I'm confident they will reach the same conclusion I have after figuring out (possibly the hard way) that Direct Earth return is impossible. People are being way to fast to grasp at nebulous ideas and speculations even if they come from Musk as THE ONE AND ONLY way it will be done, Musk like any good programmer tries to think of the simplest possible system he thinks could possibly work, we saw that with F9 reuse plans, they are now WAY more complex then originally planned, MCT will be the same.

Quote from: nadreck on 08/24/2015 05:12 PMAs opposed, I believe 50t could be the dry mass of a 820t fully loaded and fueled MCT, note that at most there would be 1.3km/s of ΔV or so a re-entry mass around 230t, and a landing mass approaching 150t.Does it really need that much delta-v to go from terminal velocity to landing?

As opposed, I believe 50t could be the dry mass of a 820t fully loaded and fueled MCT, note that at most there would be 1.3km/s of ΔV or so a re-entry mass around 230t, and a landing mass approaching 150t.

I am with Gucky on this one. I dont think that the mission plan you described is realistic at the beginning.1. Aerobraking has never been used before, for good reason.

2. SEPs are for small payloads, not ones of many mT. The amount of solar arrays needed to get any meaningful thrust would be ginormous.

3. Using first SEP and then chemical propulsion is really inefficient.

4. Your mission plan is as complicated as it possibly can get. Simple is most often more important than efficient. Any complication in the flight plan means a rats hole of additional design and engineering work and launch mass to avoid failures. The more things you have the more things can break. And if anything breaks, your mission is toast.

5. The answer to "direct return is impossible" would not be to make the plan more complicated but to reduce the payload mass. The 100 persons per MCT figure is probably something for the far future. Not something for the first try.

The plan you described is maybe something for the far future, when Humanity has figured Mars transport out and is on its way to optimize the process. When failure modes are known and engineers learn to cut the right corners. I am very skeptical for the beginning though.

Ok, lets make a back-of-the-envelope calculation.A recent all-electric satellite Eutelsat 115 WEST B (2,205 kg) uses about 18 kW (4x 4.5 kW thrusters) to reach GSO. It takes 8 Month, so that is a reasonable timeframe for the not yet crewed MCT. Each thruster weights below 16 kg, say 15. So the mass of the spacecraft without the thrusters is 2141 kg. The solar arrays provide 18 kW of power for the thrusters. Wikipedia says that one gets about 300W/kg and 300W/m^2. That makes 60 kg of solar panels and 60 m^2 area. So the spacecraft without the engines and without the panels is 2081 kg. Now, scaling that to the mass of the MCT of about 475mT in your design. Then the SEP tug would need 475/2* 18 kW= 4.25MW of solar, or 472/2 * 60kg approx 14 mT of solar panels which have a surface area of about 14000 m^2. Say launch that thing with an BFR that has a payload bay of 20m hight. Then the solar arrays need to have a wing span of about 700m. Since the individual elements cant be larger than the diameter, say 10m, that are 70 segments. Say we have 2 equal sized 350m panels (one on either side) and you use ISS type solar arrays, that would fill two boxes of about 5m height each.I admit, I thought that the solar panels alone would be far larger than that. I thought that this alone would invalidate your concept. It is at these considerations borderline possible. But I did not include everything that needs to be included. I did not include the Xenon mass, the mass of the SEP tug structure, any electronics or transformers. No heat control system or other things that are needed. sources:https://directory.eoportal.org/web/eoportal/satellite-missions/a/all-electrichttps://en.wikipedia.org/wiki/Solar_panels_on_spacecrafthttps://en.wikipedia.org/wiki/Integrated_Truss_Structure#Truss_subsystems

Something I haven't seen many discussions on: Abort ModesIs this somewhere where SEP may be superior? If you use a 6 months fast Hohmann transfer, you will naturally return to Earth in 2 years if you miss Mars for what ever reason. Swing out to 2 AU then back in again to meet up with Earth (source: the Case for Mars/Mars Direct).This is fine for initial exploration, as you will only have a small crew (~4 people) as you only need to carry 2 years worth of supplies for 4, and you would arguable have to do that anyway. But trying to carry 2 years worth of supplies for 100 people, and things get out of hand. Roughly 1000kg per person of supplies and you're looking at over 100 tonnes of just food, water and oxygen. We could just assume that nothing will go wrong, but it's not going to be fun when something does.

That's why IMO it makes sense to launch several MCT's in the same launch window - a small fleet - that way they could support and assist each other if a problem should occur.

Your basing your calculations on a freight SEP vehicle on a com-sats performance? That's like using a water bottle rocket to estimate the performance of a launch vehicle. Try using one of a million SEP design concepts for lunar tugs, ARM or Boeing's recent paper rather then this nonsense.

Quote from: Semmel on 08/25/2015 07:51 PMOk, lets make a back-of-the-envelope calculation.A recent all-electric satellite Eutelsat 115 WEST B (2,205 kg) uses about 18 kW (4x 4.5 kW thrusters) to reach GSO. It takes 8 Month, so that is a reasonable timeframe for the not yet crewed MCT. Each thruster weights below 16 kg, say 15. So the mass of the spacecraft without the thrusters is 2141 kg. The solar arrays provide 18 kW of power for the thrusters. Wikipedia says that one gets about 300W/kg and 300W/m^2. That makes 60 kg of solar panels and 60 m^2 area. So the spacecraft without the engines and without the panels is 2081 kg. Now, scaling that to the mass of the MCT of about 475mT in your design. Then the SEP tug would need 475/2* 18 kW= 4.25MW of solar, or 472/2 * 60kg approx 14 mT of solar panels which have a surface area of about 14000 m^2. Say launch that thing with an BFR that has a payload bay of 20m hight. Then the solar arrays need to have a wing span of about 700m. Since the individual elements cant be larger than the diameter, say 10m, that are 70 segments. Say we have 2 equal sized 350m panels (one on either side) and you use ISS type solar arrays, that would fill two boxes of about 5m height each.I admit, I thought that the solar panels alone would be far larger than that. I thought that this alone would invalidate your concept. It is at these considerations borderline possible. But I did not include everything that needs to be included. I did not include the Xenon mass, the mass of the SEP tug structure, any electronics or transformers. No heat control system or other things that are needed. sources:https://directory.eoportal.org/web/eoportal/satellite-missions/a/all-electrichttps://en.wikipedia.org/wiki/Solar_panels_on_spacecrafthttps://en.wikipedia.org/wiki/Integrated_Truss_Structure#Truss_subsystemsWhat you did is assume away several factors: required delta V & burn duration, which determine both required power and required propellant. The SEP mass fraction of the MCT being identical to the 702SP is not a safe assumption.

Not sure this has been discussed here. Is there a chance BFR gets a grasshopper type development vehicle?

I know, its a back of the envelope analysis. Its quick and dirty, thats what estimations are for. Its a calculation to see if the concept makes sense in the most simplistic way. I expected that an SEP tug for such a huge payload is physically impossible to launch and I tried to prove that. I did not achieve that goal. But that is not a prove that the concept works. Its just a failure to prove that it does not work. I hope you understand the difference.Also, my initial information on Impalers design did only state "from LEO to HEO" which is as fuzzy as it can get and I cant possibly deduce dV requirements from that. I just assumed that GSO is roughly equivalent to what he envisions as HEO.I still dont like Impalers concept for the other reasons I stated, even if a SEP tug is physically possible.

Ok, so you admit that you've already decided you don't like my proposed mission architecture before you did any actual research (and regardless of what the research says), and are now trying to 'prove' it is not feasible via some sloppy back of a napkin calculations that seem to be uninformed by ANY actual studies on large SEP vehicles of which their are dozens. And you expect me to do your research for you by providing these links? I think not.