that at some point its using more thrust to lift the mass of the fuel then adding delta-v.

[vidboi]

Thanks all... following-up...

This would imply that you should always use the engine with the highest ISP for your environment (atmosphere or vacuum), therefore, that the 5xT45s are always the way to go out of the 3 I listed above. But, as you imply later in your post, TWR is a significant factor. The Skipper and the 5xT45s just don't have the thrust to lift the heavy loads, which I think is why so many people use MainSails, which are horribly inefficient. Then does it become a space and weight constraint? You can't fit 7.5 T45s onto the bottom of an orange tank so you can't get to the thrust of a MainSail with the efficiency of a T45.

Mainsails have the best thrust-weight ratio out of all the engines, so for lower delta-v stages using a mainsail can actually be more efficient. Given that it is often more efficient to stage quickly rather than have high delta-v stages (especially in asparagus set-ups) using mainsails can actually be more efficient as the engine contributes less to the overall mass of the stage. Very efficient launchers such as the Nova use mainsails for the asparagus boosters and T30s/T45s for the central stack to make the most out of this behaviour. Another good example is how below a certain delta-v requirement (I think it's around 800m/s) 24-77s are more efficient than LV-Ns.

[CalculusWarrior]

To summarize AmpsterMan's post:

MOAR thrust = less delta-v, more TWR

MOAR Isp = more delta-v, usually less TWR

MOAR engines = less delta-v, more TWR (but usually less so than adding purely more thrust)

Mainsails are generally used for their insane thrust. If your payload is too heavy for a Skipper to lift, use these massive engines. That being said, I prefer using a mixture; a Mainsail on my central stage, with two Skippers on side boosters. I found that using three Mainsails is simply too much thrust (for an optimal ascent, try to keep your velocity below 200 m/s before 10 km), and I had to throttle down to halfway. The Skippers are much more fuel efficient, which helps a lot in terms of delta-v.

In the end, you don't want a TWR that is over 2 when you start your initial ascent, however a higher one is always helpful after the gravity turn, in case you pitch down too much. Hope this helps!

[Wayfare]

[*]Why would anyone ever use anything but a Skipper on an orange tank? (and a follow-up, why ever use a MainSail?)

Because you don't just want to send an engine and its fuel into orbit - you want to launch a payload as well. That high TWR on the Mainsail allows you to stick a lot of extra mass on top. Your first stage is going to have to carry a lot of rocket (second stage, maybe even third stage) if you're launching a significant payload and that's where the Mainsail really shines.

[Oddible]

Very Good Explanation AmpsterMan but I think he wanted a handy Guideline instead of difficult equations.

Absolutely not, I have plenty of blunt goals from the cheat sheet I posted. What I'm looking for is clearer rational of why I'm choosing certain engines. I was just looking at Temstar's Zenith rocket designs and he seems to use 2 T30s and 1 T45 on the bottom of his core as opposed to the 5 T45s. This is a significant gap in my knowledge. Why use 3 engines and not 5? I think one fundamental I'm missing here is speed / vertical velocity and finding some balance between velocity and fuel efficiency.

[Oddible]

for an optimal ascent, try to keep your velocity below 200 m/s before 10 km

There, that's a piece I was missing. So either build a lower thrust rocket or throttle down? A little of both? Do engines change efficiency based on their throttle?

In the end, you don't want a TWR that is over 2 when you start your initial ascent, however a higher one is always helpful after the gravity turn, in case you pitch down too much. Hope this helps!

Interesting. This is exactly the kind of help I'm looking for. Ideally I want to build small efficient rockets not behemoths that consume ungodly resources.

[RRoan]

You can't fit 7.5 T45s onto the bottom of an orange tank so you can't get to the thrust of a MainSail with the efficiency of a T45.

I can get 12 T30s and 4 T45s onto a single orange.

[Oddible]

I can get 12 T30s and 4 T45s onto a single orange.

Screenshot or it didn't happen

No seriously... structures please!

[Pixel of Life]

Choosing between less thrust and throttling down - less thrust is most likely better, because it means you won't overstress your rocket nearly as easily, and it allows you to control the speed more accurately.

[Jarin]

Screenshot or it didn't happen

No seriously... structures please!

Also, how much mass does the absurd strutting arrangement add to the ship?

[numerobis]

Why use 3 engines and not 5?

Because 3 engines is less mass than 5. Your optimal speed at low altitude, when you are going vertically, is the terminal velocity at that altitude: the faster you go, the less you lose to gravity [linearly so], but the more you lose to drag [quadratically so], thus the optimal speed is where gravity = drag. So you need a TWR of about 2 on average: one for gravity, one for drag. Any more thrust than that, and you should throttle back to save fuel. Or, even better, drop an engine so you no longer need to haul it up.

After you start turning, the optimal speed is still a matter of discussion, but again, you generally want to have as little engine mass as possible.

[regex]

I find the Skipper to be an excellent short-range interplanetary engine; it gets a decent fuel economy and the burn times aren't painful like you get from nuclear engines. I wouldn't use it going to Jool or Moho, though.

I also find it useful when the ascent stage ends up having way too much TWR for the delta-V I need.

[Tiron]

Well keep in mind, the Skipper's relatively new. The Mainsail's been around a lot longer.

But also, the reason why you'd use 3 engines rather than 5 if you can get away with it? The engines themselves have weight, and that weight detracts from your delta-V. For the same setup with the same engines with the same Specific Impulse (what Isp stands for), fewer engines will generate more delta-V, but less thrust. This is why the small radials can be more efficient than a LV-N: The LV-N is really freaking heavy (2.5 tons if I recall).

One other thing to keep in mind is that Isp isn't static: It changes as you climb. The two values specified for our engines are 'at 1 atm' (Sealevel pressure), and 'In Vacuum'. It ramps smoothly between the two as the pressure changes...and the pressure decreases logarithmically as you climb. If I've done the math right here, on Kerbin you're down to 0.135 atmospheres or so by the time you get to 10000m. It really doesn't take much altitude before your engines are closer to the 'vacuum' efficiency than the 'Atmosphere' efficiency...which is also lower on the mainsails and the skippers than the smaller engines.

That said, I've had up to 7 engines on the bottom of an orange tank before. I lack patience.

[Pbhead]

For me the Thrust to lag ratio is more important for my lifters.

Mainsails just plain have the best thrust for the lowest part count.

[FleetAdmiralJ]

[*]Why would anyone ever use anything but a Skipper on an orange tank? (and a follow-up, why ever use a MainSail?)

There are times when I just need the Mainsail's thrust to get something up. I prefer the skipper but sometimes it just won't do the job.

By the way, what are you using to get 5 engines? I know the core parts only has pieces to allow attaching 4 medium engines to a big tank as far as I'm aware

[Tiron]

There are times when I just need the Mainsail's thrust to get something up. I prefer the skipper but sometimes it just won't do the job.

By the way, what are you using to get 5 engines? I know the core parts only has pieces to allow attaching 4 medium engines to a big tank as far as I'm aware

There's a lot of ways. The 'original' method was to use the small strut cubes as an attachment point, but you can also use reversed tail connectors (which look quite good) or the new-ish radial connector port as well.

People have been doing engine clusters for awhile now.

[Oddible]

So you need a TWR of about 2 on average: one for gravity, one for drag.

This is news. So I'm not looking for a TWR of 1.xx, I'm looking for a TWR of 2 in atmosphere? This explains a lot.