Author Topic: MCT Speculation and Discussion Thread 4  (Read 503668 times)

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2080 on: 05/28/2016 02:48 PM »
We were discussing the hover slam landing, which is essentially at burnout since only marginal fuel is remaining.

That's also open throttle on all engines. I'm sure they would be throttled to within crew acceptable limits for landing.

Liftoff accelerations would probably net less than 1/3g with a full fuel load.

Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #2081 on: 05/28/2016 04:47 PM »
Humans can handle 8-9 gees just fine if oriented the correct way.

Not for very long, though.  NASA limited sustained gees on Apollo flights to 4.5 g and Shuttle flights to 3 g for very good physiological reasons.  Spacex should be aiming for a similar sustained g load for manned launches and reentries for the same reasons.

Sure, for long term/sustained... but ~4 g for a hoverslam landing just isn't that big of a deal at all.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2082 on: 05/28/2016 05:08 PM »
Humans can handle 8-9 gees just fine if oriented the correct way.

Not for very long, though.  NASA limited sustained gees on Apollo flights to 4.5 g and Shuttle flights to 3 g for very good physiological reasons.  Spacex should be aiming for a similar sustained g load for manned launches and reentries for the same reasons.

Sure, for long term/sustained... but ~4 g for a hoverslam landing just isn't that big of a deal at all.
8gees would be fine, too. But I suspect if they limit it, they'll do it for structural reasons, i.e. they could have a lighter structure if they limited hoverslam acceleration to, say, 5 gees vs 8 gees.

Humans can withstand WAY higher limits than we often assume, and I think the weak point may actually be the non-human parts.

For Shuttle, if I recall correctly, the seats were NOT oriented in the most optimum position for withstanding gee-loads. They needed a good orientation for gliding the orbiter back, so they couldn't be lying on their back like for capsules.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2083 on: 05/28/2016 08:42 PM »
I'm still looking for a clarification by envy887, is this a calculation of the vehicles acceleration rate during a hover-slam landing?  I had though he meant a launch but as a landing g force that makes a lot more sense. 

Still we need to remember that the crew/cargo inside the vehicle gets an additional local g worth of force upon them (sanity check, if the rocket were hovering we would feel 1 g, not weightlessness).  So that means a force of 5.5 g at Earth and around 4.1 g on Mars.  In practice I'm sure you would see throttling of engines and a gentler deceleration in the 3 g neighborhood.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #2084 on: 05/28/2016 10:40 PM »
I'm still looking for a clarification by envy887, is this a calculation of the vehicles acceleration rate during a hover-slam landing?  I had though he meant a launch but as a landing g force that makes a lot more sense. 

Still we need to remember that the crew/cargo inside the vehicle gets an additional local g worth of force upon them (sanity check, if the rocket were hovering we would feel 1 g, not weightlessness).  So that means a force of 5.5 g at Earth and around 4.1 g on Mars.  In practice I'm sure you would see throttling of engines and a gentler deceleration in the 3 g neighborhood.

Not sure how you are calculating that. Everything else about the vehicle in question being equal (particularly its mass and engine thrust) an occupant of that vehicle will experience the same feeling of acceleration at full thrust whether they are taking off from Earth, going for TMI from LEO, Landing on Mars or taking off from Mars. As was pointed out above as the propellant mass drops off as you are burning it, then acceleration increases. Also has been discussed if your vehicle has the same thrust and mass taking off from Earth or Mars the gravity loss would be much less from Mars. A craft that could take off with a TMR (thrust to mass ratio) of 1.3 would accelerate at .3g initially as it took off from Earth and at .92g from the surface of Mars. The gravity loss at the start of the launch would go from 77% on Earth to 23% on Mars. Now once you start pitching over from the vertical, and also as propellant load drops off, your real acceleration relative to the point you started from increases and the acceleration the occupants feel moves up at the same curve (presuming the same thrust, ISP and vehicle mass) on either location. Throttling would most likely happen on the MCT though, and also dropping the number of engines firing.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2085 on: 05/28/2016 11:10 PM »
I'm still looking for a clarification by envy887
....
See my reply above. I was figuring a hover-slam at burnout mass, not a launch.

Not sure how you are calculating that.
....
He added local gravity to my calculations above, which were based on my rough estimates of MCT dry mass, engine count, and engine thrust. That is indeed the perceived acceleration during landing.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #2086 on: 05/28/2016 11:15 PM »
Not sure how you are calculating that.
....
He added local gravity to my calculations above, which were based on my rough estimates of MCT dry mass, engine count, and engine thrust. That is indeed the perceived acceleration during landing.

Different payloads? if the mass and thrust are the same then perceived acceleration will be the same.  If the craft is nearly empty (of propellant) has the same payload and can thrust at 4g in free fall, then in a hover slam the passengers experience 4g on Mars or Earth but on Mars they are decelerating at 3.62g and on Earth at 3g.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #2087 on: 05/28/2016 11:31 PM »
What do you mean 3.8g and 4.5g, thouse are incredible acceleration rate and likely beyond what crew could tolerate,

I don't think so... 4 g isn't that big of a deal with correct positioning. The Shuttle was limited to 3, but Apollo astronauts took way more g's than that.

Humans can handle 8-9 gees just fine if oriented the correct way.


Not for very long, though.  NASA limited sustained gees on Apollo flights to 4.5 g and Shuttle flights to 3 g for very good physiological reasons.  Spacex should be aiming for a similar sustained g load for manned launches and reentries for the same reasons.

Mercury-Atlas astronauts withstood 11g. But they were test pilots made of The Right Stuff.
« Last Edit: 05/28/2016 11:35 PM by TomH »

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2088 on: 05/28/2016 11:34 PM »
Not sure how you are calculating that.
....
He added local gravity to my calculations above, which were based on my rough estimates of MCT dry mass, engine count, and engine thrust. That is indeed the perceived acceleration during landing.

Different payloads? if the mass and thrust are the same then perceived acceleration will be the same.  If the craft is nearly empty (of propellant) has the same payload and can thrust at 4g in free fall, then in a hover slam the passengers experience 4g on Mars or Earth but on Mars they are decelerating at 3.62g and on Earth at 3g.
Yes. 100t to Mars, 25t to Earth.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2089 on: 05/29/2016 06:29 AM »
Not sure how you are calculating that.
....
He added local gravity to my calculations above, which were based on my rough estimates of MCT dry mass, engine count, and engine thrust. That is indeed the perceived acceleration during landing.

Different payloads? if the mass and thrust are the same then perceived acceleration will be the same.  If the craft is nearly empty (of propellant) has the same payload and can thrust at 4g in free fall, then in a hover slam the passengers experience 4g on Mars or Earth but on Mars they are decelerating at 3.62g and on Earth at 3g.
Yes. 100t to Mars, 25t to Earth.

nadreck your forgetting that the passengers are not part of the vehicle, the vehicle acceleration upward towards the occupants but the occupants are still affected by gravity and are accelerated downward into the vehicle too, the sum of the forces are what they feel.

Consider a vehicle putting out just enough thrust to hover, inside you would feel the local gravity field acceleration, any acceleration of the vehicle is going to be added to that, so if the vehicle accelerates upward at 1 g your going to feel 2 g inside.

Formula for how quickly the whole vehicle accelerates upward:  (Thrust - (Gravity * Mass) ) / Mass

Formula for the force felt inside the vehicle by passengers:  (((Thrust - (Gravity * Mass) ) / Mass ) + Gravity
which can be shortened by cancellation to just Thrust / Mass

Thus the passengers would experience a force of 4.2g when landing on Mars due to the expected landing mass of 200 mt, on Earth the landing mass is expected to be a lower 125 mt resulting in a higher 6.7g force, but this is due ONLY to the lower mass and has nothing to do with differences in gravity fields. 

And again I reiterate I expect throttle-down of rockets to give a gentler ~3g landing at all locations.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2090 on: 05/29/2016 08:03 AM »

Impaler, the vehicle is also fighting against gravity, it's burning 1g just to stay up.

Hence, a rocket capable of accelerating at say 3g's will, using the same amount of thrust, accelerate linearly at 2g's relative to the ground while under a 1g gravitational acceleration in the opposite direction. The rocket (and passengers) will experience the same 3g's as they would in open space, even though the rocket is accelerating upwards linearly at 2g's from Earth.

A rocket capable of accelerating at 2g's will, using the same amount of thrust, linearly accelerate at 1g relative to the ground. The rocket and passengers will experience the same 2g's as they would in space, even though the rocket is rising at just 1g from Earth.

A rocket capable of accelerating at only 1g will not linearly accelerate relative to the ground. It will hover, making no net linear acceleration, and yet the rocket and the passengers experience the same 1g as they would in space.

And a rocket with its engines off will fall at 1g towards the ground (ignoring air resistance.) Yet the rocket and passengers will experience zero-g, just as if the rocket was drifting in space with its engines off, even though they are actually linearly accelerating at 1g relative to the ground. (Until they hit the ground.)

If you calculate that a rocket is capable of accelerating at 3.5g's in open space, then it will accelerate linearly at 2.5g's on Earth, and roughly 3.1g's on Mars. The rocket and crew will experience 3.5g's.

The rocket and crew will not experience 4.5g's (3.5+1) on Earth, unless the rocket increases its thrust to 4.5g's.
« Last Edit: 05/29/2016 08:07 AM by Paul451 »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2091 on: 05/29/2016 09:00 AM »
Paul re-read what your replying too, your simply reiterating my argument and producing the same result. 

envy887's original method was incorrect and I demonstrated why his subtraction of local gravity was wrong for g-force upon crew and he was instead calculating vehicle acceleration relative to the ground in his original post.

You seem to have misinterpreted my reply as saying that crew g-forces exceed what would be felt by firing the vehicle in open space, that is not the case as I stated the force is a simple Thrust / Mass relationship. 

The correct g-forces felt in envy887's hypothetical rocket are different from his original numbers due to correction, the values of 4.2g and 6.7g with 100 and 25 mt cargo loads respectively are the rockets acceleration in open-space given the stated thrust and vehicle mass.
« Last Edit: 05/29/2016 04:32 PM by Impaler »

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #2092 on: 05/29/2016 04:03 PM »

nadreck your forgetting that the passengers are not part of the vehicle, the vehicle acceleration upward towards the occupants but the occupants are still affected by gravity and are accelerated downward into the vehicle too, the sum of the forces are what they feel.

No we are in violent agreement here, it has nothing to do with the force of gravity, you cancel it out in your own equation. It was difference in payload that gave different results which was what I was looking for and had missed in the discussion. 

Gravity only enters into the calculation of what real acceleration your vehicle has relative to a point, but as long as the  vehicle is not on the ground it makes no difference what angle the craft is pointing at. Whatever the angle, if the engines are on the occupants feel an acceleration force equal to thrust/mass(including their own mass) if the engines are off they are in free fall. 
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2093 on: 05/31/2016 07:18 AM »
To better understand the MCT (specifically the portion that lands on Mars often called the BFS) we should examine an Earth analog aircraft with a comparable payload.  The best analog is the 777F, the freight version of the Boeing 777.

http://www.boeing.com/commercial/aeromagazine/articles/qtr_02_09/article_02_1.html

http://www.boeing.com/assets/pdf/commercial/airports/acaps/777_2lr3er.pdf

The cargo capacity is 102 mt and cargo volume comes out to 635 m^3 even in the containers which means the interior volume of the vehicle is slightly higher due to square-ish containers not fully filling the round fuselage.  This yield a density of just 160 g/L, most people don't realize how low density most air cargo is and space cargo is likely to be comparably low density requiring high volume to utilize.




This gives us a good idea what kind of physical volume and containerization system is going to be needed for any kind of efficient logistics.  Now consider total vehicle cost, Musk expects to ultimately build rockets for about the same cost as jets and this one costs around $300 million, we can expect the Mars landing vehicle to have a per unit cost around that once produced in volume.  The mass is 144 mt empty which is a bit more then most are expecting for the BFS, if vehicle cost is directly proportional to dry weight then prices of $200 million would be consistent with at 100 mt dry mass, note that the per unit cost to make the Shuttle orbiter was around $200 million though that was decades ago with lots of inflation between then and now, it was certainly not the cost of a comparable jet aircraft of the day so this would be consistent with a considerable decline in price if a space vehicle matches a commercial aircraft in cost per pound.

« Last Edit: 05/31/2016 07:27 AM by Impaler »

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2094 on: 05/31/2016 12:53 PM »
on Earth the landing mass is expected to be a lower 125 mt resulting in a higher 6.7g force, but this is due ONLY to the lower mass and has nothing to do with differences in gravity fields. 

And again I reiterate I expect throttle-down of rockets to give a gentler ~3g landing at all locations.

6.7g would be the vacuum burnout acceleration with 25t payload given my estimated vehicle parameters, but at Earth landing the ambient sea level pressure will reduce thrust. I estimated 230t SL and 280t vacuum thrusts, so unthrottled perceived acceleration would only be ~5.5g and local acceleration (relative to Earth's surface) would be ~4.5g assuming thrust and gravity are antiparallel.

I agree it would likely be throttled, not only to keep passengers happy, but to give more control authority.

Online Toast

Re: MCT Speculation and Discussion Thread 4
« Reply #2095 on: 05/31/2016 02:55 PM »
Humans can handle 8-9 gees just fine if oriented the correct way.

Not for very long, though.  NASA limited sustained gees on Apollo flights to 4.5 g and Shuttle flights to 3 g for very good physiological reasons.  Spacex should be aiming for a similar sustained g load for manned launches and reentries for the same reasons.

Sure, for long term/sustained... but ~4 g for a hoverslam landing just isn't that big of a deal at all.

To add to this, Soyuz capsules experience double that acceleration when landing, so it really isn't a problem so long as it's limited in duration.


Offline docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2096 on: 06/02/2016 04:59 AM »
@CNBCNow
SpaceX's Elon Musk says could launch flight to Mars with people aboard in 2024 and they would arrive in 2025. #CodeCon
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Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 4
« Reply #2097 on: 06/02/2016 03:02 PM »
@CNBCNow
SpaceX's Elon Musk says could launch flight to Mars with people aboard in 2024 and they would arrive in 2025. #CodeCon

Interesting. That particular deadline hasn't shifted for years - if anything, it's been progressively elaborated upon.
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Offline Bynaus

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Re: MCT Speculation and Discussion Thread 4
« Reply #2098 on: 06/03/2016 12:03 PM »
After Elon's Mars schedule announcement, I was wondering about this (I don't know if this has been discussed before, I presume so but perhaps not in this specific context) :

A way to speed up the development of the first manned Mars mission (to keep up with the aggressive 2024/5 schedule) would be to bring molecular H to the Martian surface (which could also serve as a radiation shield on the way there). If the BFS must have a dV capacity of 6 km/s and has a dry mass of 200 tons, it will need about 800 tons of fuel for the return journey. Since only about 5% of a stochiometric Methane/LOX fuel is hydrogen, they could bring down about 40 tons of molecular H with each BFS, well within its payload capacity (of 100 tons). Even if we add a few percent of that for the mass of the tank, this is still very feasible.

All that the first SpaceX astronauts would then have to do after landing is react the H with the atmospheric CO2 in the Sabatier reaction - this could virtually been done without any human supervision or involvement. This is also a much simpler and cleaner (and less energy intensive) process than having the crew looking for water ice, digging it out, cleaning it, testing it for any potential chemical agents (e.g., peroxides, salts, etc.) and removing them before electrolysis, etc. This is also a technology that 1) is already in development and 2) would be reasonably easy to demonstrate with a precursor Red Dragon mission. The same cannot be said of actively digging for water ice on Mars.

While I am certain the plan is for a Mars base to eventually provide BFS's with fuel synthesized from martian water ice and atmospheric CO2, the very first landings could / should skip this step and bring the H from Earth instead.

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2099 on: 06/03/2016 01:12 PM »
...
 Since only about 5% of a stochiometric Methane/LOX fuel is hydrogen, they could bring down about 40 tons of molecular H with each BFS, well within its payload capacity (of 100 tons). Even if we add a few percent of that for the mass of the tank, this is still very feasible.
...
40 tonnes of LH2 needs a 575 m^3 tank kept below 30K... the tankage and cooling are going to weigh a lot more than a few percent.

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