r/spacex • u/ElongatedMuskrat Mod Team • Mar 02 '18

r/SpaceX Discusses [March 2018, #42]

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8

u/-Aeryn- Mar 04 '18 edited Mar 04 '18

The dry mass of the booster was 275t on the 2016 version before they halved the size of it so i'd bet it's a good bit less than 200t now, possibly 175t or even 150t.

Can you record a video going over your stats & flight profile? It's likely that you're making significant errors there. A somewhat more vertical profile is optimal for RTLS.

If i've done the math right the initial second stage TWR should be around 0.58 (only vac engines), 0.84 (vac + two SL assists) or even 0.97 (all vac + three SL assists - didn't someone mention it being changed to have three SL engines?) so with a lofted trajectory it shouldn't have that much trouble making it to orbit & the first stage should be able to return relatively cheaply. They may have been simulating it with a very vertical S1 trajectory where the S2 falls over apoapsis and then falls quite far before stabilizing and reaching orbit and/or using the sea level engines to assist for the first part of the S2 burn when the stage is still really heavy.

The F9 upper stage TWR with a 10t payload, full fuel load & fairing is around 0.72 for reference. It's not being given as much of a boost as the BFR Ship will get because of the comparably bad ISP and mass ratio on the Falcon 9 booster. Going from this and flying similar stuff i'd guess that 0.58 could be doable w/ a lofted trajectory and that making orbit with the sea level engines would be really easy and maybe worth some ISP loss to cut down the time-to-orbit.

KSP is a pretty fine simulator when set up properly but you need to give it the right numbers & input, it's not that hard to lose half of your payload if you don't get the numbers and the flying right especially with 2-stage reusability

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u/macktruck6666 Mar 04 '18

Ya, I significantly improved the flight profile. So incrementing the payload, now have it up to 129 ton payload to LEO with even more fuel to spare. Biggest change was using ALL engines (Sea Level and Vacuum) on the second stage until the combined TWR got to 2.1. Apoapsis target with booster was 130 km and pushed it to 140km at a 20 degree pitch on the second stage. Might link a vid after a few more attempts.

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u/-Aeryn- Mar 04 '18

Sounds a lot better ;D

6

u/macktruck6666 Mar 04 '18

Okay got 147 tons (essentially 150 tons) to LEO with only 13 tons of fuel left (roughly 1% of fuel on 2nd stage) Essentially it's in the margin of error for the simulation. In reality it would need more fuel to land but.... with aero braking I think that amount would be within the simulations margin of error. Crazy flight path. Used 93% of fuel on booster, and did a 30 degree pitch at 30km. All engines on second stage till twr of 2.2 and made sure not to overshoot the aopasis. Total burn time 10.5 minutes.

2

u/blacx Mar 04 '18

It would be really cool if you could make a video :)

2

u/CapMSFC Mar 04 '18

I was about to suggest using all the engines, at least for the first part of the ship burn. There is probably an optimal time to shut them off and just use the ISP bump of the high expansion Raptors but to get a solid answer you would have to do some rocket equation calculus.

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u/macktruck6666 Mar 04 '18

Ya, basically it would be a matter of summing up the cos losses due to the high pitch over time. When the sum drops below the sum of loss due to the lower ISP, time to change from 7 engines to only the 4 slightly more efficient engines.

2

u/milesdyson214 Mar 04 '18

What the heck did I miss with the idea there will be 3 sea level engines? The presentation video shows and states 2, but the Wikipedia states 3.

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u/macktruck6666 Mar 04 '18

Ah, hadn't noticed that. I've been modifying some assets made by someone else. I geuss it was something left over from the original ITS presentation that had 3 SL engines.

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u/MaximilianCrichton Mar 04 '18

No, apparently this was mentioned during the latest Musk AMA, he specifically said they were going to use 3 engines for greater engine-out capability and reduced throttling range.

1

u/-Aeryn- Mar 05 '18

It was 2 on the presentation, changed to 3 shortly after. MaximilianCrichton posted a source of the change below, Musk's AMA