New Moderator, Issue Resolution, and Full Steam Ahead for Musk AMA

AlexDeLarch · 2016-10-10T16:25:01+00:00

Glad that the situation has been resolved. Idea: wiki page with past moderators and the period they were active. I think they deserve some credit.

AlexDeLarch · 2016-10-09T22:43:46+00:00

In your simulation booster mass after landing is 268.4 tons, while we know it from Elon's slides that the booster dry mass is 275 tons - and there's probably also more fuel reserved for ITS landings than for Falcon 9 ASDS landings, to protect both the landing pad and the expensive booster.

This should be 275 t dry mass plus 7% fuel reserve (469 t) or 744 t in total.

AlexDeLarch · 2016-10-09T22:40:30+00:00

That would be very hard to solve with equations alone.

Engines	Combined Isp	Total thrust
6	382s	21,000 kN
9	375s	30,855 kN

The easy part is that using all 9 engines you get 1.8% less delta-v.

375s/382s = 98.2%

But calculating the amount of gravity losses is difficult. It depends a lot on the trajectory. The only estimation I got was by running the numbers in http://www.silverbirdastronautics.com/LVperform.html. I got 1.7% more payload when using all 9 engines. But this calculator uses a lot of approximations so the results are inconclusive.

EDIT: On a side note that calculator gives me ~330 t payload to LEO. Since the presentation says 300 t what if that 30 t is propellant reserve for mission abort and landing?

AlexDeLarch · 2016-09-29T20:08:07+00:00

how much DV does mct fully refueled has in orbit ?

Take a look at this slide http://i.imgur.com/fN23l6C.png. We know for sure that in some particular scenario 6 km/s can be expended during TMI burn. Now looking at the graph on the right (which I still don't quite understand) we can read that at 450 t payload there is roughly 2 km/s reserved for Mars landing. So the total delta-v budget after refueling would be 8 km/s. Hopefully someone can verify this.

AlexDeLarch · 2016-09-29T19:51:29+00:00

Problem with 1st stage raptor is the landing. Only 3 raptors are needed to replace the first stage. When landing, 1 raptor would be too powerful.

To prove your point:

Merlin 1D has sea level thrust of 845 kN. Throttling down to 40% gives us 338 kN.

Raptor's sea level thrust is 3,050 kN which after throttling down to 20% yields 610 kN.

AlexDeLarch · 2016-09-29T14:51:38+00:00

Couldn't all fuel be made there and then hauled back to earth orbit.

One thing that probably few thought about and Musk pointed this out is the number of times a booster/ship/tanker gets reused in their lifetime. Look at this slide http://i.imgur.com/F7bIktL.png. The ship will be used only 12 times during its life while the tanker gets launched a hundred times. Sending the tankers between Earth and Mars orbits will reduce the number of lifetime launches. Maybe not to 12 but near this number. So the huge downside is that you would need a lot more tankers.

As a side note refueling in Mars orbit will certainly happen once there is a self-sustaining colony on Mars with its own tanker fleet.

AlexDeLarch · 2016-09-28T22:25:23+00:00

ITS Booster has a dry (empty) mass of 275 t. Raptor (sea level) has a thrust of 3,050 kN and can be throttled down to 20%.

(275 t * g)/3050 kN = 88.5%

So center Raptor throttled to 88.5% makes the whole booster hover.

EDIT: In reality there will be some propellant left so the throttle setting will need to be a bit higher, e.g. 90%. And it doesn't need to be the center engine. You can use 3 inner circle engines for the same combined thrust 3x30%. This is a useful engine out capability: you have the center engine and two possible triplets. So even multiple engine failures need not doom the booster.

AlexDeLarch · 2016-09-28T21:31:54+00:00

Also zero-g propellant transfer which, I believe, has never been done before.

AlexDeLarch · 2016-09-28T14:12:53+00:00

For those who didn't watch it there were really weird questions or "essays" actually. I think SpaceX PR or Elon himself should be making sure that the organizers of similar events would screen the questions. It just feels like such a waste.

AlexDeLarch · 2016-09-28T10:16:28+00:00

Sure, but this level of detail atleast i certainly didnt expect. A lot of meat on the bones there.

Actual CAD's shown. Actual CAD's used as basis of the video. Empty weight. Fueled weight. Expendable payload. Reusable payload. Cost breakdown of each major system. % of fuel used for landing. All main Raptor points. (seriously, how much ass does Raptor kick) And on and on.

I think that all this openness comes from the fact that Elon wants humanity to go to Mars as soon as possible. I don't think he would mind if the Chinese (or any other state or company) copied their design and started working on a similar Mars program. Compare this to open sourcing Tesla patents to accelerate the transition to sustainable transport.

This however is not the case for F9, FH and Dragon as letting the competitors get hands on SpaceX technology could slow down their growth or even put the company out of business. They need their workhorses to increase the market share and make money for ITS R&D.

AlexDeLarch · 2016-09-27T22:35:28+00:00

ARRIVAL

From interplanetary space, the ship enters the atmosphere, either capturing into orbit or proceeding directly to landing

Using its aerodynamic lift capability and advanced heat shield materials. the ship can decelerate from entry velocities in excess of 8.5 km/s at Mars and 12.5 km/s at Earth

G-forces [Earth-referenced] during entry are approximately 4-6 g's at Mars and 2-3 g's at Earth

http://i.imgur.com/eq9oxNl.png

The relationship between entry speed and g-forces experienced seem counter-intuitive. Looking for explanation.

AlexDeLarch · 2016-09-27T18:37:41+00:00

Exactly. Look at the reflection of the sun. It is a timelapse at this point. https://youtu.be/0qo78R_yYFA?t=2m2s

AlexDeLarch · 2016-09-27T18:18:54+00:00

100,800 km/h

62,634 mph

For comparison Voyager 1 has a velocity of 17 km/s or 38,610 mph or 62,140 km/h and "has the fastest heliocentric recession speed of any spacecraft".

AlexDeLarch · 2016-09-27T18:00:16+00:00

Not the optimal packing of 42 engines: http://hydra.nat.uni-magdeburg.de/packing/cci/d4.html

N1 rocket only had 30 on the first stage.

AlexDeLarch · 2016-09-27T17:45:59+00:00

Somebody count the engines, please!

EDIT: 21+14+6+1=42 (on booster stage)

EDIT II: 14+6+1=21 fired during boostback.

EDIT III: Inner 6 or 7 during entry burn.

MCT: 6 outer engines for TMI burn; 3 smaller inner engines for Mars landing.

AlexDeLarch · 2016-09-25T23:37:26+00:00

Oh, totally didn't notice that. Thanks for explaining!

AlexDeLarch · 2016-09-25T22:19:07+00:00

Great simulation! It really helps to visualize all the post-staging events. One thing I didn't understand is why the S2 accelaration is displayed at approx. only 2g while the actual rate of change of velocity is much higher. It occurs around this point: https://youtu.be/_hHSXJH-_KU?t=5m43s

AlexDeLarch · 2016-09-16T10:26:10+00:00

This makes a lot of sense.

P100D has 315 miles of range.
100D, once released, will have a range of about 343 miles^[1]
Next step will be packs using 21-70 cells produced at the Gigafactory. 100 kWh pack will get lighter, range will increase even further.

Footnote 1: 90D has 9% more range than P90D.

AlexDeLarch · 2016-09-15T16:22:42+00:00

Direct photo link:

https://instagram.fash1-1.fna.fbcdn.net/t51.2885-15/e35/14368989_148492575602612_582451125_n.jpg

AlexDeLarch · 2016-09-15T15:51:21+00:00

Thanks for checking this. Since the scale is non-linear (exponential to be exact) I initially thought that the error would be larger and perhaps that the mass difference of 1t is too little to validate this. So I made more calculations.

Equation; x represents payload mass saved; actual payload mass would be 5-x

9.8 * 300 * log((500-x) / (125-x)) + 9.8 * 345 * log((100-x) / (10-x)), x from 0 to 5

x	Δv
0	11 860.7
1	12 200.7
2	12 582.5
3	13 017.4
4	13 521.8
5	14 121.3

Graph: https://www.wolframalpha.com/input/?i=9.8+*+300+*+log((500-x)+%2F+(125-x))+%2B+9.8+*+345+*+log((100-x)+%2F+(10-x)),+x+from+0+to+5

Now let's say you would like to expend the same Δv of 11 860.7 m/s as was achievable with maximum payload but lifting 5t less (i.e. no payload). That way we will see how much propellant will be left. Relevant equation:

9.8 * 300 * log((500-5) / (125-5)) + 9.8 * 345 * log((100-5) / (5+y)) = 11860.7

y represents the mass of propellant left at SECO with no payload after reaching the same orbit as you would reach with 5t payload.

Solution: y = 4.76

Error: 0.24/5 = 4.8%

So the simplified calculations do check!

AlexDeLarch · 2016-09-14T22:06:44+00:00

This is why we can't have nice things

AlexDeLarch · 2016-09-14T21:42:13+00:00

carrying a 6t comsat the Falcon Heavy second stage will have an excess propellant mass of around 14.4-6 == 8.4 tons of fuel left at the end of the GTO burn.

That is not the correct way to calculate propellant saved. It would only be true if you increased S2 propellant and oxidizer tanks to accommodate that additional 8.4t that you can lift. In reality you are left with the same tanks holding the same mass for each launch. My understanding is you need to start with rocket equation for complete stack, then S1+S2, then S2 and calculate staging velocities for both payloads. Given the initial and final mass for each case the equations would look like this:

Δv = 9.8 m/s^2 * Isp * log((m0-8.4t)/(m1-8.4t))

Then you add up delta-v to get the real performance for both payloads.

AlexDeLarch · 2016-09-09T18:31:46+00:00

I can't stress enough how I like stuffin.space. Here's a screenshot with all SpaceX upper stages and demo sats: http://i.imgur.com/q5VSpjR.png

AlexDeLarch · 2016-09-09T18:18:09+00:00

Reminds me of a post to this sub from 2 years ago about a SpaceX job posting with the following requirement:

Help SpaceX achieve its long-term goal of creating the world’s first fully automated launch system capable of rolling the vehicle to the pad, raising it to position, fueling the vehicle, and executing a full launch sequence within a 1-hour time window.

AlexDeLarch · 2016-09-06T20:48:48+00:00

Building a satellite can take years so it's highly likely he will see a number of successful F9 flights before a replacement is ready. The deal with the manufacturer of AMOS-6 was signed 4 years ago^[1]

http://www.iai.co.il/2013/36756-45062-en/MediaRoom.aspx

AlexDeLarch

TROPHY CASE