RSS RSO Lunar Mission

[TeeGee]

Hey everyone!

I've got a few questions for the Realism community out there on how to best accomplish a Lunar landing. SO FAR, I've tried to create a single launch rocket with both the TLI booster, lander and CM all as payload, but didn't want to create a 3 stage rocket to do so. I was VERY close to creating a 2 stage LF rocket to carry payload to the Mun, but was short delta v. One option was to use boosters to help push the rocket up, but opted not to.

I then decided to launch with 2 seperate rockets, one with the CM on one of my SSTO rockets. And the other payloads on a 2 stage rocket (including delta v to circularize AND TLI).

Does everyone think I should do it in ONE rocket launch ala Saturn V OR should I do 2 launches...?

Im also struggling to find the best delta v loadout for a lunar mission. I have a delta v map to tell me how much I need, but I usually leave extra fuel to screw around with. How much did each stage on apollo have? Did the LM really have 4500 m/s delta v to land and takeoff?

[Hattivat]

I'm still pretty new to RSS/RO so you may take my advice with a pinch of salt.

Personally, I've found it the easiest to just copy the Saturn V approach. That is: 1st stage: kerolox to lift off the ground, 2nd stage hydrolox to get most of the acceleration needed for orbit, third stage TLI. It is absolutely essential to save as much as you can on the weight of your CSM and LM, even small savings there translate into massive savings on first stage weight and thrust.

[TeeGee]

I agree with you 100%, but I am trying to improve on what NASA has done with their Saturn V rockets. The proposed SLS rocket is 2 stage and lifts 130 tons into LEO, thats more than what Saturn V could lift... Im trying to do better.

And how about Max TW? If I use boosters, should I be aiming for a MAX thrust to weight of greater than 1? I'm trying to optimize delta v gains and was wondering what I should be aiming for with a booster design?

[Hattivat]

My rockets usually have initial sea-level TWR of 1.2 and second stage (or core engine after booster separation) initial TWR of about 0.9. MAX TWR should definitely be more than 1. Not sure if it's optimized though, as I said, I'm still pretty new to RSS/RO.

[TeeGee]

Okay, well... I put on the F 1 rocket, MAN THAT THING CAN PUSH! I've created a 2 stage Moon rocket, the first stage has 9000 m/s delta v, the second stage TLI booster has 4600 m/s (WITH the CM and LM above it), the CM has 2850, LM has 4600 respectively. If any of my numbers seem off, please let me know. I'm going to do some tests in orbit UNmanned before I head out to the Moon... man that F 1 engine blew me away... literally.

Why didn't the apollo program just dump the 2nd stage altogether and ride the F1 all the way up? It could have...

[Hattivat]

Lol, that sounds great. Just make sure that your second stage engine can ignite more than once (it seems you will need to fire it briefly to circularize). The numbers seem good to me.

As for Apollo... they probably wanted to get more stuff (like TV cameras) than truly necessary to the lunar surface. There was a proposal for a Gemini-based mission that wouldn't have needed the Saturn V at all, google it.

[TeeGee]

Yeah Im using the J2X LF engine as my second stage, she can ignite 3 times. What Im worried about is fuel flow stability. I'm using balloon cryogenic tanks and am worried that it wont be pressurized for ignition... what real fuels tank settings should I be using for orbital ignitions OTHER THAN SERVICE MODULE tank? especially for hydrolox fuels.

[Hattivat]

Fuel flow is a minor problem, you just need something to accelerate for a few seconds to make it stable again. There are two solutions:

1. Small SRBs (KW rocketry provides dedicated ullage boosters I think).

2. Have a reserve of RCS fuel and hold the H key for a while (right click the engine and wait until it says that the fuel flow is stable).

[TeeGee]

Fuel flow is a minor problem, you just need something to accelerate for a few seconds to make it stable again. There are two solutions:

1. Small SRBs (KW rocketry provides dedicated ullage boosters I think).

2. Have a reserve of RCS fuel and hold the H key for a while (right click the engine and wait until it says that the fuel flow is stable).

Is that why during staging the srb ullage rockets on the decoupled stages fire? I thought it was just for separating the 2 modules...

How did the saturn 5 reignite the 3 stage LF rocket?

EDIT: SO THAT'S what a ullage motor does! Cool!!

Edited June 30, 2014 by TeeGee

[Hattivat]

Is that why during staging the srb ullage rockets on the decoupled stages fire? I thought it was just for separating the 2 modules...

SRB rockets on decoupled stages are called separation rockets. Ullage rockets are the ones that fire on the stages that remain. In my case, I fired ullage motors just before my TLI ignition, which happened over an hour after 2nd stage separation.

As for Saturn V: http://en.wikipedia.org/wiki/Ullage_motor

[Hodo]

I have sent a few probes to the Moon in RSS RO with a 2 stage rocket. But it was a VERY light cargo load, less than 5 tons. The key is to see how much mass you can shave off of the cargo. I had to start thinking about how LITTLE I can get away with in space and survive.

[TeeGee]

I have sent a few probes to the Moon in RSS RO with a 2 stage rocket. But it was a VERY light cargo load, less than 5 tons. The key is to see how much mass you can shave off of the cargo. I had to start thinking about how LITTLE I can get away with in space and survive.

Why do I get that sinking feeling....? I double checked my numbers, I have enough delta v and TW to make it to orbit, the F1 rocket can push its payload and fuel up in 2 stages... can't it?

[Vigelius]

Things to consider:

i. As you say, the F1s can seriously push (and they aren't throttleable). If you're trying to 2-stage it, by the time your huge first stage depletes it will have some mental TwR going on. That puts a lot of strain on your stack, and might make controls difficult to use.

ii. Hydrolox is much more efficient than kerolox, and second stage engines are much lighter than the F1s. You only really need the massive pushing power for as long as it takes to get into a position where a hydrolox stage is able to take over, any more seems a bit like using a hammer to crack a nut, and you're carrying a lot of weight.

iii. Doing it on a huge second stage runs into the same TwR issue, hence the division of a Saturn V into a second stage with five J2s and a third stage with only one.

iv. The lighter your craft becomes, the more critical 'dead weight' becomes. Since your upper stage doesn't need a TwR beyond about 1, any engine pushing you beyond that essentially becomes dead weight and will be unnecessarily reducing your total dV (for that stage, and for any stages before it). Clearly adding an extra stage is also adding weight, so there will be some critical point at which it becomes worthwhile. I'm sure someone more numerically minded than me has already done the maths - I haven't, but my second stages usually cut out with a final TwR of about 2.5-3

Hope that helps a bit!

[TeeGee]

So u think that when I reach max twr with my f1, my stacks could crumple? I never thought of that.. So in reality the reason they opted for a 3 stage was to minimize the usage of the F1 beyond initial launch.

That engine scares me.

[Hattivat]

Yes, they did it to reduce max TWR, although I think it had more to do with crew safety than with danger of "crumpling". You see, max TWR equals max acceleration in g's. If your max TWR on a stage is 10, for example, that means you are subjecting your crew to 10g of acceleration for a few moments, which is undesirable.

[TeeGee]

I retract my previous statement, you can't make a 2 stage to orbit lunar launcher with the F1, the F1 doesn't have enough ISP (and in turn, delta v) to reach orbit and maintain a TW ratio of greater than 1. I was just looking at my rocket, I had the TW settings set to MUN, no SLT for Kerbin. It NEEDS to be at least 3 stages OR use a booster to push that much weight into orbit.

I'm thinking about using LF boosters like in the energia launcher to cut out the 2nd stage rocket, but I haven't decided yet. I just don't know how my rocket weighs soooo much on top, its destroying my TW ratio at the bottom stages.

I've already created a 2 launch, orbital rendevous lunar mission but I think that would waste too much fuel in orbit to dock. 1 rocket is easier.

[Vigelius]

Don't be afraid to make the upper stages fatter than you think they should look with those smaller engines on them - every increment of extra width will give as much extra capacity as several increments of extra height. When I first tried I ended up with a rocket that looked like the Eiffel tower, and flew about as well!

I use the Nova-Punch Bearcat 5x (which basically looks and pushes like five F1s) doubled in size to 10m (conveniently, the same max diameter as the procedural interstage fairing) as my guideline. I travel up the stack setting sensible diameters; same width for the second stage, a little less for the third. I then travel back down the stack setting lengths to achieve the initial TwR I want for each stage; about 1 for third and second, about 1.2 for first. Fine-tune the more forgiving upper stages a bit for dV, the end result is something that more or less looks and behaves like a Saturn V.

At that sort of size though, simplicity is king. I had a few configurations which were much more complex, and they all either blew up ('crumpled' ) as soon as the thrust came on or folded at the top of stage one as soon as I tried to start the gravity turn. It's a pretty close-run thing.

[metaphor]

One of the reasons the Saturn V had 3 stages was so that the acceleration near burnout wouldn't get too high for the astronauts. There's also the issue that with only 2 stages, each one has to have more delta-v, so the dry mass of each stage is more important (that's especially true for kerolox stages which have lower Isp). Even the SLS really has 3 stages, since the boosters are basically the first stage (the boosters weigh much more than the core).

[TeeGee]

One of the reasons the Saturn V had 3 stages was so that the acceleration near burnout wouldn't get too high for the astronauts. There's also the issue that with only 2 stages, each one has to have more delta-v, so the dry mass of each stage is more important (that's especially true for kerolox stages which have lower Isp). Even the SLS really has 3 stages, since the boosters are basically the first stage (the boosters weigh much more than the core).

I tried creating a 2 stage rocket with enough delta v to reach orbit, then for TLI but didnt have enough thrust. How is that possible? Well, I tried puting 9000 m/s delta v in the 1st stage WITH the 5000 delta v in the TLI stage, the TW was 0.36 at SL.

[TeeGee]

Yeah... I was using a 4 F1 stage 1 booster... oops.

[Sirine]

Try aim higher, then later will be much simpler.

Example:-

Build a (400tons payload) RSS lifter to Lower Earth Orbit. Then you will have no problem reaching the Moon (Mun).

[TeeGee]

Try aim higher, then later will be much simpler.

Example:-

Build a (400tons payload) RSS lifter to Lower Earth Orbit. Then you will have no problem reaching the Moon (Mun).

How can I do that?

[Vigelius]

I tried creating a 2 stage rocket with enough delta v to reach orbit, then for TLI but didnt have enough thrust. How is that possible? Well, I tried puting 9000 m/s delta v in the 1st stage WITH the 5000 delta v in the TLI stage, the TW was 0.36 at SL.

Kerolox is more dense and less fuel efficient than hydrolox, so the equivalent top part of your first stage would have been much heavier than the equivalent hydrolox second stage to achieve the same dV. Not to mention that you're carrying those super heavy engines all the way up. And yeah, only having 4/5 of them probably didn't help either!

Try aim higher, then later will be much simpler.

Example:-

Build a (400tons payload) RSS lifter to Lower Earth Orbit. Then you will have no problem reaching the Moon (Mun).

I can't help feeling 400t might be waaay excessive! My lunar stack was 180t by the time it got to parking LEO, and I'm pretty sure that could be trimmed down. Perhaps design a launcher capable of 200t for a bit of wiggle room, but a 400t launcher would be an absolute monster - I'm not sure the game could deal with something that big! Massive respect if you can prove me wrong though...

[TeeGee]

Kerolox is more dense and less fuel efficient than hydrolox, so the equivalent top part of your first stage would have been much heavier than the equivalent hydrolox second stage to achieve the same dV. Not to mention that you're carrying those super heavy engines all the way up. And yeah, only having 4/5 of them probably didn't help either!

I can't help feeling 400t might be waaay excessive! My lunar stack was 180t by the time it got to parking LEO, and I'm pretty sure that could be trimmed down. Perhaps design a launcher capable of 200t for a bit of wiggle room, but a 400t launcher would be an absolute monster - I'm not sure the game could deal with something that big! Massive respect if you can prove me wrong though...

Solution: http://en.wikipedia.org/wiki/M-1_(rocket_engine)

Best 1st stage engine ever designed.

I was laughing SOOO hard when my rocket started going dry, the G load on my kerbals was reaching 15!! They almost died before SENCO. I decided to use SSME rocket engines for my V2STO for that very problem... I guess staging the Apollo rocket wasn't because of lack of thrust or performance, but because of crew safety.

Edited July 1, 2014 by TeeGee