(Help) Fly me to the moon!

[rogerwilco]

Okay, so after you guys were so helpful on my orbit question I want to ask something a little more academic.

I've been reading about the deltaV requirements for different stages/transfers and I know what I want to acheive. I am using the numbers from the dV chart and adding 30% for my inept rocket handling.

So I have the following route (working backwards for mass):

Moon Lander - Landed: 830 (To moon orbit) + 1200 (To kerbin orbit) = 2030m/s required.

I have created a lander with 2250 m/s and it weight 4.5 Tons (module, fuel, engine, paracute, science etc).

So now I have a mass of 4.5 Tons to get to Moon orbit, so I need a Kerbin -> Moon transfer stage.

This requires 1200 (to Moon orbit) and 210 (capture Moon orbit) gives a requirement of 1410 m/s.

Using a T400 and T800 tank and a T45 engine (1,500 m/s) I have a transfer stage weighing 8.3 tons, so I have a total of 17 tons to lift into a stable Kerbin orbit, so that I can launch my transfer stage to the moon.

Great! Just need a lifter.... This is where my numbers get unstuck and what I think should work, doesn't always.

I know I need 4500 m/s to get to Kerbin orbit, now when I use this configuration which I believe should work, I sometimes use way more deltaV than 4500, and often my Peri decays before I can get actually get to the Apo on the other side to raise it! Whilst I almost always get to 70km, when firing prograde it doesnt always provide enough force to stablize the orbit...

Any pointers here? I think I'm on the right track, just missing something critical.

Thank you for your help.

Edited February 27, 2015 by rogerwilco

[Kelderek]

At first glance, your second stage (after the SRBs are ejected) looks like it has too little thrust. Your TWR for that stage starts at 1.07 and that is extremely low. Remember that 1.0 TWR is the bare minimum to move upward at all in Kerbin's gravity, and 1.07 is not much of an improvement. If you have access to the Mainsail engine I would consider using that instead of a skipper for that stage. Otherwise you would need to come up with an alternate solution that gives you more thrust. Your first stage is fine for thrust, it's the second one that is the issue I think.

You have plenty of DV to get to orbit, but you are wasting too much of it by fighting against gravity too much. By adding more thrust, you will burn fuel more efficiently. Keep in mind though that adding more thrust will likely also add to the mass of your vessel and that will affect your DV numbers. I would suggest you aim for at least 1.4 or 1.5 TWR for your second stage. I wouldn't go much higher than 2.0 though as you might start losing efficiency to atmospheric drag.

[rogerwilco]

Hi Kelderek,

Thanks for your response, I do not have that engine but I think it is an upgrade. I shall go with that and bear in mind the TWR target of 1.5 for the middle atmosphere.

[Aethon]

Just a quick point (pressed for time).

Why would you orbit Kerbin after your Mun return? Just set your Periapse for 20km and let the atmosphere decelerate you. That should save dv for the whole journey.

[Randazzo]

Kelderek is correct, your TWR is too low. Personally I think it's too low on all stages. My own personal preference is no lower than 1.5 unless -absolutely- necessary and then no lower than 1.3 under any circumstances.

As suggested, try a mainsail instead of a skipper if you have it available. Also, swap out that LV-T45 for a 30. The isp is the same, but the 45 is heavier and has less thrust, and while it does have a gimbal, my experiences with it leave me feeling that it's not a worthwhile tradeoff.

I also notice that you appear to have either two goo canisters or two rcs tanks on one side of your rocket. Is this intentional to pull it into a gravity turn?

And one final note... are there no struts on those SRBs? You, sir, are a madman.

Edited February 25, 2015 by Randazzo

[capi3101]

You're flying stock aero, right? If so, you can get away with building a liquid-fueled asparagus-staged booster using Temstar's guidelines. I'll walk you through it for this craft.

Okay - so the paywad is 17 tonnes to LKO. Go through the steps -

1) Assume 15% payload fraction. No problem: 17 / 0.15 = 113.33 tonnes.

2) Assume 1.65 TWR at launch. Again, no problem: TWR = T/mg, therefore T = TWR*mg = 1.65 * 113.33 * 9.8 = 1832.6 kN

3) Assume 22% of the thrust is in the core. 1832.6 * .22 = 403.172 kN. Okay - so for the core, you want a Skipper Engine. The numbers would suggest 62% thrust; I'd set it so the last booster stage gives you a 1.5 TWR my own self.

4) Divvy the remaining thust among the boosters. Let's do six. That works out to (1 - 0.22) / 6 = .13, so 13% of the total thrust per booster. So 1832.6 * 0.13 = 238.238. I'd submit you could use a combo of either an LV-T45 and two 24-77 engines (240 kN), or a single LV-T30 with a single 24-77 (235); the first gives you more steering, the second saves you mass you can use for fuel or sundry items.

5) Determine your fuel tank mass - we subtract off the mass of the payload and engines from the theoretical mass, minus two tonnes for sundries (decouplers, sepratrons, maybe a probe core if you want to de-orbit the final stage and batteries to power it, etc.). That comes out to 133.33 - 17 - (A Skipper, 6 LV-T30s and 6 24-77s = 3 + 7.5 + .54 = 11.04) - 2 = 103.29 tonnes. If you divvy that evenly among seven stacks (three booster pairs and the core), that works out to 103.29/7 = 14.756 tonnes per stack. A set of X200-32 fuel tanks at about 80% of their rated capacity should do the trick. Alternatively, stacks consisting of an X200-16, an X200-8 and an FL-T200 will do the trick; myself, I always like the option with the lowest part count).

That oughta do the trick stock. If you want to do a serial-staged rocket, what you can do is shoot for a three-stage booster with 1/6 of the necessary launch delta-V (758.33) in the bottom stage, 1/3 (1516.67) in the middle and 1/2 (2275) in the upper. That's again assuming the 4550 stock aero figure; if you're using FAR, you can shoot for 3500 instead and adjust the delta-V per stage accordingly. I've also heard in FAR that you can get away with a 1.2 initial TWR per stage; I myself have just started tinkering with FAR, so I can't confirm that one way or the other. Stock you want 1.5 if you can get it.

[Kelderek]

If you swap to a mainsail as I suggested before, your first stage will have an even higher TWR than the 2.05 you have now. You may want to consider adjusting the thrust on the SRBs down a little, this will increase their burn time and keep your TWR lower for the first stage. I usually shoot for a TWR of 1.6 to 1.7 for the first stage on the launch pad. Changing the thrust on the SRBs will not lower the DV, it will just make them burn longer at lower thrust and that will help prevent you having too much drag from the atmosphere. Do not adjust the mainsail thrust when you add it, leave that at 100%. You shouldn't need to change much else.

Adding some struts to those SRBs might be a good safety measure though.

[Empiro]

You're miscalculating the amount of dV you need to get back from the Mun. You only need about 1000 m/s, not 2000. I also suggest replacing the Skipper engine in the first stage with a Mainsail. It will reduce your total delta-V, but you should still have enough because you only need 8000 m/s or so (including a generous safety margin).

I'd actually keep the SRBs at full thrust, but throttle down the Mainsail to keep under terminal velocity. You want to burn the SRBs at max so they run out quickly and you can ditch them at earliest opportunity. These big SRBs tend to burn out right around 10km, which is perfect. After 10 km, max throttle and keep your AP 45 seconds ahead of you until it reaches 90km or so.

[Torquemadus]

The lifter is a stage-and-a-half to orbit design. It has lots of heavy fuel in the core stage, but most of the thrust comes from the SRBs.

I'm playing all stock, so I built a quick mock-up of the rocket to test it. I tried launching the stack without the SRBs. As you can imagine, it didn't have enough thrust to lift itself off the pad. I experimented with removing some of the fuel from the core stage to see how much I had to burn off to get a decent TWR. The core only produces a satisfactory TWR after the vast majority of the fuel is gone. This makes it a poor contender for use as a first stage.

A big problem here is the short burn time of the SRBs. Ideally, to be of best use as a first stage, the SRB should keep burning long enough to lift the stack out of the atmosphere and give it a decent apoapsis. Having done this, the second stage would then be used to circularise into orbit. The Skipper is intended for use as a second stage engine.

Unfortunately, because the SRB burns too quickly, it burns out before the stack has cleared the denser layers of the atmosphere. This causes two problems. The first is that the high TWR produced by the SRB when it is low on fuel is wasted fighting against terminal velocity. The second is that since the rest of the stack also hasn't fully cleared the atmosphere, it hasn't yet accelerated to a significant speed. This means that a huge chunk of the deltaV needed to achieve orbit has to come from the rest of the stack after SRB separation.

The solution to this is to thrust limit the SRBs. As a rule of thumb, I thrust limit my SRBs to 70% (feel free to experiment). By adding moar boosters, but keeping them thrust limited, you can delay their burnout. This means that they keep burning long enough for the stack to clear the noticeably dense and draggy lower layers of the atmosphere. As the SRBs burn off their fuel, the TWR of the stack increases. This means that the stack experiences a few brief and precious moments when the nearly empty SRBs are producing large amounts of thrust, with negligible resistance from the atmosphere to oppose them. If this is done correctly the stack can be placed on a sub-orbital trajectory with an apoapsis at the desired altitude, while already traveling at a significant fraction of orbital velocity. This greatly reduces the work that has to be done by the second stage to reach orbit.

[rogerwilco]

Wow, loads of input - thank you so much guys.

You've give me lots of think about. I think I have over estimated the amount of dV required, but I wanted to make sure I didn't just fall short because of a lousy orbit entry or some such. Maybe I'm misinterpreting the chart, I'll read up about that.

Everyone says move to Mainsail, I will do that, capri3101 thanks for your detail, I will have to spend some time thinking about what you've said until I get it.

According to some comments, the main reason I'm using the liquid engine in the first stage is so I can gimbal the thrust for me to gravity turn, otherwise the solid boosters just sit there going straight up

I understand that my TWR must be much higher than it is, but I also understand that there is no benefit with a stupidly high value here.

Randazzo, they are two monopropellant tanks, not sure why they are not symmetrical it was not intentional...

So I did actually make it to the moon and back (after hours) this evening. The change was I modified the transfer stage to use two T800 tanks, rather than T800 +T400 so I had enough juice in these to totally kill the horizontal momentum of the Mun, and most of the vertical. This meant I had 2000 dV on my way back, and like someone said above, I only used about 1,400 dV.

Thanks guys, very rewarding when it actually works!!

[Pecan]

...So I did actually make it to the moon and back (after hours) this evening...

Congratulations! Obviously, I'm too late to help then.

Oh well, my first comment was going to be "You are on the right track" anyway. Feel good about that, you did understand (almost) everything already :-)

ETA: Incidentally, I had a mess around with an 18t payload and to keep it low part-count would probably go with core+2 stacks of orange tube and skipper engines. With large probe core and decoupler for the payload, radial decouplers, fuel-lines to core and separatrons for the booster stacks that's a launch TWR of 1.47 and well over 5km/s dV. Over-engineered, but simple.

Edited February 26, 2015 by Pecan

[gc1ceo]

I always give my orbit lifters a budget of 4650 so I can factor in mistakes, a need for small inclination changes, etc and I try and shoot for a minimum of 1.5 TWR if not an outright 2.0.