Author Topic: MCT Speculation and Discussion Thread 4  (Read 621425 times)

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1140 on: 11/11/2015 03:10 PM »
I've done calculations, and yes, it's possible to build an MCT with very good mass fractions (and by mass fraction, I mean without cargo or supplies, etc). I can show you my work if you'd like. But it'll take a while.

But honestly, DC-Y/I is even more ambitious but is still possible. I don't get this idea that you can't build good mass fractions. If it were true, then a reusable upper stage on a reusable TSTO rocket would never be possible (and I do get the idea that many people think that is true).

Also, I object firmly to the word "possible." What you actually mean is "feasible," not "possible." There is no first-principles reason why MCT couldn't achieve a mass ratio as good as 10 (when lightweighted, no cargo, etc). At very least, you could use propellant densification techniques if you were marginal.

So show me a first-principles reason why a mass ratio of 10 is impossible.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1141 on: 11/11/2015 08:04 PM »
By all means show your work, I expect to see a mass allotment for each subsystem and a justification for each which totals to a vehicle capable of all the mission DeltaV segments your proposing.

Their are an ENDLESS list of 'more ambitious' vehicles that can to trotted out to make anything else look easy in comparison, but the problem is they are made of PAPER.

Delta Clipper Y was NEVER BUILT, all we saw was the sub-scale non-orbital test vehicle DC-X and it's purpose was to demonstrate VTVL and fast turn-around which it did, it did not come within a mile of the DeltaV needed for orbit.  Basically it did nothing more then SpaceX Grasshopper has done.

From http://www.astronautix.com/lvs/dcx.htm

Quote
The DC-X was specifically not designed to demonstrate the most critical issue for any practical SSTO: structural mass fraction

Did you even know that the DC-X dry mass fraction was a colossal 48% of the take off weight 9100 kg dry 18,900 gross take off.  According to http://www.astronautix.com/lvs/dcx.htm

On top of that it used the RL10A-5 engine which only get 368s in Vac (on HYDRO-LOX), that's a DeltaV of just over 2.6 km/s.  The full scale vehicle was aiming for a dry mass of 5x as much and a mere 10 mT payload, it would have needed a different engine, even with a good 450 ISP the vehicle would have a launch mass of around 500 mT to reach orbit and would need >50 RL10B-2 (the best in production engine from P&W) or 4 Shuttle main engines, which is also the only Hydro-Lox engine designed to operate continuously from Sea-level to to vacuum while being restart able & reusable, all necessary features of DC-Y.


Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1142 on: 11/11/2015 08:48 PM »
Again, first-principles reason why it's not possible. Mere historical analogy is not good enough to show it can't be done.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1143 on: 11/11/2015 09:22 PM »
First off you are the one repeatedly trying to argue from historical vehicles, pointing out various metrics of performance and claiming they are achievable all in a single vehicle without consideration of how they conflict.

Now with that line of logic thoroughly refuted your now abdicating any connection with real world materials or systems, and claim I need to show that the laws of physics alone invalidate your claims?

This is preposterous and insulting because you know very well that their is no first principle of physics that limits a vehicles DeltaV, only the speed of light is denied to us by the laws of physics.

You can always propose exponentially smaller and smaller dry mass fractions and higher and higher T:W ratio engines made from any theoretically possible material and any theoretically possible propellant we can imagine that will make any DeltaV achievable.  An anti-matter rocket made of neutronium is certainly allowed by the laws of physics but has no place in this this thread.

If this is your position then you have business posting in this thread as you have left the bounds of rational engineering discussion.  And I am still waiting for your actual calculations.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1144 on: 11/11/2015 09:45 PM »
First off you are the one repeatedly trying to argue from historical vehicles...
Because if you're trying to prove something is possible, a single example from history is sufficient. If you're trying to prove something isn't possible, historical analogy isn't useful.

Yup, there's asymmetry there.

Historical analogy may help you throw doubt on the feasibility of something, but it doesn't touch the possibility of something.
« Last Edit: 11/11/2015 09:46 PM by Robotbeat »
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1145 on: 11/11/2015 09:50 PM »
First off you are the one repeatedly trying to argue from historical vehicles, pointing out various metrics of performance and claiming they are achievable all in a single vehicle without consideration of how they conflict.

Now with that line of logic thoroughly refuted your now abdicating any connection with real world materials or systems, and claim I need to show that the laws of physics alone invalidate your claims?

This is preposterous and insulting because you know very well that their is no first principle of physics that limits a vehicles DeltaV, only the speed of light is denied to us by the laws of physics.

You can always propose exponentially smaller and smaller dry mass fractions and higher and higher T:W ratio engines made from any theoretically possible material and any theoretically possible propellant we can imagine that will make any DeltaV achievable.  An anti-matter rocket made of neutronium is certainly allowed by the laws of physics but has no place in this this thread....
Okay, show that you cannot achieve a rocket that does a 10:1 mass fraction with available materials and using chemical propulsion.

And yes, you CAN get better and better mass fractions as we make manufacturing and materials science advances. Heck, we've barely touched a lot of materials science advances, not even achieving the levels of performance we can get out of the materials we already have.

This is my point: you can't sandbag a design, then claim it's not possible to significantly exceed that design. What you're doing is not demonstrating how high of performance you can get, instead, you are trying to show what can be done without any degree of cleverness. MCT will likely involve cleverness.

Do you have any reason to believe that SpaceX is not trying to advance the state of the art in materials science or mass fraction? Because I have evidence that they ARE.
« Last Edit: 11/11/2015 09:51 PM by Robotbeat »
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1146 on: 11/12/2015 03:58 AM »
Where do you get this belief that the burned of proof is on me?  You are the one claiming cutting edge performance on par with what a SSTO vehicle needs.  YOU need to prove that envelope pushing is possible and can be done at reasonable costs, I am STILL waiting for your numbers that you hinted at several posts ago.

You are basically admitting that your logical process consists of pushing every performance margin to to braking point and claiming THAT will be the MCT design, while I am intentionally try to find a conservative design that would not require as much 'cleverness' (all though far from none).  Other word for 'cleverness' is TIME and MONEY, and 'blowing up a few in development'.

If your next response doesn't contain actual calculations then this discussion is pointless.  I repeat SHOW YOUR WORK.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1147 on: 11/13/2015 12:03 AM »
Where do you get this belief that the burned of proof is on me?...
Because you claim it's not possible. Not just unfeasible, but not possible.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1148 on: 11/13/2015 12:05 AM »
Where do you get this belief that the burned of proof is on me?  You are the one claiming cutting edge performance on par with what a SSTO vehicle needs.  YOU need to prove that envelope pushing is possible and can be done at reasonable costs, I am STILL waiting for your numbers that you hinted at several posts ago.

You are basically admitting that your logical process consists of pushing every performance margin to to braking point and claiming THAT will be the MCT design
No I'm not. I don't claim to think I can predict MCT, just establish some bounds about what COULD be done.
Quote
.while I am intentionally try to find a conservative design that would not require as much 'cleverness' (all though far from none)....
...that line of thinking would never have produced Falcon 9 v1.1 or Full Thrust, which is necessary to do their partial reusability.


I said it'd take time. I may get to it this weekend.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1149 on: 11/13/2015 01:12 AM »
Where do you get this belief that the burned of proof is on me?  You are the one claiming cutting edge performance on par with what a SSTO vehicle needs.  YOU need to prove that envelope pushing is possible and can be done at reasonable costs, I am STILL waiting for your numbers that you hinted at several posts ago.

You are basically admitting that your logical process consists of pushing every performance margin to to braking point and claiming THAT will be the MCT design
No I'm not. I don't claim to think I can predict MCT, just establish some bounds about what COULD be done.
Quote
.while I am intentionally try to find a conservative design that would not require as much 'cleverness' (all though far from none)....
...that line of thinking would never have produced Falcon 9 v1.1 or Full Thrust, which is necessary to do their partial reusability.


I said it'd take time. I may get to it this weekend.

Their is the rub, SpaceX didn't START at Full-thrust, they started at something like half as much and built up too it over several years.  They were CONSERVATIVE initially, which is exactly what I'm saying should be done, their will certainly be an attempt to improve performance over time and that could yield higher payloads, faster transit times or more safety and engine-out margin.  Your trying to pretend that a conservative start permanently cripples any future potential for improvement.


As for the numbers, sorry I though you had them ready already, we will rejoin this topic when you have them ready.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1150 on: 11/13/2015 01:20 AM »
SpaceX only launched 5 F9 v1.0s. They were basically all dev vehicles. If you want to argue the first couple MCTs will have sandbagged weight fractions, then that's fine. I won't contradict you. But actual operational MCTs capable of the full mission cycle and of carrying passengers in both directions... That I am certain will /require/ cutting-edge capability to be both economical and safe.
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 GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #1151 on: 11/17/2015 02:52 PM »
Just a musing, because I know the following concept has some big 'issues'. Nevertheless it's been in my head despite my own objections, and could do with others agreeing it isn't useful/workable :)

We've spoken about a BFR launching
- MCT cargo, MCT passenger.
- tankers for refueling (also versions of MCT)
and some have added:
- existing payloads, with multiple satellites in a single fairing etc
- big payloads (space station components etc)

If you were going to try to use earth based requirements to help fund your MCT... could a tanker upper stage be made that ALSO launches a single satellite (up to FH size)? 

You'd have 1 payload/satellite of variable mass, sitting on top of the tanker second stage, with the tanker filled to use up the rest of the launch capability (secondary payload). Instead of launching tanker-only missions a launch could position the payload, and then continue by shipping the reduced tanker payload to a depot.

ie: 2 payloads configured in a novel way.

I believe the difficulties of making this work outweigh the benefits.
..... There's less tanker fuel going up and it's going towards the wrong place. For each half-filled tanker you're doubling your refuelling stops and doubling your tanker numbers waiting to offload. The launch difficulties of a BFR are far higher than a F9 or FH (for now). And it locks you in to satellite launch timing rather than fuel launch requirements (or requires you to leave the tanker waiting in orbit).

So just thinking aloud... I'm 95% against but my 5% wants to play it out.
« Last Edit: 11/17/2015 02:53 PM by GregA »

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1152 on: 11/17/2015 03:32 PM »
If you were going to try to use earth based requirements to help fund your MCT... could a tanker upper stage be made that ALSO launches a single satellite (up to FH size)? 

Two reasons at least why it does not make sense. Minor, it would need development of a small fairing, which is still big and expensive. Such a fairing would otherwise not be needed. Major, as you already mentioned, the orbits will not match. Fuel will go to near equatorial LEO or to the easiest LEO reachable from the launch site. The payload will need a different inclination and transfer orbit. Changing inclination in LEO is HARD.

Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #1153 on: 11/22/2015 07:20 PM »
I think he was referring to the launch vehicle, aka make an even large BFR to launch an enormous MCT which has the propellant capacity to do a direct departure to mars.  In other words he thinks doubling the size of the launch rocket is worth it to avoid making a SEP tub because you know 'fewer vehicles' logic.  ::)

Well, if you already have to build a custom factory and a custom launch pad, what's the marginal cost of increasing the rocket size? Materials are cheap.

Quote
Total cost is the metric we care about, if it be 1 vehicle or 10.

That's where I disagree. Development cost, early in the program, is far more critical since it will be easier to get investment once a functioning Mars transport system is demonstrated.

They may well use SEP tugs eventually, but the initial system should work without them.


Quote
Keep in mind that tank is 100 mT larger and lacks any insulation.

100 mT larger? I expect the propellant mass of MCT to be greater than that of F9.

 (if Isp is 380 and delta-v is 6 km/s, then mass ratio needs to be 5: with empty mass = 50 tons and payload mass = 100 tons that's 600 tons of propellant. If the empty mass is 100 tons that's 800 tons of prop. If the delta-v is larger it will be more, quite possibly over 1000 tons.)

Quote
Finally I expect their to be multiple smaller tanks through the vehicle to accommodate good weight distribution and avoid the cargo-holds large volume, this will cut down on the efficiency of the tank.

Why would it need multiple smaller tanks? I would expect it to be shaped rather like a Blue Origin New Shepard or Apollo Lunar Module - relatively wide fuel tank + engines at bottom, capsule at top.

I think you may be over complicating things.

[quote[
You can't just scale the vehicle to conquer thouse factors, the parasitic stuff all grows with overall vehicle mass (except for com/computer systems which are negligible already).[/quote]

Well I was thinking in square/cube law, minimum gauge terms but maybe not.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #1154 on: 11/25/2015 09:38 PM »
If you were going to try to use earth based requirements to help fund your MCT... could a tanker upper stage be made that ALSO launches a single satellite (up to FH size)? 

Two reasons at least why it does not make sense. Minor, it would need development of a small fairing, which is still big and expensive. Such a fairing would otherwise not be needed. Major, as you already mentioned, the orbits will not match. Fuel will go to near equatorial LEO or to the easiest LEO reachable from the launch site. The payload will need a different inclination and transfer orbit. Changing inclination in LEO is HARD.

True, but let's assume for a moment they'll be launching from the Cape.  Most sats going to GTO launch from there, unless they are going to a a polar orbit, then they launch from Vandy.

You would need either a PLF that can mount on the nose of a reusable tanker stage.  Or perhaps a payload bay in the nose that could deploy a sat out a hatch in the nose.  Something like the Rocketplane Kistler K-1 was going to do. 
The stage with a D4H or FH class sat going to GTO (<13mt so far, historically) could launch directly to a transfer orbit, just as most sats going to GEO/GSO do now.  Once in the correct location, it deploys the sat which then does it's own propulsive acting to put it in the correct GEO/GSO orbit.  Again, like is usually done now.
The transfer orbit will take the tanker stage back close to Earth where it can do a small burn to deorbit itself and come back to the launch pad.   The trick is to have it come back and deorbit at the right time to is gets back to the launch site.
That's probably one reason SpaceX decided not to pursue a reusable F9US.  Other than Dragon, most payloads F9/FH will be launching will be going BLEO, and so getting them back could be tricky, where it's much easier if you aren't trying to get the upper stage back. 

Two things bring down costs.  Resuability and economics of scale.  If you make stages cheaper and churn out more, you can bring the cost per unit down, and SpaceX seems to have taken that path with F9US.

But, and internal payload bay on the nose of an MCT tanker could look something like this...with a payload bay behind the nose cap instead of a docking hatch.  The stage could use solar panels like this that could retract, or depending on how long it'll be up there, maybe methalox fuel cells or IVF to make electricity from residuals propellants so there's no solar panels to need to mess with for such missions.


Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1155 on: 12/02/2015 08:04 PM »
I did a small side calculation to see what kind of cargo delivery capacity MCT would have for a lunar landing.

Again 75 mt dry mass, maximum propellant load of 300 mt, I'll be assuming NO lunar derived propellants and a similar SEP tug that delivers to EML1.  Any crew reach EML1 via a Dragon capsule.

Lunar landing from EML1 is 2520 m/s, Earth return is via direct assent of 2740 m/s to an atmospheric intersecting swing buy in which aerobraking brings us to a LEO orbit.  As in the return from Mars the vehicle will be comply empty of propellants at this point and require a small additional refueling to land.

Working backward the empty vehicle would 80 mt propellant to perform Earth return so this much needs to be retained at landing.  If the vehicle starts full fueled at EML1 then it can land offload 85 mt cargo and still make Earth return.  If the SEP tug were to move the MCT past EML1 and down to a Low lunar orbit (decent of 1870 m/s at the cost of ~800 m/s at low thrust) then the cargo delivery would nearly double to 160 mt.

Now lets look at a manned landing an return, I'll assume the same 25 mt return mass goal for Mars that Musk has stated so this will presumably the mass of a habitation module being carried inside the MCT.  To return to Earth with this module the propellant requirement grows by 33% as our dry mass has increased from 75 to 100 mt.  At 107 mt of propellant from EML1 this is starting to cut into payload quite a lot, we have only 33 mt of cargo along with the 25 mt habitat. 

The benefit of coming from LLO is considerable for this mission, the MCT could offload 108 mt while carrying the 25 mt habitat their and back.  But the cost in time of having a SEP tug move the vehicle from EML1 to low lunar orbit is likely to be 2-3 months, while the crew habitat is certainly capable of this period of life-support (it can go to mars and back) it may not be worth it if the lunar stay is only going to be of only modest length.  If on the other hand the lunar stay is several months this delay might not be a problem.  Alternatively crew rendezvous could be moved to LLO itself to remove this period from the crew's mission duration.

In any event this type of lunar mission, even without crew would be an excellent shake-down mission for the MCT as it would duplicate many of the aspects of a Mars mission.

Offline Zed_Noir

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Re: MCT Speculation and Discussion Thread 4
« Reply #1156 on: 12/03/2015 07:28 AM »

...

In any event this type of lunar mission, even without crew would be an excellent shake-down mission for the MCT as it would duplicate many of the aspects of a Mars mission.

Have you consider using unmanned MCT tankers in LEO and LLO to top off the MCT lunar lander's prop tanks? In case of LLO for both descent & ascent from the Lunar surface.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1157 on: 12/03/2015 07:52 AM »
Have you consider using unmanned MCT tankers in LEO and LLO to top off the MCT lunar lander's prop tanks? In case of LLO for both descent & ascent from the Lunar surface.

I have thought of a fully refueled tanker and a fully fueled MCT going to TLI together. The tanker then transfers its fuel to the MCT and does a direct return to earth while the MCT goes to land and relaunch from the moon. It's the most fuel efficient mission profile. The tricky part is rendezvous and refuelling on the way to the moon.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1158 on: 12/03/2015 08:09 AM »

...

In any event this type of lunar mission, even without crew would be an excellent shake-down mission for the MCT as it would duplicate many of the aspects of a Mars mission.

Have you consider using unmanned MCT tankers in LEO and LLO to top off the MCT lunar lander's prop tanks? In case of LLO for both descent & ascent from the Lunar surface.

No that would be very inefficient.  The MCT has a 75 mt dry mass which would be moved all the way to LLO to do a job that could easily be done by a mere tank with a ~5% dry mass fraction.

I would put that tank on a SEP tug which is how both the propellants and MCT will be moved from LEO to the staging point for the lunar landing.

If we need more propellant then I would just move the staging point closer to the moon as I described, this easily brings the cargo delivery mass to over the 100 mt design goal and the MCT will almost have a volume limit just like every vehicle which means we can't just arbitrarily increase useful cargo even if we have the DeltaV to push it on paper it still needs to fit into the vehicle.

Speculation about 'MCT Tanker' is in my opinion misguided, the vehicle would be a terrible tanker due to it's dry mass which is highly specialized for other functions.  The LEO tanker will be a stretched 2nd stage which will transfer propellant to SEP tugs which will move propellants beyond LEO.
« Last Edit: 12/03/2015 08:15 AM by Impaler »

Offline Zed_Noir

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Re: MCT Speculation and Discussion Thread 4
« Reply #1159 on: 12/03/2015 10:02 AM »
Have you consider using unmanned MCT tankers in LEO and LLO to top off the MCT lunar lander's prop tanks? In case of LLO for both descent & ascent from the Lunar surface.

I have thought of a fully refueled tanker and a fully fueled MCT going to TLI together. The tanker then transfers its fuel to the MCT and does a direct return to earth while the MCT goes to land and relaunch from the moon. It's the most fuel efficient mission profile. The tricky part is rendezvous and refuelling on the way to the moon.

Obviously you dual launched the MCT and the tanker from a couple of nearby launch facilities at the same time.  ;)

Unless you think there will only be one launch pad for the BFR.  :o
« Last Edit: 12/03/2015 10:11 AM by Zed_Noir »

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