[0.20.2] Zenith rocket family (modernised for 0.20.x with perfect subassembly)

[DerekL1963]

Does anyone actually ever use the RCS systems on these boosters for translation? Now that we can steer with reaction wheel we don't need steering authority from RCS any more. And with a pair of R24-77 for deorbit you can translate retrograde fine too. So really the only thing that'll be missing from the new build is the ability to translate forwards and sideways. But the only use I can think of for that is if you're actually attempting to dock a payload to something with the booster still attached.

For my part, all but the aft most pair of RCS nozzles go away - if I use RCS at all, that pair works perfectly with the nozzles on the payload and they're more than sufficient to turn all but maybe the Nova and Supernova. (Though I usually strip the probe cores and associated equipment because I play with debris off and no Clean Space Act.) I rarely use RCS nowadays because reaction wheels on the payload and gimbaling the inner engines gives more than sufficient control authority, even when coasting to apoapsis.

Fewer parts is always good, especially since they'll be on the part of the booster that goes all the way to orbit...

If you do upgrade them, would it be too much to ask to standardize on clipping the probe cores under the upper decoupler? It's a real PITA to get at the ones buried under the engines.

[Temstar]

If you do upgrade them, would it be too much to ask to standardize on clipping the probe cores under the upper decoupler? It's a real PITA to get at the ones buried under the engines.

Yes from the look of things I'll probably wait for 0.22 for the upgrade so it comes up at the same time as stock subassembly. In the upgraded version all boosters are now controlled with 2.5m guidance package under the decoupler. I've had a few people asking for this feature since it turns out there's actually a non-negligible need for these rockets to lift payloads that don't actually have an upward facing control part (big rovers, whole surface bases, etc). One other bonus of relying on reaction wheel is that these rockets are now all fin-less as aerodynamic control surfaces are no longer required for roll control.

I'll post a picture of the upgraded Nova when I get home.

[TMS]

I've got a feeling that the torque values of the reaction wheels may be nerfed as part of the SAS refresh.

[Temstar]

Here is the upgraded Nova:

Assuming no surprised in terms of ASAS weight, all Zenith rockets after the upgrade will have identical guidance package at the front. It consists of 2.5m guidance unit, 2.5m battery, 2.5m ASAS and two built in RTGs for power. The small solar panels in the above picture are also gone.

For deorbiting, here are the two retrograde engines in action. In theory you could of course flip the rocket around and use the main engines. But without RCS to back away from the payload after release I feel that it's probably a bit unrealistic to fire the main engines right in the face of the payload. Hence these small engines.

I've got a feeling that the torque values of the reaction wheels may be nerfed as part of the SAS refresh.

Yeah that's what's holding me back right now. I'll wait till 0.22 to see what the changes are and then release the update.

[rottielover]

In extremely pleased that this much beloved line of launchers will be getting an update. Thank you for the excellent design, and for sharing!!!

[DerekL1963]

Assuming no surprised in terms of ASAS weight, all Zenith rockets after the upgrade will have identical guidance package at the front. It consists of 2.5m guidance unit, 2.5m battery, 2.5m ASAS and two built in RTGs for power. The small solar panels in the above picture are also gone.

Nice... that will make life much easier for those of us who hack your boosters for our own purposes.

For deorbiting, here are the two retrograde engines in action. In theory you could of course flip the rocket around and use the main engines. But without RCS to back away from the payload after release I feel that it's probably a bit unrealistic to fire the main engines right in the face of the payload. Hence these small engines.

My only concern with that would be that Mechjeb sometimes activates those engines (I've had that problem in the past). I've never tried grouping them with a staging action though, that should prevent that from happening. (Will have to test.) Since you're saving so many parts, I wonder if spending a few on Seperatons wouldn't be a bad idea. (They'd give some retrograde thrust to boot, so the mass penalty isn't a total loss.) Then you could flip the stage and use the mains.

[stu6000]

have gotten a few good ideas from these designs.

[lewisd]

Wow, thanks so much for all the time and effort you've put into this. This is wonderful! The Supernova is a perfect starting point for getting my 32 Kebal colony to the Mun. It weighs a bit over the payload capacity, so I've added some boosters to help get things into LKO, before setting off for the Mun.

Edit: I meant to add, it's working great for me with MechJeb2. I get a bit of destructive wobble on the last stage, but I seems like disabling the gimbals on 2 of the 3 engines cuts that down to almost nothing, while still giving MechJeb enough to work with to keep things on the calculated course.

Edited September 14, 2013 by lewisd

[The14th]

I just realised I never actually posted my appreciation in this thread. I love the Zenith Series, and rely on them heavily (pun intended) whenever I need a lifting stage that's actually reliable!

I really appreciate the effort you've gone into to make the sub-assembly versions work among other things. I'll be looking forward to seeing where you take it when 0.22 comes out.

Thanks for your awesome work.

[capi3101]

Went ahead and used a Zenith booster for a design I did over the weekend; it worked as advertised. At this point I'm thinking my earlier problems may have just been in transferring a craft straight from 0.20 to 0.21 without making any adjustments (if that makes any kind of sense).

I was going to ask if you had any kind of ready formula for figuring out how much fuel an individual asparagus stage requires, Temstar. I know how your base formulas work (15% payload fraction, 1.6-1.7 launch TWR, 22% of total thrust in the core/centerline stge). Do you have a particular delta-V goal for each individual stage, or do you just go with (derived booster mass - engine mass - staging equipment mass - RCS mass - control unit mass)/number of stacks? I ask for my own knowledge; been working on formulas for single-stage boosters myself lately (and boy do they suck).

[Temstar]

Well, if you know 78% of your lift off thrust comes from the outer ring of engines all that's left to figure out is how many engines/boosters. Basically the idea is more is better. The more the booster you have the smaller each slice of the rocket equation will be. However you don't want to make your rocket to be too complicated so I stick with 6-8 boosters in a ring.

[HoY]

Apart from some very specific rendezvous situations, there is never a good reason to launch retrograde. As said though, it's a common mistake to make and there is a persistent amount of disinformation out there on the interwebs so I don't blame you.

I'll let this old gem speak for itself:

old post im quoting but i want to mention that starting in a retrograde orbit (270º) is better for solar powered ships heading Outwards in the Kerbol system, as your prograde burn will be on the light side of the planet rather than the dark side

[capi3101]

Alright - let's say my total payload 138.175 tonnes - the formulas say:

Rocket: 921.167 tonnes (booster = 782.992 tonnes)

1.6-1.7 TWR: 14459 kN, 15363 kN

22%: 3181-3380 (a pair of Mainsail and an LV-30 should do the trick, 13.25 tonnes)

Lessay I d721.o eight boosters; then it's 1410-1498 kN per booster (i.e. a single mainsail each, eight mainsails total, 48 tonnes)

So that leaves 721.742 tonnes to divvy up among nine stacks (80.19 tonnes per stack if it's done evenly).

So is that how I determine the fuel (two jumbos and an X200-16 or thereabouts in this case), or is there a more general formula you've developed?

[Temstar]

Unfortunately there's no clear formula for dividing up the stacks. However do realise that the stacks are not suppose to be divided evenly. The centre stack should have much more than 22% of the total fuel since:

1. It has bigger engines, so it can carry more fuel for a given TWR
   
2. It needs less TWR, since it's an upper stage
   

I think generally Zenith family core stage has around a TWR of 1.2 - 1.3 at the start of the core stage burn. This results in a TWR curve of something like:

Ignition: 1.7

1st booster pair separation: 1.6

2nd booster pair separation: 1.5

3rd booster pair separation: 1.2

The reason for this is that first and second pair of boosters are used for the vertical climb stage where engines are fighting gravity and drag so they need good TWR. Once you cut loose the 2nd pair of boosters the air is pretty thin and you start gravity turn, by the time the last pair of boosters burn out you are getting close to horizontal (so minimal gravity loss) and in near vacuum. With both gravity drag and air drag near zero you can then finish the climb to orbit at a leisurely pace. Having a low TWR for final stage means you're not lugging as many engines up to the final orbit (so more payload) and the gentle TWR means it's easier to achieve circular orbit.

Say we aim for core stage TWR of 1.2 at separation for your rocket. That means core stage + payload is around 276 tons. With a 138 ton payload you are left with 138 tons of rocket. So minus the weight of your engines and guidance systems and you are left with the target wet weight of fuel tanks.

[capi3101]

I was going to say somewhere between 33% and 40% of the total fuel in the core stage - that's just based on my own observations of staging in general, though. Sounds about right.

[lewisd]

I know I've said it before, but after my playing KSP last night, it bears repeating:

Temstar, I love your rockets!

I've been using them for more than just lifting to LKO. I'm building a station in keosynchronous orbit, and I've been using the larger ones to get station parts out there, with the last stage of the Zenith lifter getting me part way from LKO to KSO. Yesterday, after lifting something to orbit, I realized that the space between LKO and KSO was getting a bit littered with discarded boosters. Of course, because of the way I've been using them, they were all out of fuel. Bipropellant fuel, at least...

As it turns out, every single booster left in space had enough monopropellant to use a long (couple minutes) RCS burn at periapsis to put them in a lithobraking orbit.

So, as I said: I love your rockets!

[Nephf]

That Supernova

[DerekL1963]

As it turns out, every single booster left in space had enough monopropellant to use a long (couple minutes) RCS burn at periapsis to put them in a lithobraking orbit.

Why would you burn a periapsis? Doing so at apoapsis is far more efficient.

[lewisd]

Why would you burn a periapsis? Doing so at apoapsis is far more efficient.

You're absolutely right. Apoapsis is where I burned. I always get those terms backwords.

[deepspacecreeper]

im getting trouble with self-designed mega launchers breakin up on the launchpad, so i tried out ur rockets. they work good.

just one question shown in below post.

Edited November 14, 2013 by deepspacecreeper

[deepspacecreeper]

Unfortunately there's no clear formula for dividing up the stacks. However do realise that the stacks are not suppose to be divided evenly. The centre stack should have much more than 22% of the total fuel since:

1. It has bigger engines, so it can carry more fuel for a given TWR
   
2. It needs less TWR, since it's an upper stage
   

I think generally Zenith family core stage has around a TWR of 1.2 - 1.3 at the start of the core stage burn. This results in a TWR curve of something like:

Ignition: 1.7

1st booster pair separation: 1.6

2nd booster pair separation: 1.5

3rd booster pair separation: 1.2

The reason for this is that first and second pair of boosters are used for the vertical climb stage where engines are fighting gravity and drag so they need good TWR. Once you cut loose the 2nd pair of boosters the air is pretty thin and you start gravity turn, by the time the last pair of boosters burn out you are getting close to horizontal (so minimal gravity loss) and in near vacuum. With both gravity drag and air drag near zero you can then finish the climb to orbit at a leisurely pace. Having a low TWR for final stage means you're not lugging as many engines up to the final orbit (so more payload) and the gentle TWR means it's easier to achieve circular orbit.

Say we aim for core stage TWR of 1.2 at separation for your rocket. That means core stage + payload is around 276 tons. With a 138 ton payload you are left with 138 tons of rocket. So minus the weight of your engines and guidance systems and you are left with the target wet weight of fuel tanks.

but is the TWR supposed to be reduced when gravity turn has started?

the eff. thrust to fight gravity, (the thrust used in calc. of TWR) is thrust*sin(vertical angle of rocket)

if i have a mainsail, the thrust is 1500*sin(30)=750 when the rocket's vert.angle is 30 degrees

[Temstar]

Yes TWR becomes less important the more horizontal your rocket becomes since the more horizontal the less loss due to gravity drag.

Put it this way, it's not so much "upper stage need less TWR", instead it's more "upper stage doesn't need as many engines since it's thrusting sideways". By fitting less engines to the upper stage (the part of the rocket that you can get away with that sort of thing) you decrease the weight of the upper stage without hurting the performance much which results in both smaller stages underneath/around it, and bigger payload to orbit since the dry weight of that upper stage is now lower.

[deepspacecreeper]

Yes TWR becomes less important the more horizontal your rocket becomes since the more horizontal the less loss due to gravity drag.

Put it this way, it's not so much "upper stage need less TWR", instead it's more "upper stage doesn't need as many engines since it's thrusting sideways". By fitting less engines to the upper stage (the part of the rocket that you can get away with that sort of thing) you decrease the weight of the upper stage without hurting the performance much which results in both smaller stages underneath/around it, and bigger payload to orbit since the dry weight of that upper stage is now lower.

do you mean that the reduced gravity and drag cancels out the reduced thrust that fights gravity?

Edited October 15, 2013 by deepspacecreeper

[BigD145]

It would be nice to see this updated with parts earlier in the .22 tech tree. As is you can unlock 95% of the tree and still not have access to these designs. It's usually because even the subassemblies have giant or miniature unmanned pods that exist late in the tree. I know the tail connector is used a lot, but the pod/guidance is the biggest hindrance for career mode. Time to alter these a bit since official subassemblies don't need pods.

[capi3101]

I'm up for leaving the Zenith series alone, personally. Perhaps what's needed is a series of boosters based on the same set of principles upon which the Zenith is based, just with parts from lower areas of the tech tree.

The FTX-2 Fuel Ducts become available with Tier 5; proper asparagus staging isn't even possible before then.