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The depictions of the BFR for earth to earth travel include the 1st stage booster. I would think if you are just going ballistic, the upper stage would have enough delta V to go it alone, for closer destinations at least. That would simplify the operation a lot. Can anyone calculate the range it would have as a single stage to not-quite-orbit passenger vehicle?

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    $\begingroup$ Probably zero - upper stages typically have vacuum optimized engines and a thrust to weight ratio that would barely get them off the launchpad. As for simplicity, modifying an upper stage to work at sea level would be am entirely new vehicle rather ran simply reusing / mass producing the existing orbital launcher. $\endgroup$ – Quentin Clarkson Dec 19 '17 at 6:04
  • $\begingroup$ @QuentinClarkson the BFS Will need sea-level landing engines and it was stated that first test flights will be suborbital "hops" without the booster. $\endgroup$ – jkavalik Dec 19 '17 at 8:04
  • $\begingroup$ @jkavalik though the suborbital hops might be done with only a small load of fuel on board. I imagine the BFR second stage w/ payload would have ∆v in the ballpark of 6000m/s, whereas the minimum for a suborbital hop is something like 1400m/s. So it's entirely possible that even with vacuum optimized engines it could launch when only 25% fueled and that would be enough to go straight up 100km, but not enough to have any real range. Suborbital hops do get much more expensive in terms of ∆v if you want to go anywhere other than straight back down. $\endgroup$ – Blake Walsh Dec 19 '17 at 11:42
  • $\begingroup$ @BlakeWalsh might be. But afaik the main problem with vacuum optimized engines is that the bells might not survive atmospheric firing, not performance itself. And the BFS is shown to have both types of engines (iirc 2SL + 4Vac on the images) $\endgroup$ – jkavalik Dec 19 '17 at 11:49
  • $\begingroup$ @jkavalik I wouldn’t expect the landing engines to be much use at launch - On F9 a single throttled down engine is more than enough to lift a near empty tank, but it takes 9 engines at full thrust to get off the launchpad. $\endgroup$ – Quentin Clarkson Dec 19 '17 at 12:12
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From Elon Musk AMA on r/space

Will be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.

Next step will be doing orbital velocity Ship flights, which will need all of the above. Worth noting that BFS is capable of reaching orbit by itself with low payload, but having the BF Booster increases payload by more than an order of magnitude. Earth is the wrong planet for single stage to orbit. No problemo on Mars.

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/

The "vacuum" or high area ratio Raptors can operate at full thrust at sea level. Not recommended.

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodhsw6/

So for testing purposes even longer hops should be possible if they plan orbital launches for it. Maybe they will produce "for testing" vac nozzles which will be safer to use at sea level?

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Let's see if this is even possible. There are 3 engines on the upper stage of the BFR that are rated for atmospheric. Each engine, at ground level, has about 3 MN of thrust. The fully loaded weight is about 1335 tons. Gravity is pulling on that mass at a force of about 12 MN.

Bottom line, the second stage can't lift off of the ground fully fueled. It could lift off if it doesn't have a full fuel load, but that would severely limit the range.

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  • $\begingroup$ I was wondering about the vacuum engines and the drastically lightened mass ratio for touchdown, so I thought it might not have liftoff thrust. But that would have been a separate question and I figured just blundering into this question would also answer that one. $\endgroup$ – Johnny Robinson Dec 20 '17 at 0:12
  • $\begingroup$ 1) would "fully loaded" even be required? This is a suborbital trajectory, not a flight to Mars. Since your argument is based on mass, it's necessary to know the required mass rather than just maxing it out and restricting to three (or perhaps two) engines. 2) wouldn't simply switching to shorter nozzles make the other four engines also useful for lift-off? I don't think it's an open-and-shut case like this. $\endgroup$ – uhoh Dec 20 '17 at 12:45
  • $\begingroup$ Remember that the second stage is designed to launch from Mars and return all the way to the surface of the Earth including propulsive landing. If the issue is just nozzle length, I think the answer to this question needs a more quantitative analysis. See for example space.stackexchange.com/a/8807/12102 There could be an intermediate nozzle length, or a mix of lengths. I think the engines are almost the same. $\endgroup$ – uhoh Dec 20 '17 at 13:33
  • $\begingroup$ The weight on Mars is much less, and using a vacuum engine on the surface of Mars could be done. On Earth, however, it isn't really an option. $\endgroup$ – PearsonArtPhoto Dec 20 '17 at 13:58
  • $\begingroup$ The reference to Mars launch returning all the way to Earth surface is a reminder of the substantial capability in general, and would already have lift-off capable systems as part of its design. Weight is also much less if you don't top-off the tank and thrust is larger if you use more engines, ergo this TTW argument does not fly. $\endgroup$ – uhoh Dec 21 '17 at 0:54

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