If the MCT is completely refueled on Mars for launch back to earth, all of the fuel will not be needed to get the TEI burn. There will be quite a bit of fuel left. So the MCT could fire and slow down before earth reentry, or it could slow down using aerocapture for a few orbits to slow down. Also, there will probably be only a crew return, not 100 people.
Yeah but,..Any chance they would use both. We have seen the results of firing an engine in a braking burn. (F-9 booster recovery attempts) Also they showed computer modeling of hypersonic reentry with engine firing. ISTM the results show the engine pushes the superheated plasma from reentry, away from the spacecraft.So say they use an appropriately sized engine to push the plasma away from the spacecraft, but the majority of the braking is done by the atmosphere. Would lead to less wear on the pica-X heat shield and is reusable by refueling the engine.
Quote from: R7 on 06/16/2015 12:40 pmTry crushing a beer can containing several bars of pressure against your forehead.Or better - don't.
Try crushing a beer can containing several bars of pressure against your forehead.
Quote from: guckyfan on 06/16/2015 03:41 am The hardest reentry is back to earth. If it can do that, Mars speed is no problem. But it CAN'T that's the rub, the vehicle can't be the large low density tank people are imagining, it would be crushed. The Apollo heat shield alone was 15% of the mass, the structure was 27% and this was to for a compact and easy to protect shape. So we can't just wave our arms and say MCT will be able to do this.How much retro-propulsion do you think we can do upon return to Earth? Any propellent to do this with is added to our DeltaV from Mars surface which is already 6-7 kms for a direct Earth return. The propellent fraction is already near the limits of credibility.
The hardest reentry is back to earth. If it can do that, Mars speed is no problem.
I think that is what he was referring to, and empty MCT. Empty of fuel and cargo, thus being much lighter and easier to crush when slamming into the earths atmosphere, even with a good heat shield. Thus the idea of a retro burn to slow down before entering earths atmosphere. Also, coming from Mars one MCT at a time, an orbit or two would help in enabling a more precise landing at the launch site, instead of the middle of the ocean or the jungle somewhere.
Quote from: spacenut on 06/16/2015 01:50 pmIf the MCT is completely refueled on Mars for launch back to earth, all of the fuel will not be needed to get the TEI burn. There will be quite a bit of fuel left. So the MCT could fire and slow down before earth reentry, or it could slow down using aerocapture for a few orbits to slow down. Also, there will probably be only a crew return, not 100 people. Crew?Why?MCT should be able to return to Earth empty. (And as needed provide occasional return transport for humans needing to return)On the way out assuming several 10s of passengers it would be astounding if there were not several engineers capable of specialized training as "flight engineers" to repair anything repairable by humans. No astronaut corps test pilots needed, just FEs similar to on the shuttle.Crew mass & life support is wasted mass and money.
Quote from: spacenut on 06/16/2015 04:40 pmI think that is what he was referring to, and empty MCT. Empty of fuel and cargo, thus being much lighter and easier to crush when slamming into the earths atmosphere, even with a good heat shield. Thus the idea of a retro burn to slow down before entering earths atmosphere. Also, coming from Mars one MCT at a time, an orbit or two would help in enabling a more precise landing at the launch site, instead of the middle of the ocean or the jungle somewhere. Being empty will not make the structure weaker. MCT isn't an Atlas fuel tank.
While machines are pretty good at doing their jobs, nobody will want to risk a several billion dollar investment on the possibility that a IC chip will fry because of a stray cosmic ray and send the craft wandering off into space or worse, come c rashing down on Earth at 6 to 7 KMS.
If MCT isn't pressurized, there would be other, bigger problems to worry about.
It would literally be crushed like an empty beer can against ones forehead when it hits the Earth's atmosphere.
Quote from: Impaler on 06/16/2015 02:01 amAt these kinds of packaging densities you would need 1200-5000 m^3 to use that mass effectively for launching satellites. But their is no way the vehicle can have such a huge cargo hold, it would make the overall vehicle too large and require too much structural mass to make it survive re-entry.This is an entirely wrong assumption. An Ariane V, per your words, has ~390 cubic meters of volume and can manage to bring two satellites into GTO and costs about $200 million per launch, at a price of $100 million per satellite. A falcon 9 has ~275 cubic meters and can get a single satellite into GTO in fully expendable mode at a price of $90 million per launch/satellite.Therefore, the only thing a MCT needs to do in order to be competitive as a satellite delivery platform is lower the cost of getting satellites into GTO, not increase the amount of satellites into GTO and since the goal of the MCT is to be rapidly reusable, a "gas & go" type of system, the only costs involved with the launch would be processing, operation and the cost of the methalox rather than building an entire vehicle, the MCT is cheaper and if the internal volume of its cargo bay is only equal to that of an Ariane V fairing, it can still get two satellites into GTO so the only way a MCT is not competitive is if the cost of its launch approaches the $200 million mark.And given the MCT's much touted price tag of $50 million per 100 people ($500 000 per passenger) I just don't see that as a likely event.Quote from: Impaler on 06/16/2015 02:01 amCommercial communication satellites are LOW DENSITY, just look at the size of current payload fairing and you can see that their is no way you could put 100 mT of satellites into the kind of volumes were looking at for a MCT cargo-hold.Falcon 9 payload fairing has a volume of ~275 m^3 and it launches only 5 mT to GTO, Ariane 5 has ~390 m^3 and launches a maximum of 12 mT to GTO. Shuttle had ~300 m^3 payload bay and could carry 24 mT to LEO. At these kinds of packaging densities you would need 1200-5000 m^3 to use that mass effectively for launching satellites. But their is no way the vehicle can have such a huge cargo hold, it would make the overall vehicle too large and require too much structural mass to make it survive re-entry.Volume is VERY important, MANY space launch systems face volume limitations, Dragon capsule for example is volume rather then mass limited for most cargoes that need to be launched to ISS.You would never pack 100 metric tonnes' worth of satellites in a MCT. Mostly because an MCT without refuelling can do much less than that in tonnes to GTO. (I remember that a number of 10-15 tonnes was mentioned somewhere in the previous topic, which amounts to 2 conventional satellites or 3-4 SEP ones, so about an Ariane 5 worth of payload to GTO). The reason why you'd want to do this without refuelling is to lower operating costs and complexity of the mission.So you pack it with as many satellites as you can given the volume and mass constraints and that still makes the volume of the MCT a non-issue because it's still cheaper than the alternatives and has enough of a capability for at least 2 satellites.The only time when the volume of a MCT becomes an issue is if you consider a commercial depot stationed in LEO, to which the MCT could deliver its full payload in mass and from which tugs would take over. But even then it'd still be cheaper
At these kinds of packaging densities you would need 1200-5000 m^3 to use that mass effectively for launching satellites. But their is no way the vehicle can have such a huge cargo hold, it would make the overall vehicle too large and require too much structural mass to make it survive re-entry.
Commercial communication satellites are LOW DENSITY, just look at the size of current payload fairing and you can see that their is no way you could put 100 mT of satellites into the kind of volumes were looking at for a MCT cargo-hold.Falcon 9 payload fairing has a volume of ~275 m^3 and it launches only 5 mT to GTO, Ariane 5 has ~390 m^3 and launches a maximum of 12 mT to GTO. Shuttle had ~300 m^3 payload bay and could carry 24 mT to LEO. At these kinds of packaging densities you would need 1200-5000 m^3 to use that mass effectively for launching satellites. But their is no way the vehicle can have such a huge cargo hold, it would make the overall vehicle too large and require too much structural mass to make it survive re-entry.Volume is VERY important, MANY space launch systems face volume limitations, Dragon capsule for example is volume rather then mass limited for most cargoes that need to be launched to ISS.
Using a BFR and MCT to launch satellites is comparable to using the Queen Mary as an ore barge.We should also acknowledge that and move on.It will also refuel at least twice, so Apollo-like 'missions' to Mars are off the table... for SpaceX anyway.
That brutal 12-14 kms entry to Earth is something everyone who is talking about this direct Earth return is glossing over, that is beyond Apollo speeds, the only thing that can survive that kind of heat, dynamic pressure and g-force is a dense capsule with thick heavy ablatives.This is why it is not valid to design spacecraft by only looking at Delta-V and tank sizes and imagining that a giant 2nd stage can do the job of direct Earth return from Mars surface just because it can hold the propellents to launch to Earth. It would literally be crushed like an empty beer can against ones forehead when it hits the Earth's atmosphere.A 2nd stage that can return from Earth orbit is a vastly simpler thing to do because the speed is half (and the energy is a quarter), and it is fairly easy to slow down the 2nd stage by several kms with residual propellents, and to employ disposable things like parachutes because it only needs to perform ONE landing before servicing rather then two, and lastly it can be made much less reliable in landing because it's unmanned, no one dies horribly if it crashes or burns up on reentry unlike MCT.
Quote from: lamontagne on 06/15/2015 05:21 pmQuote from: JasonAW3 on 06/15/2015 03:25 pmQuote from: Robotbeat on 06/15/2015 11:58 amWhat difference is there between MCT and a reusable upper stage? Just the habitable portion on top. They will need similar performance (~6.5-7km/s). Both need reentry and landing capability (legs, etc).The problem with Shuttle is there were only a few of them made, no custom ones. With MCT, thousands will be made, so no problem making some that lack the habitable portion or that act as tankers or that are only used for cargo. The requirements for these things are similar but the MCTs can be modified to fit the purpose instead of having one vehicle type do everything at once.I don't know about THOUSANDS of MCT's being built, dozens and maybe hundreds, but by that time, I expect newer designs will superceed the current design scheme.Sharing the development costs between a large number of ships is a key requirement. A 10 billion dollar development cost over 100 ships is 'only' 100 million dollars per ship, and if each ship can do 50 trips, then it's 2 million $ per trip, and a small portion of the 50 million dollar fare per ship. So perhaps 100 ships per design generation?50 reuses is far too many. My guess is the MCTs may last about 3 decades, one reuse every ~2 years (every synod), so only 12-15 reuses is practical.Musk has said 80,000 people per year (and ten times as many cargo shipments), which is 1000 Passenger MCTs at once, plus 10,000 cargo MCTs (or actually, there ways around this, but it remains to be seen if they're worth it). So yeah, at any one time, there would need to be thousands of MCTs.
Quote from: JasonAW3 on 06/15/2015 03:25 pmQuote from: Robotbeat on 06/15/2015 11:58 amWhat difference is there between MCT and a reusable upper stage? Just the habitable portion on top. They will need similar performance (~6.5-7km/s). Both need reentry and landing capability (legs, etc).The problem with Shuttle is there were only a few of them made, no custom ones. With MCT, thousands will be made, so no problem making some that lack the habitable portion or that act as tankers or that are only used for cargo. The requirements for these things are similar but the MCTs can be modified to fit the purpose instead of having one vehicle type do everything at once.I don't know about THOUSANDS of MCT's being built, dozens and maybe hundreds, but by that time, I expect newer designs will superceed the current design scheme.Sharing the development costs between a large number of ships is a key requirement. A 10 billion dollar development cost over 100 ships is 'only' 100 million dollars per ship, and if each ship can do 50 trips, then it's 2 million $ per trip, and a small portion of the 50 million dollar fare per ship. So perhaps 100 ships per design generation?
Quote from: Robotbeat on 06/15/2015 11:58 amWhat difference is there between MCT and a reusable upper stage? Just the habitable portion on top. They will need similar performance (~6.5-7km/s). Both need reentry and landing capability (legs, etc).The problem with Shuttle is there were only a few of them made, no custom ones. With MCT, thousands will be made, so no problem making some that lack the habitable portion or that act as tankers or that are only used for cargo. The requirements for these things are similar but the MCTs can be modified to fit the purpose instead of having one vehicle type do everything at once.I don't know about THOUSANDS of MCT's being built, dozens and maybe hundreds, but by that time, I expect newer designs will superceed the current design scheme.
What difference is there between MCT and a reusable upper stage? Just the habitable portion on top. They will need similar performance (~6.5-7km/s). Both need reentry and landing capability (legs, etc).The problem with Shuttle is there were only a few of them made, no custom ones. With MCT, thousands will be made, so no problem making some that lack the habitable portion or that act as tankers or that are only used for cargo. The requirements for these things are similar but the MCTs can be modified to fit the purpose instead of having one vehicle type do everything at once.
Do we have a propellant depot in LEO?
I also take it as a given that at least initially many MCTs would be staying on (or at) Mars at least for a few years. They might be a source of spare engines too if some didn't pass inspection on Mars.
This would suggest that although we intend to ramp up cargo and personnel flights to Mars with each successive launch season, we could do it with a steady state of MCT production, say one a month or less. That makes planning production simple. Now as soon as we have a few spare MCT's hanging around in LEO waiting for launch season, someone else might want to lease a few to start up lunar ISRU for fuel and maybe something else.
Quote from: nadreck on 06/16/2015 07:17 pmDo we have a propellant depot in LEO? It is part of the plan laid out by Elon Musk. However for the first few conjunctions when only 2-4 flights go to Mars, it may be easier to just refuel directly in LEO. When the number of flights increases, depots will soon become necessary.Edit: I could imagine that a manned MCT would be refuelled draining a full cargo MCT that would then be fuelled up for a second time. That may count as a kind of depot and avoid a delay for the crew.
Quote from: nadreck on 06/16/2015 07:17 pmI also take it as a given that at least initially many MCTs would be staying on (or at) Mars at least for a few years. They might be a source of spare engines too if some didn't pass inspection on Mars. Nothing to have a disagreement on but I believe only the first 2 or 3 would stay, probably forever. I believe it is safer to send them back after unloading and only a few weeks stay on Mars rather than having them there for a full synod and then relying on their continued function and safety without means for a thorough inspection.
Quote from: nadreck on 06/16/2015 07:17 pmThis would suggest that although we intend to ramp up cargo and personnel flights to Mars with each successive launch season, we could do it with a steady state of MCT production, say one a month or less. That makes planning production simple. Now as soon as we have a few spare MCT's hanging around in LEO waiting for launch season, someone else might want to lease a few to start up lunar ISRU for fuel and maybe something else.Sounds good. I would guess though that it will be some time until production rate is ramped up to one a month. Depending on funds.
This discussion gets me to another thought. I had anticipated cargo flights might not be pressurized. But they will likely need to be pressurized, not only for the benefit of the cargo but to give them stability.Getting slightly OT, I wonder if equipment will have to be pressurized all the way, which would make unloading quite difficult. Or if it could be exposed to the near vacuum of Mars for a short time during unloading. Some equipment would be designed to work on the surface, no problem there. But a lot of stuff would go into habitats.