Jump to content

[It's finally here!] Apollo Replica "MUNBUG XII" Saturn V, CSM, Lander, Buggy

Mulbin
 Share

Recommended Posts

Giggleplex777

Giggleplex777

Posted
Posted (edited)
OK, I'm now experimenting with reduced fuel and smaller engines in the second stage with some success.

Out of interest does anyone have the TWR for each stage? Also kerbal equivalent DV for each stage would be useful (not the real DV of saturn V stages... but the equivalent percentage for the kerbal universes lower DV).

Some estimates for the TWR:

First stage: 1.2-3.5 before CECO, up to 4.0 after

Second stage: 0.8-1.9 before CECO, up to 1.9 after

Third stage: 0.6-0.75

The first stage has to do a lot of work to get the second stage high and fast enough due to its TWR. Not sure how much delta-v it requires, but I'm guessing that the rocket should begin the pitchover maneuver later (pitching no more than 5-10 degrees until 20km) so the rocket doesn't start falling into the atmosphere.

Edited by Giggleplex777
Link to comment
Share on other sites
sgt_flyer

sgt_flyer

Posted
Posted (edited)

well, for the S-IC + everything above (here, with a 46 tons payload to TLI- we have 2946.8 tons, with 34020 kN of thrust. which gives us 34020/(2946.8*9.87) = 1.16 TWR for the Saturn V at liftoff.

for the S-II + everything above, we have 646.8 tons, with 4400 kN of thrust - 4400/(646,8*9.87) = 0.68 TWR at ignition.

for the S-IVb + payload, we have 166,8 tons, with 1000kN of thrust - 1000/(166,8*9.87) = 0.6 TWR at ignition.

(which matches more or less the acceleration curves from apollo 8)

http://en.wikipedia.org/wiki/Saturn_V#mediaviewer/File:Apollo_8_acceleration_2.svg

regarding the delta-v repartition,

the real Saturn V has around 23% of the delta-V for the first stage, and 38.5% of the delta-V for each of the S-II and the S-IVB.

(the S-IVB should have used a bit less than 1/4th of it's total delta-V at circularization in real life.)

so if we transpose this to KSP, (which would need around 5800 m/s of delta-V to Free return with minimal margins)

we have 1334 m/s of delta-V for the first stage, 2233 for the second stage (so we are at 3567 m/s at S-IVb ignition)

and we have 2233m/s remaining for circularization and TMI. - so here, we need to use at least 1000 to 1200 m/s of delta-V from the S-IVb to circularize in LEO, and the remaining 1000m/s to make your free return.

(though with KSP's atmo, and curvature, you might need to trade more delta-V on the S-IC to gain a bit more altitude before going to low thrust upper stages - and diminish a bit the S-IVB's delta-V)

Edited by sgt_flyer
Link to comment
Share on other sites
Giggleplex777

Giggleplex777

Posted
Posted (edited)

Yeah, I based my estimates on the graph which, although slightly higher than the actual values, is probably more reasonable for a rocket in KSP with stock aerodynamics. I would recommend having more delta-v in the first stage because the second stage would be greatly affected by the thicker atmosphere at lower altitudes. 1330m/s would probably get you to about 16km, but since don't have different liquid fuel types in stock KSP, the mass ratio for the two stages are a lot more similar than in real life. The first stage probably wouldn't be able to push the rocket at 4Gs by the end of its burn.

Edit: A test with a rocket with a TWR of 1.3 (max 2.0) and ~1350m/s of delta-v:

Liftoff

jzdwHJe.png

Engine cutoff

n5cxMF3.png

Expect a similar result with a larger rocket.

The problem is that the lower atmosphere is so "soupy" that it takes much more delta-v to get out of (about 40% of the delta-v to LKO is expended between 0-15km).

Edited by Giggleplex777
Link to comment
Share on other sites
Mulbin

Mulbin

Posted
  • Author
Posted

OK that's great, thanks guys. My new second stage (and everything above) has a TWR of 1.00 on the ground, 1.47 max and DV of 1,900... so I'm in the right zone. Not too concerned with total accuracy but nice to know it's behaving roughly as it should. Also... inbound engine cutoff!... currently set to about 40 seconds before outbound cutoff (shorter than the real thing as the entire LEO insertion is shorter).

Link to comment
Share on other sites
WooDzor

WooDzor

Posted
Posted
No... definitely not with the amount of weight from the I-beams. Just to clarify I'm not using 5 of the large engines on the first stage... I'm using 10! 5 wouldn't get it off the ground so 5 Mainsails?... no chance! :)

To get a comparison take your first and second stages off and just put your S-IVb, payload and fairings on the pad, then check the weight... mine weighs 102(out of 720) tons!

Wow! that is almost twice the weight of my S-IVb + payload. And I thought I had gone overboard. I managed to do everything on mainsails except for the S-IVb because I like to have my flag at that point and i hate waiting for my apoapsis to reach the Mun.

Good to have you back. I salute you sir!

Link to comment
Share on other sites
Mulbin

Mulbin

Posted
  • Author
Posted (edited)

Does anyone have any reliable info on the number and position of ullage and retro rockets on each stage?

EDIT - I have found a schematic for the first stage retro motors which are housed near the base of the rocket (8 motors arranged in 4 pairs).

Edited by Mulbin
Link to comment
Share on other sites
sgt_flyer

sgt_flyer

Posted
Posted (edited)

http://upload.wikimedia.org/wikipedia/commons/b/b2/Saturn_v_schematic.jpg

Here's the high resolution of wikipedia's schematic.

Basically :

S-1C :

8 solid retrorockets into the S-1C engine's extension cones

S-II :

8 solid ullage rockets placed onto the interstage ring (dropped with the ring) (i think they decreased the number of ullage rockets from 8 to 4 after several launches)

http://2.bp.blogspot.com/-s_1VTb99PcQ/TeeLSCuIk6I/AAAAAAAAFgE/UjAtU-K_6Ss/s1600/Saturn%2BS_WEB.jpg

4 solid retrorockets in the interstage between S-II and S-IVB

S-IVB :

2 solid ullage rockets (attached with a decoupling system on the S-IVB, dropped after usage) - used just after S-IVB separation from the S-II for the J-2 first ignition (on later models, i think they stopped using these, and only used the liquid APS) Saturn 1B used 3 of these. (Here, we can see them on the S-IVB of a saturn 1B http://commons.m.wikimedia.org/wiki/File:Saturn_S-IB_SIV-B.jpg)

2 low thrust liquid ullage engines inside the S-IVB APS (those two APS also housed the RCS for the S-IVB roll control when engine was active / attitude control while coasting) - used prior to TMI engine ignition, and also after the TMI for crashing the S-IVB on the moon.

http://www.collectspace.com/ubb/Forum14/HTML/000878.html

Edited by sgt_flyer
Link to comment
Share on other sites
MK3424

MK3424

Posted
Posted

I remember somewhere a anekdote on the ullage motors on the Saturn V: Because they needed to make the launcher lighter, they starting to use less ullage motors on the interstage but it had almost dramatic consequences: when staging, the launcher nearly collided with the spent stage on Apollo 15 i think it was (correct me if i'm wrong).

Link to comment
Share on other sites
sgt_flyer

sgt_flyer

Posted
Posted (edited)

Well, on apollo 15 the problem was that the S-1C did not shut-off correctly for 4 seconds after staging - with the power of the four F-1 with no upper stages and an almost empty S-1C, i don't think any amount of ullage or retrorockets will help the S-II to escape the lower stage :P (ok the F-1 were only at 2% thrust for those 4 seconds, but still : p)

Edited by sgt_flyer
Link to comment
Share on other sites
Mulbin

Mulbin

Posted
  • Author
Posted

Well Munbug XII is finished, but hit a strange bug, about halkf of my lander fuel is disappearing before separation, might need to run the whole mission again to work out when the fuel is going missing! Have they altered the way docking ports and fuel crossfeed works?

Link to comment
Share on other sites
Rune

Rune

Posted
Posted
Well Munbug XII is finished, but hit a strange bug, about halkf of my lander fuel is disappearing before separation, might need to run the whole mission again to work out when the fuel is going missing! Have they altered the way docking ports and fuel crossfeed works?

That's a root/clipping issue, I think. Ever pulled a part, only to have a completely unrelated group of parts come off with it? Sometimes, when you drop a new part on a complicated build using nodes, it will pick up a weird parent (and that is why the tanks are draining into one of the lower stages). It usually happens when you are highlighting some other part instead of the one you are going to attach to. The fix is complicated (the only sure way is to rebuild the whole thing), but you can diagnose by picking suspicious parts to see if they pull the lander tanks with them.

Rune. The joys of the VAB bugs!

Link to comment
Share on other sites
Guest
This topic is now closed to further replies.
 Share
Go to topic listing
×
×
  • Create New...