Author Topic: SpaceX technically only needs to build like 2 BFSes and 1 BFR...  (Read 30591 times)

Offline Robert827

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Before flying the first crew, I suspect at least one unmanned passenger BFS round trip to Mars and back would be needed.

Offline jpo234

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Before flying the first crew, I suspect at least one unmanned passenger BFS round trip to Mars and back would be needed.

Will this be possible? It is my understanding that this would require autonomous refueling on Mars.
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Offline Bynaus

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I'd think the ratio would be something like 1 BFR: 1 Passenger MCT: 2-4 Tanker BFS: 10-20 Cargo MCTs.

I think it really depends what we are looking at: early missions? fully established Mars colonial transport architecture? Obviously, in the long run you want plenty of BFSs (if you want to move 50'000 people to Mars per year, there is no going around it!), but Robobeat was going after the minimum number absolutely necessary to complete a human mission to Mars - and the minimum number is exactly what we can expect for the very first missions.

For these very first missions, you clearly don't need "10-20 Cargo MCTs". You can transport all the cargo you need for a scout class mission in a single BFS - just tune down the number of crew members to 4-10 or so, and use the rest of the capacity for cargo. Also, I don't think you need so many tankers if you only have one BFR to launch them anyway. I doubt that the "turn-around" time of the BFS tankers is going to be much longer than the BFR.

The absolute minimum is: 1 BFR, 1 tanker BFS, 1 BFS. And thats what we are likely going to see for 2022/2024 (or whenever the first mission happens).
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Offline guckyfan

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Before flying the first crew, I suspect at least one unmanned passenger BFS round trip to Mars and back would be needed.

Will this be possible? It is my understanding that this would require autonomous refueling on Mars.

Why would that even be desirable?

Mars ascent is by far the easiest part of the mission. The hardest are Mars descent and earth descent. Mars descent will be tested at least twice with cargo BFS and earth descent from interplanetary speed can be tested with a loop around the moon plus maybe an additional acceleration before atmospheric entry to correctly simulate interplanetary speed. Again putting demands on SpaceX that NASA mission plans don't include.

Online JamesH65

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Before flying the first crew, I suspect at least one unmanned passenger BFS round trip to Mars and back would be needed.

Will this be possible? It is my understanding that this would require autonomous refueling on Mars.

Why would that even be desirable?

Mars ascent is by far the easiest part of the mission. The hardest are Mars descent and earth descent. Mars descent will be tested at least twice with cargo BFS and earth descent from interplanetary speed can be tested with a loop around the moon plus maybe an additional acceleration before atmospheric entry to correctly simulate interplanetary speed. Again putting demands on SpaceX that NASA mission plans don't include.

So landing on Mars is harder than taking off? Really? Landing on Mars has been done before. No-one has EVER launched from Mars. Earth descent, also been done before.

I'd say it was all very difficult. I wouldn't like to put degrees of risk on it.

Offline jpo234

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Landing on Mars has been done before.

There was never a propulsive landing without chutes. The  landing vehicle always disintegrated during EDL, only the payload survived.

No-one has EVER launched from Mars.

Things that make a Mars launch easier compared to Earth: No athmosphere and wheather to speek of, shallower gravity well.

Earth descent, also been done before.

Once again: Never a propulsive landing of a complete spacecraft from interplanetary speeds.

I'd say it was all very difficult. I wouldn't like to put degrees of risk on it.

I think we can all agree to this.
« Last Edit: 07/04/2016 04:30 pm by jpo234 »
You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Online JamesH65

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Landing on Mars has been done before.

There was never a propulsive landing without chutes. The  landing vehicle always disintegrated during EDL, only the payload survived.

No-one has EVER launched from Mars.

Things that make a Mars launch easier compared to Earth: No athmosphere and wheather to speek of, shallower gravity well.

Earth descent, also been done before.

Once again: Never a propulsive landing of a complete spacecraft from interplanetary speeds.

I'd say it was all very difficult. I wouldn't like to put degrees of risk on it.

I think we can all agree to this.

Do we know that RD won't be using drogues or chutes of any kind?

My point was though, that as a race, we have experience of landing things on planets (from interplanetary speeds), but nothing on launching.

And is the weather on Mars really that benign? Gravity well is a given of course.

Offline guckyfan

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Do we know that RD won't be using drogues or chutes of any kind?

Yes. With the weight of Dragon it will still be much too fast to use parachutes when it needs to brake for landing.

My point was though, that as a race, we have experience of landing things on planets (from interplanetary speeds), but nothing on launching.

We have plenty of experience on launching. From a very difficult place to launch, the earth. And some on the moon. We need to do some test on relaunch without prior refurbishment. That too will be done on earth with relaunching second stages. Much harsher environment than Mars.

And is the weather on Mars really that benign? Gravity well is a given of course.

Yes, strength of wind is much less. As far as we know, no landing on Mars failed due to weather.

Offline jpo234

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Do we know that RD won't be using drogues or chutes of any kind?

Why RD? I thought this is about the BFS part of the MCT...
And yes, RD will be all about propulsive landing.

My point was though, that as a race, we have experience of landing things on planets (from interplanetary speeds), but nothing on launching.

We know how to launch from the third planet, aka Earth and its moon.

And is the weather on Mars really that benign? Gravity well is a given of course.

Martian athmospheric pressure on the ground is 0.6% of the pressure at sea level on earth. On earth this would count as a low quality vacuum.
You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Offline Ludus

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I'd think the ratio would be something like 1 BFR: 1 Passenger MCT: 2-4 Tanker BFS: 10-20 Cargo MCTs.

I think it really depends what we are looking at: early missions? fully established Mars colonial transport architecture? Obviously, in the long run you want plenty of BFSs (if you want to move 50'000 people to Mars per year, there is no going around it!), but Robobeat was going after the minimum number absolutely necessary to complete a human mission to Mars - and the minimum number is exactly what we can expect for the very first missions.

For these very first missions, you clearly don't need "10-20 Cargo MCTs". You can transport all the cargo you need for a scout class mission in a single BFS - just tune down the number of crew members to 4-10 or so, and use the rest of the capacity for cargo. Also, I don't think you need so many tankers if you only have one BFR to launch them anyway. I doubt that the "turn-around" time of the BFS tankers is going to be much longer than the BFR.

The absolute minimum is: 1 BFR, 1 tanker BFS, 1 BFS. And thats what we are likely going to see for 2022/2024 (or whenever the first mission happens).

So I agree Robotbeat is right about the minimum I just think that 10:1 ratio is a given and may even be larger in the early stages. If you are sending 1000 passengers in 10 MCTs you need to send 100 Cargo only ships. If you are sending only 1 MCT it's got to be mostly devoted to cargo with only a few passengers.

The turn around of a BFR can potentially be much faster than a tanker. After all it's back on the pad 15 minutes later. A tanker has to go into orbit and maneuver to dock with it's target and then transfer propellant, then maneuver into a proper orbit for reentry and return to the pad. This likely takes days. So a single BFR can optimally handle the launch of several Tankers in a continuous operation. In practice the difference between 15 minutes and several days cycle time won't matter until the system is running very smoothly. I was just allowing for that optimum use. A single BFR can potentially handle several tankers and dozens of BFSs of different sorts.

Whether SpaceX builds a minimum would depend on what other uses the system has by then. Despite being focused on Mars I think they happen to also be building out a general purpose space transport system that will end up having a lot of other profitable uses.
« Last Edit: 07/04/2016 09:34 pm by Ludus »

Offline Ludus

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Also worth noting that if they go modular, a passenger module is just another kind of cargo module/container (albeit with very special requirements for power, comms, etc)  This was proposed for Shuttle, back in the day...

This makes sense and would eliminate the special class of passenger MCTs completely. Then there would only be tankers and Cargo, where the cargo module was sometimes a self contained passenger Hab.

There would be no issues with LAS designs as long as passengers don't go up or down from orbit in the Hab.

Tankers just seem so simple and efficient as pure second stages with without separate payload and they'd be launched so frequently compared to other variants that the specialization makes sense eventually. To start I'd think propellant tanks for transfers would just be another kind of cargo module.
« Last Edit: 07/04/2016 09:23 pm by Ludus »

Offline TomH

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So landing on Mars is harder than taking off? Really? Landing on Mars has been done before. No-one has EVER launched from Mars. Earth descent, also been done before.

Yes, far, far more difficult. Whether something has been done before does not necessarily have a direct correlation to how difficult it is or isn't. A human has never wiggled a pinkie finger on Mars before. That does not mean it will be difficult to do once a human is there. A vehicle departing Mars will be out of the atmosphere quickly with an exceptionally low Max Q as well as low gravity losses. OTOH, EDL on Mars is difficult in that the atmosphere is just dense enough to burn up a craft with no TPS, yet parachutes alone cannot land a craft; retropulsive landing is necessary. If RD comes in too high, it will skip off the atmosphere. If it comes in too perpendicular, it will hit like a big bug on your windshield. It has to dive deeply enough to create atmospheric drag, with an almost blunt angle of attack and literally fly parallel to the ground. As drag induces negative ΔV, the angle of attack must be constantly and precisely changed such that lift always equals the acceleration of G. As the vehicle decelerates, the angle of attack is changed by shifting the CG via a precise movement of mass on the interior of the vehicle.

With all due respect, those who are familiar with Mars EDL know that it is far more difficult than Mars departure. In fact, Mars EDL is one of the hardest things there is in space exploration. As a teacher and a scientist though, I always encourage others not to simply take the word of one person, whether it is me or someone else. Go do in depth research and discover it for yourself.
« Last Edit: 07/05/2016 12:26 am by TomH »

Online JamesH65

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Do we know that RD won't be using drogues or chutes of any kind?

Why RD? I thought this is about the BFS part of the MCT...
And yes, RD will be all about propulsive landing.

My point was though, that as a race, we have experience of landing things on planets (from interplanetary speeds), but nothing on launching.

We know how to launch from the third planet, aka Earth and its moon.

And is the weather on Mars really that benign? Gravity well is a given of course.

Martian athmospheric pressure on the ground is 0.6% of the pressure at sea level on earth. On earth this would count as a low quality vacuum.

Sorry, with my statement about launching, I was clearly talking about off Earth, and aside from the Moon landing launches, there is little to go on. The launches are completely unattended for one, which has never happened before, even on Earth. No chance to pop over to the rocket to attach anything! A considerable number of challenges to overcome, but I am very optimistic. Lots of smart people working on it.

Offline Robotbeat

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Actually, there have been unattended rocket launches from a planetary body. The Soviet lunar sample return missions.
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Offline jpo234

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Sorry, with my statement about launching, I was clearly talking about off Earth, and aside from the Moon landing launches, there is little to go on. The launches are completely unattended for one, which has never happened before, even on Earth. No chance to pop over to the rocket to attach anything! A considerable number of challenges to overcome, but I am very optimistic. Lots of smart people working on it.

I think we are in violent agreement now. In Reply #41 I questioned an unmanned round trip test of the BFS because of the very issues you are pointing out.

Always under the assumption that everything goes right, I think 2022 will see an unmanned BFS test that will return only after the manned landing in 2025. And that's why initially there will be at least 3 BFS:
1 unmanned BFS for 2022, 1 manned BFS for 2024/25 and one tanker to LEO for both missions.

The 2022 BFS can return once the crew arrives in 2025 and unloads its cargo and refuels it.

Edit: The 2022 BFS will have to generate the fuel for the return flight after the landing, so that the fuel is ready in 2025.
« Last Edit: 07/05/2016 11:35 am by jpo234 »
You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Offline Robotbeat

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These are all things that anyone doing a sample return mission has to solve. And they are solvable. They have even been done before.
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Offline jpo234

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These are all things that anyone doing a sample return mission has to solve. And they are solvable. They have even been done before.

I'm not sure what "all things" are, but autonomously refueling a spacecraft from an autonomously deployed ISRU plant and then autonomously launching it has definitely never been done before.
You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Online JamesH65

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Actually, there have been unattended rocket launches from a planetary body. The Soviet lunar sample return missions.

Thanks, wasn't aware of that. Need to look it up now!

Offline cuddihy

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Offline whitelancer64

Luna 20 https://en.m.wikipedia.org/wiki/Luna_20

Also Luna 16 and Luna 24 were successful sample return missions. A total of 300 grams of lunar regolith sent to Earth.
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