Author Topic: MCT Speculation and Discussion Thread 4  (Read 505399 times)

Offline Eerie

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Re: MCT Speculation and Discussion Thread 4
« Reply #2640 on: 08/26/2016 02:57 PM »
I have a totally crazy idea, feel free to shoot it down.

The idea is a Tri-propellant (but not really) MCT.

Assuming that:
1. Methalox engines are required for utilizing Mars-side ISRU.
2. Most payload mass will only go one way: Earth to Mars.

How much sense would it make to use Hydrolox engines for the Mars-bound half of the trip?
Basically, you use highly efficient Hydrolox engines to get to Mars, use ISRU there to fill the hydrogen tank with methane, and use Raptors to go back home (and I know, the tank volume ratios are problematic).

You may say, but SpaceX doesn't have Hydrolox engines. True, but Blue Origin does. One good enough to land, too. :-)
« Last Edit: 08/26/2016 02:58 PM by Eerie »

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2641 on: 08/26/2016 02:58 PM »
I have a totally crazy idea, feel free to shoot it down.

The idea is a Tri-propellant (but not really) MCT.

Assuming that:
1. Methalox engines are required for Mars-side ISRU.
2. Most payload mass will only go one way: Earth to Mars.

How much sense would it make to use Hydrolox engines for the Mars-bound half of the trip?
Basically, you use highly efficient Hydrolox engines to get to Mars, use ISRU there to fill the hydrogen tank with methane, and use Raptors to go back home (and I know, the tank volume ratios are problematic).

You may say, but SpaceX doesn't have Hydrolox engines. True, but Blue Origin does. :-)
You'd actually get much more performance by filling the tanks with methane than with hydrogen.

Hydrogen has terrible density, so you could fit a LOT more methane in there than hydrogen.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Eerie

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Re: MCT Speculation and Discussion Thread 4
« Reply #2642 on: 08/26/2016 03:16 PM »
You'd actually get much more performance by filling the tanks with methane than with hydrogen.

Hydrogen has terrible density, so you could fit a LOT more methane in there than hydrogen.

This doesn't make sense. There is a reason Saturn 5, STS, SLS and the Centaur all use hydrolox...

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2643 on: 08/26/2016 03:18 PM »
Hydrogen already has a lot of negative design trades, and if you add in the requirement that the system also has to burn methane equally well, there's no good reason to use hydrogen at all.

And the tank volume ratios are more than slightly problematic. The hydrogen tank to put 180 tonnes through TMI and EDL would be more than 7 times larger in volume than the methane tank needed to return a 80 tonne vehicle from Mars surface to Earth's surface. It would be larger in volume than most estimates for the entire BFR booster.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2644 on: 08/26/2016 03:21 PM »
You'd actually get much more performance by filling the tanks with methane than with hydrogen.

Hydrogen has terrible density, so you could fit a LOT more methane in there than hydrogen.

This doesn't make sense. There is a reason Saturn 5, STS, SLS and the Centaur all use hydrolox...

The main reason is that obscene costs weren't a deal-breaker for those programs.

Offline Eerie

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Re: MCT Speculation and Discussion Thread 4
« Reply #2645 on: 08/26/2016 03:24 PM »
Hydrogen already has a lot of negative design trades, and if you add in the requirement that the system also has to burn methane equally well, there's no good reason to use hydrogen at all.

And the tank volume ratios are more than slightly problematic. The hydrogen tank to put 180 tonnes through TMI and EDL would be more than 7 times larger in volume than the methane tank needed to return a 80 tonne vehicle from Mars surface to Earth's surface. It would be larger in volume than most estimates for the entire BFR booster.

...on the positive side, a huge MCT means a great ballistic coefficient for Mars aerobraking/landing. :-)

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2646 on: 08/26/2016 03:38 PM »
You'd actually get much more performance by filling the tanks with methane than with hydrogen.

Hydrogen has terrible density, so you could fit a LOT more methane in there than hydrogen.

This doesn't make sense. There is a reason Saturn 5, STS, SLS and the Centaur all use hydrolox...
And if you switched those stages over to methane with the same total tank volume, the stages would weigh more, but they'd give you a much higher performance. More payload to higher delta-v, GIVEN the same starting point.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Online IntoTheVoid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2647 on: 08/26/2016 03:40 PM »
Assuming that:
1. Methalox engines are required for utilizing Mars-side ISRU.
2. Most payload mass will only go one way: Earth to Mars.
False assumption.
Mars ISRU is not capturing existing methane as we do on earth. Mars ISRU envisions building the methane from water derived H2 and atmospheric CO2. If the system had H2 engines and tanks you'd refill them with H2 and skip the methane altogether.

In other words, if the trades worked in hydrogen's favor, there's nothing preventing them from building a hydrogen rocket without the methane. Since they have chosen to build a methane rocket, one might presume that the trades did not work in hydrogen's favor. Quite possibly for some of the reasons mentioned above.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2648 on: 08/26/2016 03:42 PM »
Assuming that:
1. Methalox engines are required for utilizing Mars-side ISRU.
2. Most payload mass will only go one way: Earth to Mars.
False assumption.
Mars ISRU is not capturing existing methane as we do on earth. Mars ISRU envisions building the methane from water derived H2 and atmospheric CO2. If the system had H2 engines and tanks you'd refill them with H2 and skip the methane altogether.

In other words, if the trades worked in hydrogen's favor, there's nothing preventing them from building a hydrogen rocket without the methane. Since they have chosen to build a methane rocket, one might presume that the trades did not work in hydrogen's favor. Quite possibly for some of the reasons mentioned above.

Good point, however remember that hydrogen is terrible for storing on Mars. And for the same mass of propellant, if you used hydrogen on Mars, it'd require much more water to be mined than if you used methane.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #2649 on: 08/26/2016 05:28 PM »
You'd actually get much more performance by filling the tanks with methane than with hydrogen.

Hydrogen has terrible density, so you could fit a LOT more methane in there than hydrogen.

This doesn't make sense. There is a reason Saturn 5, STS, SLS and the Centaur all use hydrolox...

It makes complete and perfect sense. The tanks are volume constrained. Hydrogen has high ISP but very low ISP Density. Given equal masses of H2 vs CH4, the H2 has more energy. However, you need around five or more times the volume to contain that Hydrogen. Given equal volumes of H2 and CH4, there is far more impulse energy packed into the methane. If you fly to Mars on H2, you need fuel tanks roughly 5 times the size that BFS currently has. Once you put CH4 into them on Mars, you only fill them 20% full for equal amount of needed energy. Now you have a whole bunch of wasted mass in the form of the oversized fuel tank.
« Last Edit: 08/26/2016 05:46 PM by TomH »

Offline Eerie

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Re: MCT Speculation and Discussion Thread 4
« Reply #2650 on: 08/26/2016 06:29 PM »
Thanks for demolishing my proposal. :-)

So, I am correct to understand that the increased mass of the tanks would eat all the performance gains from using Hydrolox?

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2651 on: 08/26/2016 06:43 PM »
Thanks for demolishing my proposal. :-)

So, I am correct to understand that the increased mass of the tanks would eat all the performance gains from using Hydrolox?
It depends. If your initial mass isn't a constraint, then for the same tanks, you're better off with methane.

If your initial mass is your greatest constraint (and you're able to vary tank size), then hydrogen provides better performance.

If neither are really constraints, and you're just asking what the burn-out mass of an empty stage would be using either technology (and a very small payload), then it's kind of a toss-up.

With ISRU on Mars, initial mass of the stage is not such a concern. Maybe ISRU power is the greatest constraint, in which case methane may be the better option (though this depends on how fast you intend to launch the ascent vehicle back to Earth).
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2652 on: 08/26/2016 07:27 PM »
Approaching 30 days from what (we hope) is the big reveal, I thought it a good time to revisit and post revised BFR/MCT speculation before any info leaks out.  Trying to stay within the parameters of what Musk has said as I best understand.  A TSTO vehicle launched by a re-useable, single core BFR that puts the BFS a.k.a. the MCT into LEO where it is re-fueled, travels to and lands on Mars where it is again refueled for the journey back to Earth carrying a quarter of the outbound “cargo” mass.  The outbound cargo masses 100 tonnes which I assume means either cargo or people or a combination thereof.  BFS/MCT mass not included in the 100T.

Myriad unknowns led by the dry mass of the BFS.  Rocket equation dictates various mass assumptions here can produce wildly different answers.

My predictions, metric unless otherwise stated:
1.   Entire launch vehicle BFR+BFS masses under 5,000T.  Guestimate ~4,500T.
2.   BFS dry mass < 100T, my pick is 85T carbon composites BUT heavier than some predictions because ruggedized to allow for minimal maintenance.
3.   BFR absolutely > 10m diameter to fit enough engines. Likely between 12.5 and 15m.  My guess 15m.  Allows addition of more engines in the future.
4.   My guestimate BFR+BFS stack <100m height.  Certainly <125m.
5.   Sticking with the “over 230T” Raptor thrust Elon mentioned, I get 25-27 engines.  My guestimate is 26 with “over 230T” as 235T in my spreadsheet.  Around 13.5 million Lbs force.
Engine # most likely wrong because…
6.   Predict that Raptor engine design goal thrust changed to higher than 230T previously stated, but only by several 10s of tonnes, not hundreds.
7.   BFS with 5 Rvac engines
8.   RTLS minimizes cost, turnaround time, effort.  Changed my opinion from max payload ASDS for those reasons.  Just make the BFR bigger. Stages low and slow ~2.2 Km/sec.  “Easy” recovery & re-flight vs F9 GTO flights.
9.   Initial BFR test flights likely equipped with less engines and less payload.
10.   Large crew volume design >2,000m3.  Initial flights with less people & people space but more cargo space.
11.   Initial crewed Mars mission will carry 6-12 people.  10 is my latest #. Why?
NASA & other nations will buy seats. 
http://forum.nasaspaceflight.com/index.php?topic=40683.msg1557261#msg1557261
12.   SEP still under development awaits later opposition cargo transits
13.   BFS will have “exotic” upper mounted engines for rough terrain Mars landing &takeoff (just echoing others’ analysis here)
14.   BFS will be a lifting body for EDL, but not a scaled up Dragon capsule shape.  It will look badass.

You know we’re totally screwed trying to predict Musk because he already warned us,
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

I’ve attached a spreadsheet showing different assumptions, BFS mass, etc.

Anyone else want to update their speculations?
« Last Edit: 08/26/2016 07:30 PM by philw1776 »
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2653 on: 08/26/2016 08:32 PM »
Lastly some estimates on mass, as noted earlier the leg system on F9 masses 2500 kg, carbon fiber fabric 5 layers thick totaling 1 kg/m^2 and covering the space between a 15 m base and a 51 m total diameter would be 1800 m and kg bringing total mass to 4.3 mt, a very modest amount that would be far lighter then carrying extra propellant.  The propellant needed for just the last bit of deceleration and up to landing is going to need to do around 800 m/s based on extrapolation from Red Dragon, and that's going to come out to around 20 percent propellant fraction minimum.  I'm estimating 200 mt at entry and 40 mt of landing propellant.

Reducing dry mass will have priority since the architecture is constrained by the return leg. The main benefit of something like HIAD would be its lower mass compared to a mid L/D aeroshell.

That is very shortsighted their are important trades to be made for each unit of dry mass, simply aiming for the lowest possible dry mass would give us an expendable centaur stage.  A decelerator is clearly going to save mass at Mars entry, I'd even argue that entry is impossible without it.  And given the entry speed that will be experienced at Earth return it will be useful their as well by allowing lower peak g-forces during entry and a lower terminal velocity in the lower atmosphere which means less propellant needed at landing and more safety margin.  If the decelerator can cut the deceleration needs of the terminal landing burn by just 150 m/s it's more efficient then it's mass in propellant.

Offline Oersted

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Re: MCT Speculation and Discussion Thread 4
« Reply #2654 on: 08/26/2016 11:10 PM »
Regarding decelerator devices...
Isn't it somewhat counter-intuitive to put them at the front of the vehicle? Wouldn't such a vehicle normally flip around to have the area of maximum resistance at the back?
I am thinking that any large-scale decelerators should be deployed at the rear of the vehicle, as a form of drogue chute. That would give a heatshield of vehicle diameter at the front and a large-diameter deployable decelerator trailing behind, to create a stable configuration. And no, I am not just talking about a parachute, because I think of a much more robust device, be it with fins or petals or whatever the aerodynamicists can come up with.

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2655 on: 08/27/2016 12:41 AM »
Regarding decelerator devices...
Isn't it somewhat counter-intuitive to put them at the front of the vehicle? Wouldn't such a vehicle normally flip around to have the area of maximum resistance at the back?
I am thinking that any large-scale decelerators should be deployed at the rear of the vehicle, as a form of drogue chute. That would give a heatshield of vehicle diameter at the front and a large-diameter deployable decelerator trailing behind, to create a stable configuration. And no, I am not just talking about a parachute, because I think of a much more robust device, be it with fins or petals or whatever the aerodynamicists can come up with.

The decelerator designs are generally inherently stable in either direction once they have commenced their descent into the atmosphere. The hypersonic heating rate is proportional to velocity cubed, and to the inverse of the square root of the atmospheric density divided by the leading edge radius.

HeatingRate = 1.83e-4 * Math.Pow(speed, 3) * Math.Sqrt(atmosphericDensity / HeatingRadius());

So, the larger the leading edge radius (i.e. the blunter the body), the lower the heating rate. Although perhaps counter-intuitive, this is why the decelerator is best positioned at the front of the vehicle. I've attached a few design alternatives below.

Offline CyclerPilot

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Re: MCT Speculation and Discussion Thread 4
« Reply #2656 on: 08/27/2016 01:23 AM »


Approaching 30 days from what (we hope) is the big reveal, I thought it a good time to revisit and post revised BFR/MCT speculation before any info leaks out.  Trying to stay within the parameters of what Musk has said as I best understand.  A TSTO vehicle launched by a re-useable, single core BFR that puts the BFS a.k.a. the MCT into LEO where it is re-fueled, travels to and lands on Mars where it is again refueled for the journey back to Earth carrying a quarter of the outbound “cargo” mass.  The outbound cargo masses 100 tonnes which I assume means either cargo or people or a combination thereof.  BFS/MCT mass not included in the 100T.

Myriad unknowns led by the dry mass of the BFS.  Rocket equation dictates various mass assumptions here can produce wildly different answers.

My predictions, metric unless otherwise stated:
1.Entire launch vehicle BFR+BFS masses under 5,000T.  Guestimate ~4,500T.
2.BFS dry mass < 100T, my pick is 85T carbon composites BUT heavier than some predictions because ruggedized to allow for minimal maintenance.
3.BFR absolutely > 10m diameter to fit enough engines. Likely between 12.5 and 15m.  My guess 15m.  Allows addition of more engines in the future.
4.My guestimate BFR+BFS stack <100m height.  Certainly <125m.
5.Sticking with the “over 230T” Raptor thrust Elon mentioned, I get 25-27 engines.  My guestimate is 26 with “over 230T” as 235T in my spreadsheet.  Around 13.5 million Lbs force.
Engine # most likely wrong because…
6.Predict that Raptor engine design goal thrust changed to higher than 230T previously stated, but only by several 10s of tonnes, not hundreds.
7.BFS with 5 Rvac engines
8.RTLS minimizes cost, turnaround time, effort.  Changed my opinion from max payload ASDS for those reasons.  Just make the BFR bigger. Stages low and slow ~2.2 Km/sec.  “Easy” recovery & re-flight vs F9 GTO flights.
9.Initial BFR test flights likely equipped with less engines and less payload.
10.Large crew volume design >2,000m3.  Initial flights with less people & people space but more cargo space.
11.Initial crewed Mars mission will carry 6-12 people.  10 is my latest #. Why?
NASA & other nations will buy seats. 
http://forum.nasaspaceflight.com/index.php?topic=40683.msg1557261#msg1557261
12.SEP still under development awaits later opposition cargo transits
13.BFS will have “exotic” upper mounted engines for rough terrain Mars landing &takeoff (just echoing others’ analysis here)
14.BFS will be a lifting body for EDL, but not a scaled up Dragon capsule shape.  It will look badass.

You know we’re totally screwed trying to predict Musk because he already warned us,
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

I’ve attached a spreadsheet showing different assumptions, BFS mass, etc.

Anyone else want to update their speculations?

Wow.   Time flies.  September is almost here.  Great list.  I agree with most of what you have but had a few differences.

I think the dry mass needs to be 70k or less.  This mass has to be accelerated so many times, there is immense financial and archetecture-feasibility pressure to trim dry mass. The return leg is a killer and the return cargo mass quickly falls to zero for some synodes if the dry mass is too high.

For BFS engines, I think 4 or 5 is a good number.  4 around the perimeter could allow for any one engine to be out and maintain a symmetric thrust.  With 5, having one central engine helps obviously,  but I think adding a 5th eats up dry mass budget.  Having a 5th raptor helps with gravity losses at earth launch, but is mostly dead weight the rest of the trip.

I think if the raptor was slightly less powerful, 5 would be perfect.  I think the raptor will actually start with lower thrust than has been publicly shared.  They will increase chamber pressure on later iterations to increase thrust and isp.

As for exotic upper mounted engines, I used to think they were required, but now I think terminal landing will be done on raptors alone (again to save dry mass).  For this to be possible from the first BFS landing will require scouting an appropriate flat solid rock surface and a dragon placed rover to sweep away any rocks/pebbles and place beacons to allow for guidance to the landing pad within a few meters.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2657 on: 08/27/2016 02:28 AM »
Regarding decelerator devices...
Isn't it somewhat counter-intuitive to put them at the front of the vehicle? Wouldn't such a vehicle normally flip around to have the area of maximum resistance at the back?
I am thinking that any large-scale decelerators should be deployed at the rear of the vehicle, as a form of drogue chute. That would give a heatshield of vehicle diameter at the front and a large-diameter deployable decelerator trailing behind, to create a stable configuration. And no, I am not just talking about a parachute, because I think of a much more robust device, be it with fins or petals or whatever the aerodynamicists can come up with.
That's an option, but would have lower drag and wouldn't protect the rest of the vehicle.
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To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2658 on: 08/27/2016 03:20 AM »
As for exotic upper mounted engines, I used to think they were required, but now I think terminal landing will be done on raptors alone (again to save dry mass).  For this to be possible from the first BFS landing will require scouting an appropriate flat solid rock surface and a dragon placed rover to sweep away any rocks/pebbles and place beacons to allow for guidance to the landing pad within a few meters.

Whilst I agree that Raptors alone would save dry mass, as well as removing the dependency on two successfully serialised miracles, if a Red Dragon could safely land on an unprepared Martian surface, why couldn't a BFS, especially if it resembles a scaled up Dragon?

Offline Chris_Pi

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Re: MCT Speculation and Discussion Thread 4
« Reply #2659 on: 08/27/2016 03:57 AM »
Whilst I agree that Raptors alone would save dry mass, as well as removing the dependency on two successfully serialised miracles, if a Red Dragon could safely land on an unprepared Martian surface, why couldn't a BFS, especially if it resembles a scaled up Dragon?

A Red Dragon's heat shield and motors don't need to survive past touchdown. Safe for it is no destructive motor failures. Gouging up the heatshield and throwing rocks up into motors is fine as long as it can land once. A vehicle that has to take off again and do another re-entry needs an intact heatshield and motors for Earth return.

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