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Elon Musk Reddit AMA

Dkmdlb

By Dkmdlb
in Science & Spaceflight

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Frozen_Heart

Frozen_Heart

Posted
Posted
Just some rough numbers here. Using the Saturn V as a template, we can make an estimate of the size of the rocket that will send SpaceX to Mars because we know that they plan on using 9 Raptor engines in the first stage, and we know the thrust they anticipate they will get out of the Raptor. Also, let's assume they use a single core instead of 3 identical cores Falcon heavy style.

According to wikipedia, the Saturn V weighed 2,970,000 kg at launch, and the F-1 engines (of which there were 5 on the first stage) could generate 7,740,000 newtons of thrust.

The launch TWR of the Saturn V was 1.3ish.

5 x 7,740,000 newtons / 2,970,000kg / 9.8 m/s^2 = 1.3

Let's see what SpaceX could do with 9 raptor engines producing a launch TWR of 1.3

9 x 8,200,000 newtons / mass of rocket / 9.8 m/s^2 = 1.3

9 x 8,200,000 newtons / 9.8 m/s^2 = 1.3 x mass of rocket

9 x 8,200,000 newtons / 9.8 m/s^2 / 1.3 = mass of rocket

And we get 5,790,000 kg, which is 6,385 tons, for the launch mass. That's in line with what everybody is saying.

Musk quoted the Raptors thrust as 500,000lbf rather than the previously stated 1,800,000lbf. The Raptors thrust has been all over the place and I have no idea what it actually is planned to be. This is the most recent quote though so it would need a lot more than 9 engines to accomplish the 100 tons to Mars.

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Elthy

Elthy

Posted
Posted

If you split the vehicle in a payloadsection (launched in one piece) and a transferstage (also launched in one piece) you wouldnt need a complicated docking port, just something to bolt those parts together. With wireless datatransmission you wouldnt even need a cable...

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Nibb31

Nibb31

Posted
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As do real docking ports. The ISS, however, is pretty much bolted together, not just held by the ports.

Not really, the ISS is held together with CBM ports and APAS/hybrid docking ports. The PMA between Unity and Zarya is only an APAS.

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Northstar1989

Northstar1989

Posted
Posted
Just some rough numbers here. Using the Saturn V as a template, we can make an estimate of the size of the rocket that will send SpaceX to Mars because we know that they plan on using 9 Raptor engines in the first stage, and we know the thrust they anticipate they will get out of the Raptor. Also, let's assume they use a single core instead of 3 identical cores Falcon heavy style.

According to wikipedia, the Saturn V weighed 2,970,000 kg at launch, and the F-1 engines (of which there were 5 on the first stage) could generate 7,740,000 newtons of thrust.

The launch TWR of the Saturn V was 1.3ish.

5 x 7,740,000 newtons / 2,970,000kg / 9.8 m/s^2 = 1.3

Let's see what SpaceX could do with 9 raptor engines producing a launch TWR of 1.3

9 x 8,200,000 newtons / mass of rocket / 9.8 m/s^2 = 1.3

9 x 8,200,000 newtons / 9.8 m/s^2 = 1.3 x mass of rocket

9 x 8,200,000 newtons / 9.8 m/s^2 / 1.3 = mass of rocket

And we get 5,790,000 kg, which is 6,385 tons, for the launch mass. That's in line with what everybody is saying.

You're forgetting one critical thing. The Rocketdyne F-1 engine had an ISP of 263 seconds at sea level, whereas the Raptor would have an ISP of 321 sl (363 s vac), according to the current figures on Wikipedia (your source for the thrust figures). Plug that into the Rocket Equation, and you'll find you get a DRASTIC reduction in rocket mass. And that's BEFORE you consider that we can now achieve MUCH lower dry masses for the same payload thanks to numerous advances in materials science and composites since the 1960's...

Regards,

Northstar

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  • 1 year later...
Nibb31

Nibb31

Posted
Posted (edited)
43 minutes ago, Bluegillbronco2 said:

idk if has been mentioned above but they plan on refueling the thing in orbit... drastically reducing the size of the rocket. And they plan on refueling it on mars too...

I think everybody has been pretty aware of that from the start. It's pretty much the centerpiece of the plan.

1 hour ago, _Augustus_ said:

That was it? I expected more..

I don't think there really is more to reveal. Despite what Musk said about the BFS shown as being the exact same design that will fly, we all know that it's just a notional concept at this time. Many of the engineering decisions in the design design make absolutely no sense or are badly thought out. The rest of the plan is just handwaving and wishful thinking really.

Edited by Nibb31
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p1t1o

p1t1o

Posted
Posted
On 1/6/2015 at 4:05 AM, Bill Phil said:

Why care about Aesthetics? it needs to work!!! And well!!! Aesthetics can add unneeded mass.

 

On 1/9/2015 at 3:07 AM, Skyler4856 said:

Any potential investor will care...

Yes, especially if they are at all interested in engineering or serious space travel applications, they will care...negatively.

 

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kerbiloid

kerbiloid

Posted
Posted
25 minutes ago, p1t1o said:
On 06.01.2015 at 7:05 AM, Bill Phil said:

Why care about Aesthetics? it needs to work!!! And well!!! Aesthetics can add unneeded mass.

 

On 09.01.2015 at 6:07 AM, Skyler4856 said:

Any potential investor will care...

Yes, especially if they are at all interested in engineering or serious space travel applications, they will care...negatively.

 

(Though your quote was a little necroquoting, but anyway...)

T-shirts? Souvernir rockets? Books? Toys? Thematical games? Paintings? Videos with pro&contra speakers? Space movies? Tourists? Journals?

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p1t1o

p1t1o

Posted
Posted (edited)
5 minutes ago, kerbiloid said:

(Though your quote was a little necroquoting, but anyway...)

T-shirts? Souvernir rockets? Books? Toys? Thematical games? Paintings? Videos with pro&contra speakers? Space movies? Tourists? Journals?

Oh dag-NABBIT I didn't realise lol! Schoolboy error...

But anyhoo, I cant tell which side of the fence you are on...?

Edited by p1t1o
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