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

[Brotoro]

One thing that isn't important in your sandbox, but will be important when you have to PAY for your parts: One Mainsail is much cheaper than five LV-T45 engines.

[Promii]

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.

The average TWR should be 2. Since it grows as your fuel is consumed it should start out <2 fully fueled.

[SuperFastJellyfish]

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

Struts become massless upon lifting off of the ground, AFAIK.

On topic, I try to shoot for TWR of 1.7ish on liftoff. That seems to keep the vehicle at 100% throttle while following terminal velocity pretty closely. I'm not a big fan of mods, but I have to have Kerbal Engineer Redux for the telemetry data. During ascent, it will show you your atmospheric efficiency(how close to terminal velocity you are) in real time so you can adjust your throttle as needed.

[rkman]

This would imply that you should always use the engine with the highest ISP for your environment (atmosphere or vacuum)

You should use the engine that gives the most delta-V (if it's more than required just reduce the amount of fuel), and it needs to give approximately the correct amount of thrust for the job it needs to do (e.g. twr>2 is overkill for launch from Kerbin). In practice we usually need to compromise between thrust, isp, and engine mass.

[Wayfare]

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.

QFT. In rebuilding my Munshine V for 0.21 I'm finding that the lighter lander (enabled by the half-sized 1m tanks) and the lighter CSM (enabled by the lighter lander) allows me to slap a Skipper on the TMI stage instead of a Poodle... Which cuts the burn time to the Mun by two-thirds. Fuel efficiency is overrated anyway.

[Guest]

QFT. In rebuilding my Munshine V for 0.21 I'm finding that the lighter lander (enabled by the half-sized 1m tanks) and the lighter CSM (enabled by the lighter lander) allows me to slap a Skipper on the TMI stage instead of a Poodle... Which cuts the burn time to the Mun by two-thirds. Fuel efficiency is overrated anyway.

Agreeing right back at ya, I forgot about Mun transfers. If my Mun payload is > 20 tons (roughly) I'll use a Skipper, otherwise a single -45 does the job.

The Skipper really is a fantastic engine.

[Oddible]

I have to have Kerbal Engineer Redux for the telemetry data. During ascent, it will show you your atmospheric efficiency(how close to terminal velocity you are) in real time so you can adjust your throttle as needed.

Great, I use Engineer but many of the values are lost on my still.

THANKS ALL! Got my 10Mg part up to my space station and performed my first dock! Also was able to use my 2nd stage for 99% of my intercept so I still have a fully fueled tug to go fetch the last part that I couldn't get all the way out there!

[N3Coalition]

I no longer use mainsails or skippers. Instead I've found that an engine cluster of 5 LV-T30 and 4 LV-T45s provides about 300N more thrust then the mainsail with better Isp.

I use the airplane tails and radial attachment points in 4 symmetry to attach the engines. The only problem with this design is part count.

[ThreeMartiniLaunch]

Temster, the designer of the Zenith family of lifters, posted an excellent discussion on this topic here.

[Oddible]

Temster, the designer of the Zenith family of lifters, posted an excellent discussion on this topic here.

Fantastic, this rounds things out and explains better why my final design worked. I ended up using 6 LV-T45 boosters with a MainSail core. Looking at my Engineer profile it looks like all were a bit too much (1.9-2.07 TWR rather than Temster's recommended 1.7, and ascending rather than descending).

[numerobis]

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.

This is indeed a major design point. However, I find the skipper better on this count: the mainsail tends to require a lot more struts (more than twice as many).

[numerobis]

That mainsail and 6x T45 design could be slightly improved with a simple trick: replace the T45 with T30. You already have plenty of control authority between the canards and the mainsail. Might as well cut the mass of the boosters and increase their thrust.

By the way, MechJeb 2 can show you not just the TWR at the start of the stage, but also at the end, which helps to judge whether you are overbuilding the stage.

Edited July 26, 2013 by numerobis

[jshier]

One thing that isn't important in your sandbox, but will be important when you have to PAY for your parts: One Mainsail is much cheaper than five LV-T45 engines.

For now. Since it doesn't matter at all to gameplay right now, they haven't balanced the part costs at all, so they'll certainly change when it starts to matter.

[Oddible]

That mainsail and 6x T45 design could be slightly improved with a simple trick: replace the T45 with T30.

Misspoke myself, they actually are T30s.

Played around with this setup a bit more trying to find the sweet spot since I'm a bit over powered (probably not over powered enough to worry about it).

Swapping the MainSail with 1xT45 and 4xT30s (150Mg total mass) gets me in the 1.59-1.49 range on my boosters and easily made it to my 500K station orbit.

Swapping the MainSail with a Skipper (147Mg total mass) was low, in the 1.34-.92 on my stages. This config dropped its final booster at about 12.5K which meant a lower than 1 TWR until 14K causing vertical deceleration but with the fuel loss and the change in atmosphere at 14K it started accelerating again. Dropped the core at 30K and did not make orbit. Max altitude 39K.

The conclusion here is that while the 1x45 4xT30 config may have been more efficient, it costs nearly 4x more to replace the MainSail with 5 engines.

Thanks for all the help and info!

[karolus10]

I often use an single core with 2x mainsail for first stage, So I could launch heavier payloads without need of using parallel staging .

only downside of this solution are slightly longer rockets...