Direct quote from Elon implying no second stage, just booster and mars spacecraft from an article in GQ:"Well, there's two parts of it—there's a booster rocket and there's a spaceship. So the booster rocket's just to get it out of Earth's gravity because Earth has quite a deep gravity well and thick atmosphere, but the spaceship can go from Mars to Earth without any booster, because Mars's gravity is weaker and the atmosphere's thinner, so it's got enough capability to get all the way back here by itself. It needs a helping hand out of Earth's gravity well. So, technically, it would be the BFR and the BFS." As in "Big frakking Spaceship."http://www.gq.com/story/elon-musk-mars-spacex-tesla-interview?utm_source=10370

A BFR can be conceived with lots less than 30 1st stage engines. As a point of reference, I’ve taken these Musk’s statements as a given even though I bet they’ll be modified at his supposed late 2015 (likely spring 2016) briefing.100mT land on Mars cargoLand the whole thing and re-use it; i.e. return it to Earth“there's a booster rocket and there's a spaceship. So the booster rocket's just to get it out of Earth's gravity because Earth has a deep gravity well and thick atmosphere, but the spaceship can go from Mars to Earth without any booster”Raptor thrust over 230mT; use a lot of them.380 seconds ISP vacuumMy BFR is 12.5m diameter, making a relatively squat first stage under 30m tall. Propellant tanks alone 23.5m. Delivers ~2.8Km/sec after gravity losses. The wider diameter leaves room for growth for later larger versions, allowing for lengthening the tanks in both stages, adding engines and lengthening the cargo hold. Assumed a heavy dry weight of 90mT for stage 2 MCT.No worries about pad towers for 100m high rockets. BFR with MCT 2nd stage is shorter than the 70m F9 but 10 times more massive. BFR’s multi Raptor engine driven 12.5m diameter is a nice size to fit living quarters and various colonial heavy equipment. Don’t need 15 million LBS thrust, but with mass ratios ~4.3% need a bit over 12 million LBS (54 million Newtons) thrust, 24 engines arranged in rings of 16 and 8 with room for a future center engine or 2.Given the huge delta V requirements for both Mars departure from refueling at LEO AND later functioning as a SSTO taking off from Mars’ surface and return to Earth, I put the Km/sec budget into the 2nd stage. Stage one goes low & slow, under 3 Km/sec, boosting the heavy 2nd stage before return to launch site, RTLS. The dry weight plus fuel of the returning 1st stage exceeds Raptor thrust so any 2 of the 8 engine inner ring engines throttled down provide landing thrust.Total BFR mass 4450mT or 9.8 million LBS. LEO mass fraction 4.3%.Stage One:12.5m diameter with 23.5m length propellant tanks3280mT 7.2 million LBS 1st stage fueled mass230mT thrust engines 506K LBS24 engines 56 million Newtons 12.2 million LBS Thrust; T/W 1.24Rings of 16 engines, and 8 enginesAvg ISP from sea level to vacuum 325After 1 Km/sec additional Delta V reserved for RTLS Rocket Equation gives 3.4 Km/sec but nets under 3 Km/sec Delta V after gravity lossesStage Two The MCT:Dry Mass 90mT; 100mT is cargo12.5m diameter with 7.5 m length for propellant tanks. Cargo 8.5m length ; 1040 m3 volume(Plenty of space for expanded fuel tanks in a simple modified tanker version.)1165mT fueled mass (2.6 million LBS)380 seconds ISP vac; Rvac engines assumed 14% higher thrust as with F9 FT6 Rvac engines 3.5 million LBS Thrust6.75Km/sec Delta V capability, via Rocket eq.8.4Km/sec Mars liftoff with only 25mT return cargoI also modeled another small version to see just how small a BFR could meet Elon’s goals. I optimistically lowered the MCT 2nd stage dry weight from 90mT to 75mT. This minimal, 12m diameter optimistic version needs only 19 engines with rings of 12, 6 and two center engines. The MCT stage 2 has only 5 engines, reducing the cost of building this BFR/MCT from 30 Raptors to 24. Engines are the big cost driver for mass producing BFRs.

From another thread, here is a recent quote by Musk in an interview:"So, technically, it would be the BFR and the BFS." As in "Big frakking Spaceship.""

Just to confirm: you have the BFR contributing only 1km/s delta V before separation?

philw1776: Your engine count is off because you forgot to account for lower thrust at Sea-level, 230 mt is the VAC performance goal for Raptor. I'm estimating that Sea-level thrust is ~77% of Vac which raises the engine count to ~30 to get very nearly identical total thrust as your estimate.

Q: Has the Raptor engine changed in its target thrust since the last number we have officially heard of 1.55Mlbf SL thrust?A: Thrust to weight is optimizing for a surprisingly low thrust level, even when accounting for the added mass of plumbing and structure for many engines. Looks like a little over 230 metric tons (~500 klbf) of thrust per engine, but we will have a lot of them

Quote from: Impaler on 12/12/2015 03:38 AMphilw1776: Your engine count is off because you forgot to account for lower thrust at Sea-level, 230 mt is the VAC performance goal for Raptor. I'm estimating that Sea-level thrust is ~77% of Vac which raises the engine count to ~30 to get very nearly identical total thrust as your estimate.A question. Do we know that the 230mt quoted (I assume in the reddit Q&A Elon did) was for Vac? Or has there been any more information on this elsewhere? I'm asking because Elon was answering a question that was about SL thrust. The quote I remember. QuoteQ: Has the Raptor engine changed in its target thrust since the last number we have officially heard of 1.55Mlbf SL thrust?A: Thrust to weight is optimizing for a surprisingly low thrust level, even when accounting for the added mass of plumbing and structure for many engines. Looks like a little over 230 metric tons (~500 klbf) of thrust per engine, but we will have a lot of them

It would be amazing to one day see hardware with a giant 'SpaceX BFR' Decal on the side. I wonder if the 'name' will survive that long... Of it gets renamed something else... 'Tiny' maybe.

I understand that usually it is assumed that the first stage will give the upper stage/MCT a horizontal speed of ~3km/s, similar to what the Falcon first stage does. For RTLS of the first stage that needs to be reversed, cutting into payload.I do wonder if another approach could be effective. MCT will have a very large delta-v budget. It needs it to perform its functions towards Mars. Could the first stage go up almost straight similar to the BO New Shepard, but maybe up to 150km peak altitude, eating all the gravity and air resistance losses and use the second stage for the task of building up orbital speed? On the way down with its large diameter it may not need a reentry burn or only a very small one. Reuse fuel would be mainly only the small amount of landing fuel.Did anyone of those who did thorough analysis ever consider such a scenario or am I way off?

The vast majority of the energy in a rocket launch is expended reaching orbital velocity, not orbital height. Getting to orbital velocity requires a large amount of horizontal velocity. Getting that horizontal velocity takes about the same amount of dV, no matter what altitude you are at. If you started at LEO altitude, and started accelerating horizontally, you'd still need to obtain 17,100 mph of horizontal velocity. Without the horizontal velocity, you're suborbital.

Quote from: Dante80 on 12/13/2015 04:13 PMThe vast majority of the energy in a rocket launch is expended reaching orbital velocity, not orbital height. Getting to orbital velocity requires a large amount of horizontal velocity. Getting that horizontal velocity takes about the same amount of dV, no matter what altitude you are at. If you started at LEO altitude, and started accelerating horizontally, you'd still need to obtain 17,100 mph of horizontal velocity. Without the horizontal velocity, you're suborbital.I know all of this. I even explicitly stated it in my post. So what's your point?Usually, I understand, a first stage does a lot of delta-v towards orbital speed. The Falcon 9 does much less, to facilitate RTLS and leaves more of the buildup of orbital speed to the second stage. That is not the optimum approach for expendable vehicles. It is a better approach for reusable vehicles.My suggestion was to carry the idea to the extreme. Let the first stage eat all the gravity loss and drag loss and leave all or almost all of the buildup of orbital speed to the second stage. MCT needs the big delta-v budget anyway to get to Mars from LEO and land.

"Big Falcon Spaceship"

I know all of this. I even explicitly stated it in my post. So what's your point?Usually, I understand, a first stage does a lot of delta-v towards orbital speed. The Falcon 9 does much less, to facilitate RTLS and leaves more of the buildup of orbital speed to the second stage. That is not the optimum approach for expendable vehicles. It is a better approach for reusable vehicles.My suggestion was to carry the idea to the extreme. Let the first stage eat all the gravity loss and drag loss and leave all or almost all of the buildup of orbital speed to the second stage. MCT needs the big delta-v budget anyway to get to Mars from LEO and land.