Well, many reasons why I believe there will be an incremental approach.1. SpaceX always did an incremental approach. The didnt start with Falcon 9 1.1 when they developed their first rocket. Same holds true for the BFR. They need to learn to manufacture, launch, etc. such big rockets and thats easier if they dont jump into it with both feat.
2. There is a business case. They can probably much more cheaply launch large constellations of satellites. Like one or more of the internet constellations. Maybe even modules of a new space station. Maybe even a commercial space station, serviced by Dragon2 for space tourists.
3. All the key technologies for BFR+MCT can be tested on a smaller rocket with a smaller MCT precursor. That includes:
* landing on Mars
* large scale energy production on Mars
* water production on Mars* methane/oxygen production on Mars
* landing, retanking and MCT (precursor) return to Earth
* flight operations and trajectory execution* communications* high bandwidth, high latency internet access on Mars
* booster launch and recovery (using Raptor)
* second stage recovery (if any)
* re-tanking in LEO
* first human on Mars and survivability* first habitat on Mars
* first plant growth on Mars using Mars soil* first sustainable food growth on Mars
And that are just the biggies. All this needs to be ready for colonization. And all this is easier with a smaller version of BFR/MCT than the ultimate goal noted by Musk.Once these technologies are in place and tested and ready, a full size MCT makes sense. I would not be surprised that after the logistical hassles are overcome with the first missions, a bigger version of MCT are produced, one that is closer or even capable of 100 humans to Mars. Even though this particular capability would not be used.4. I dont think that SpaceX will have the funds to do it alone. They will need the help of governments, NASA might not even be enough. When in cooperation with space agencies, pork needs to be provided. That is far easier with a precursor MCTs that focus on technology development and science rather than direct colonization of a naked planet.
I've done a mass breakdown of my proposed MCT vehicle.Thermal Protection Airoshell: 5 to 10 kg/m^2 metallic fully reusable, 650 m^2 5 MtTanks and plumbing: 5% of 300 Mt propellant load 15 MtLanding Gear: 10% of landed mass based on F9-R ratio with F9 expendable 18 MtRaptor Engines: 1.5 Mt each based on 150:1 T/W ratio and 2300 KN force x4 6 MtVernier Engines: Hover 175 Mt on Mars with 100:1 T/W ratio 1 MtMiscellaneous: Transit solar panels, thermal radiators, avionics, cargo handling 5 MtStructural Frame: 11% of Entry mass, carbon-fiber skeleton 25 MtTotal Dry Mass 75 MtCargo 100 MtLanding Propellant: Provide 800 m/s Terminal decent DeltaV 40 MtTotal Entry mass 215 Mt
Here's a fully fueled 180mT MCT outbound from LEO (100mT cargo) compared with the launch back from Mars' surface with "only" 25 mT cargo MCT Dry Wt & Cargo 180 mTS2 Mass w/MCT 1025 mT LEO departure. Mars Return is 75 mT lessS2 Mass w/MCT 2.3 Million LBSStage 2 Km/sec 6.48 Km/sec Rocket Equation LEOS2 Mars Return 25mT Cargo 8.5Km/sec Rocket EquationExponentials help when you reduce the mass. Just refuel with less propellant to reduce "excess" Km/sec.The technical challenge is a lightweight MCT vehicle able to withstand Earth re-entry if that is the goal rather than return to some high Earth/moon orbit.
If anything the 225 Mt number for BFR launch mass I'm speculating is aggressive. I'm concerned about what the 2nd stage recovery cost will be and if this will kill the performance and lower the % of launch mass reaching orbit as philw1776 thought his own analysis looked high in this regard.
Quote from: Impaler on 09/04/2015 04:48 pmIf anything the 225 Mt number for BFR launch mass I'm speculating is aggressive. I'm concerned about what the 2nd stage recovery cost will be and if this will kill the performance and lower the % of launch mass reaching orbit as philw1776 thought his own analysis looked high in this regard.I think you might have misunderstood me here. My suggestion is that ~200 tons launched from Earth to LEO seems like the limit, but that limit only has to cover spending on structure & permanently-attached habitat; One adds to that with supplemental launches and reaches ~600 tons at Mars entry, with the extra spent on food, people, ECLSS, ISRU gear, surface equipment, and descent propellent. Then you factor in propellant for Earth Departure and reach 2000-3000 tons IMLEO (4.5 to 6km/s).And that buys you a conjunction-class colony-in-a-can mission, for 25 people at least; And maybe 100 people with prelanded assets.
Humm, if I am launching from Mars surface and I want to enter an elliptical hohoman transfer around the sun I need to decrease my heliocentric velocity aka I need to be going slower then Mars itself after having left it's sphere of influence. Thus gravity loss incurred during escape may actually be beneficial IF it results in the loss in heliocentric velocity that one needs to reach Earth.
According to https://en.wikipedia.org/wiki/File%3aDelta-Vs_for_inner_Solar_System.svg you need 5.5 km/s DeltaV to reach escape
I just had a wild idea about the MCT design. What if it was inflatable by itself, without a separate habitat?Bigelow habitats generally have a solid cylindrical core surrounded by an inflatable shell. But is there any particular reason why a conical shape wouldn't work just as well? Imagine a scaled-up SuperDragon enclosed in a layer of inflatable material which is puffed-up in vacuum.The capsule would be launched deflated and only inflate once safely out of the atmosphere. When it's time to enter the atmosphere of Mars you can repack all the furniture back into the solid core and deflate the exterior. This pull brings the inflatable portion back behind the heat shield. On the surface of mars you can inflate it again and will end up with a sort of mushroom-shaped habitat. You can mount light-weight flooring inside the inflated region when on the surface..The "inflatable" portion doesn't have to touch actually the heat shield. You will have side-mounted engines surrounding the heatshield, similar to the current Dragon.Such a design requires that the skin material have some thermal resistance, similar to the sides of a normal capsule. It would likely need a fairing for flying upwards through the earth's atmosphere. The habitat also needs to support inflation/deflation while people are inside the core. Deflation seems particularly difficult. The simplest way would be to make core itself mostly airtight and pump out the air from the exterior portion.This design only makes sense for shipping large numbers of people. You give them plenty of space during the transit period and on the surface but pack them closely together for takeoff and landing. It solves the "habitable volume" problem without increasing payload diameter too much.
I was thinking of something more akin to a gravitational slingshot in which the vehicle is ahead of mars in it's orbit and mars pulls it back, the vehicle loses velocity relative to the sun and mars gains it.
In a previous article, the same author, Richard Heidmann, seems to have fixed on the idea that BFR is an SSTO, that MCT has no main engine, just belly thrusters for landing. That's led to some weird conclusions, and hence the current article.
Nice concept. But quite frankly, I like the concept developed in L2 better. Hyperion et al. seem to have a better handle on the subject. Its very interesting to the development though and only good things can come from independent groups tackle the same problem.