Author Topic: MCT Speculation and Discussion Thread 4  (Read 569827 times)

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #520 on: 09/02/2015 03:10 AM »
philw1776:  Did you take into account some propellant for landing on Earth?  In a simple direct Earth return this would be needed and in a return that tries to capture into an orbit would also need some propellant or an airo-capture maneuver. 

The easiest capture orbit would be an elliptical one, come in just above the atmosphere and do a braking burn at perigee, this should be much lower deltaV then doing the same kind of capture at Mars because Earth's gravity well it so much stronger.  I'd estimate ~1 km/s as that about what you need to do an Earth escape from a high orbit and this is basically the time-reversal of that maneuver.  From this orbit a refueling could be done and then the vehicle can do a retro-propulsion assisted decent and landing from a lower orbit and speed which would hugely reduce the difficulty.

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #521 on: 09/02/2015 05:48 AM »
S2 Mars Return 25mT Cargo  8.5Km/sec Rocket Equation

For a 80t dry mass that would mean a prop. mass fraction of 92% (with isp of 380, assuming you can fit such huge nozzles into MCT). That's what you optimally get for an expendable methalox upper stage, not for a deep space SSTE monstrosity.
« Last Edit: 09/02/2015 05:51 AM by Oli »

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #522 on: 09/02/2015 03:28 PM »
There should be no problem fitting the 5 Rvac nozzles into the 12.5m MCT or even inside a 10m.

8.5 Km/sec is probably a half Km/sec too low
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #523 on: 09/02/2015 04:05 PM »
There should be no problem fitting the 5 Rvac nozzles into the 12.5m MCT or even inside a 10m.

8.5 Km/sec is probably a half Km/sec too low

I think 7.5km/s for the return has 300m/s margin after gravity loss of 400m/s

Can someone who is saying 8.5km/s or more give me a break down of where they are budgeting the extra ΔV?
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 philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #524 on: 09/02/2015 04:17 PM »
Now I'm confused.  Just calculated Mars' escape velocity and it's only ~5 Km/sec without needed allowance for gravity losses. 
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #525 on: 09/02/2015 04:26 PM »
Now I'm confused.  Just calculated Mars' escape velocity and it's only ~5 Km/sec without needed allowance for gravity losses.

Yes but being lower escape velocity you get a lower benefit from hyperbolic velocity (Oberth effect) so you need more ΔV over escape for trans Earth injection from low Mars orbit than you do for trans Mars injection from low Earth orbit.
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 nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #526 on: 09/02/2015 04:28 PM »
But I still feel 7.5km/s ΔV is sufficient, 8.5 or 9 should actually allow a much faster return if it was available.
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 philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #527 on: 09/02/2015 04:31 PM »
But I still feel 7.5km/s ΔV is sufficient, 8.5 or 9 should actually allow a much faster return if it was available.

Agree with prior comments re:oberth effect.  Still can't find citable source for total Km/sec budget.

I did find a spreadsheet formula error on my Mars return rocket equation delta V.  Now 8.2 Km/sec.
Fixed in attached.
« Last Edit: 09/02/2015 04:32 PM by philw1776 »
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #528 on: 09/02/2015 06:26 PM »
Now I'm confused.  Just calculated Mars' escape velocity and it's only ~5 Km/sec without needed allowance for gravity losses.

Yes but being lower escape velocity you get a lower benefit from hyperbolic velocity (Oberth effect) so you need more ΔV over escape for trans Earth injection from low Mars orbit than you do for trans Mars injection from low Earth orbit.

Humm, if I am launching from Mars surface and I want to enter an elliptical hohoman transfer around the sun I need to decrease my heliocentric velocity aka I need to be going slower then Mars itself after having left it's sphere of influence.  Thus gravity loss incurred during escape may actually be beneficial IF it results in the loss in heliocentric velocity that one needs to reach Earth.

This is just the opposite of leaving Earth in which In need to have excess velocity relative to the Earth and gravity loss while escaping the Earth is counter productive.

At the NASA trajectory browser it looks like they have DeltaV between 800 and 1000 m/s for Earth return but this is from a C3=0 Mars orbit aka escape.

http://tinyurl.com/p88y3co

According to https://en.wikipedia.org/wiki/File%3aDelta-Vs_for_inner_Solar_System.svg you need 5.5 km/s DeltaV to reach escape so an additional 1 km/s would mean a total of 6.5 km/s which looks to be the best case lowest DeltaV for Earth return.  BUT it should be noted that this trajectory doesn't match Musk's stated goal of a one Synod round trip nor is it particularly fast as the return transit legs are around 240 days, the ones below 200 days are usually above 1 km/s so their is a trade off.  IF you were trying to meet all of Musk's goals the DeltaV would be significantly higher and the dry mass fraction into unrealistic areas, so some kind of descoping is needed.
« Last Edit: 09/02/2015 08:51 PM by Chris Bergin »

Offline Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #529 on: 09/02/2015 08:18 PM »
I do not intend to interrupt your current dV conversation. Its very interesting, so please go on. But I want to make a point that bugs me quite some time already.

There is one thing, I do see not enough in L2 and here in the open forums BFR/MCT designs.
They do not seem that anyone takes an evolution of the BFR+MCT design into account. I would suspect that the first versions of the BFR+MCT are much less fleshed out, much less capable than the announced 100mT cargo / 100 passengers to Mars surface. In fact, I would expect them to have less than half of that.

Just an example for early missions that do not need the full, stated MCT capacity. Its just an example, it does not need to go down that way.
* The first MCT goes to Mars and stays there. Having a fuel production plant on board. But no people and no intention to get back. It would require a way to collect water, witch probably is the largest challenge.
* The second MCT would be dedicated to make it back. Using the fuel production of the first lander, store some food, a precursor for Humens. On its way back, it might bring some rocks as well. But really, it would be a demonstrator of getting back.
* The third might be a ship that brings furthe supplies as a precurser to human arrival. It could function as an MCT in case one of the next human rated MCTs can not make it back.
* The forth might have some humans on board. The mission would be: survive and come back. And the equipment would be triple and quadruple redundant to make that happen. No base as of yet. No habitat, MCT will have to do. That and the last MCT in case something breaks.
* The fifth MCT might get the first crew that stays longer. With the mission to create a base. It might be accompanied by 2-3 cargo MCTs. That all are intended to get back.

All this will likely happen with a less capable craft, as explained above. I would expect a major redesign of the magnitude F9 to F9 1.1 and Dragon to Dragon2. But still after this redesign, BFR and MCT might not have the capability that Musk is promising and that you are guys designing right now.

My arguments completely neglects the uses of BFR/MCT for LEO. At the current state, I would expect a precursor to BFR+MCT that is capable of payload to LEO of around 70 to 100 mT. It might be enough for the Mars missions stated above after one evolution step. It also might well be used to phase out FH and be used to deploy large satellite constellations that SpaceX and others are planning. I do not believe that SpaceX will come up with a BFR+MCT design that fulfils Elons statements in the next 20 years.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #530 on: 09/03/2015 12:49 AM »
Could not agree more that the real life BFR/MCT will be an evolutionary design.  I've always said so.
I do think BFR starts out with more tons to LEO then you speculate but even Elon's #s today will likely be revised by flight time.  First crew #s to Mars will be around 8-12. my guess. Summer 2033.
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Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #531 on: 09/03/2015 04:02 PM »
I do not intend to interrupt your current dV conversation. Its very interesting, so please go on. But I want to make a point that bugs me quite some time already.

There is one thing, I do see not enough in L2 and here in the open forums BFR/MCT designs.
They do not seem that anyone takes an evolution of the BFR+MCT design into account. I would suspect that the first versions of the BFR+MCT are much less fleshed out, much less capable than the announced 100mT cargo / 100 passengers to Mars surface. In fact, I would expect them to have less than half of that.

Just an example for early missions that do not need the full, stated MCT capacity. Its just an example, it does not need to go down that way.
* The first MCT goes to Mars and stays there. Having a fuel production plant on board. But no people and no intention to get back. It would require a way to collect water, witch probably is the largest challenge.
* The second MCT would be dedicated to make it back. Using the fuel production of the first lander, store some food, a precursor for Humens. On its way back, it might bring some rocks as well. But really, it would be a demonstrator of getting back.
* The third might be a ship that brings furthe supplies as a precurser to human arrival. It could function as an MCT in case one of the next human rated MCTs can not make it back.
* The forth might have some humans on board. The mission would be: survive and come back. And the equipment would be triple and quadruple redundant to make that happen. No base as of yet. No habitat, MCT will have to do. That and the last MCT in case something breaks.
* The fifth MCT might get the first crew that stays longer. With the mission to create a base. It might be accompanied by 2-3 cargo MCTs. That all are intended to get back.

All this will likely happen with a less capable craft, as explained above. I would expect a major redesign of the magnitude F9 to F9 1.1 and Dragon to Dragon2. But still after this redesign, BFR and MCT might not have the capability that Musk is promising and that you are guys designing right now.

My arguments completely neglects the uses of BFR/MCT for LEO. At the current state, I would expect a precursor to BFR+MCT that is capable of payload to LEO of around 70 to 100 mT. It might be enough for the Mars missions stated above after one evolution step. It also might well be used to phase out FH and be used to deploy large satellite constellations that SpaceX and others are planning. I do not believe that SpaceX will come up with a BFR+MCT design that fulfils Elons statements in the next 20 years.

The question is what is the point of developing an intermediate capacity rocket for SpaceX?  The goal is a fully recoverable rocket. That's what drops the prices by orders of magnitude and makes Mars even thinkable. Once you stop destroying the rocket at each launch, then the cost become much closer to the fuel costs+ development costs.  the development costs are a huge portion for rockets that aren't used much.  So I would expect that Spacex to develop a single large core, and then use that for all possible variants.  The BFR could be used with only partial loads and part of its fuel, for example.  Or for smaller loads is could fly back to the launch pad, since this cuts down on payload by almost 50%.  125 tons with fly back, 250 tons with barge or a remote landing area.  That's a wide range of payloads.  As for the MCT itself, whatever extra capacity it has can be filled with spare parts, or even just stock materials; we're going to need a lot of spare parts for this program  :-)

If a market develops, a modification of the second stage could carry up many different payloads using adapted fairings, without breaking the bank in development costs.

In a similar vein, the shuttle almost never flew fully loaded, and increased capacity by various optimisations with the external tank and the tiles, but it always remained outwardly identical.

Michel Lamontagne

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #532 on: 09/03/2015 04:28 PM »
One precursor mission I would like to see discussed is a launch of an MCT carrying nothing more than a probe launcher and a set of landers and solar powered rovers.  If we remove the crew, stores and most of the radiation protection, how many landings could we have on Mars in a single shot?  How about 100 Mars rovers running about and looking for landing spots and such?  This is figuring 20 tons for the storing and handling mechanism, which is perhaps a bit short. 

Do 100 lander/rovers fit into a 500m3 of cargo space?

We could probably fit in a Mars sample return mission at the cost of 20 rovers!  So 80 rovers+mars sample return.
Would lose the MCT itself though.  It would become a rather nice space station in mars orbit, and a relay station for the coms.

All with a single launch.
How would that compare to sending any number of falcon Heavy missions?

Regards

M Lamontagne

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #533 on: 09/03/2015 05:01 PM »
Why only 500 cubic meters storage?  Seems way too small by over a factor of 2.
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #534 on: 09/03/2015 06:04 PM »
Why only 500 cubic meters storage?  Seems way too small by over a factor of 2.
Just a stretch goal. Total internal volume should be between 1000 to 1500, but some of it is in impractical shapes for storing vehicles, and we might want to keep some of the furnished areas for living space latter, or storing multiple communication antennas, or whatever.  5m3 per lander seemed ample.

ML

Online oldAtlas_Eguy

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Re: MCT Speculation and Discussion Thread 4
« Reply #535 on: 09/03/2015 06:27 PM »
Why only 500 cubic meters storage?  Seems way too small by over a factor of 2.
Just a stretch goal. Total internal volume should be between 1000 to 1500, but some of it is in impractical shapes for storing vehicles, and we might want to keep some of the furnished areas for living space latter, or storing multiple communication antennas, or whatever.  5m3 per lander seemed ample.

ML
Even 1000-1500m^3 is still too small for a volume needed to hold 100 people. It needs to be >2000m^3 or about 2500m^3.  A 15m diameter MCT with a 30m tall payload section has about 2000m^3. This is what I expect this section of the MCT to be like. Plus larger diameters solves some other problems such as Mars entry terminal velocity values. In general it makes the height of the MCT a lot shorter and manageable. Larger diameters also increases prop tank volume for a specified height and decrease the ratio of tank weight to volume.

Offline Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #536 on: 09/03/2015 08:58 PM »
The question is what is the point of developing an intermediate capacity rocket for SpaceX?  The goal is a fully recoverable rocket. That's what drops the prices by orders of magnitude and makes Mars even thinkable. Once you stop destroying the rocket at each launch, then the cost become much closer to the fuel costs+ development costs.  the development costs are a huge portion for rockets that aren't used much.  So I would expect that Spacex to develop a single large core, and then use that for all possible variants.  The BFR could be used with only partial loads and part of its fuel, for example.  Or for smaller loads is could fly back to the launch pad, since this cuts down on payload by almost 50%.  125 tons with fly back, 250 tons with barge or a remote landing area.  That's a wide range of payloads.  As for the MCT itself, whatever extra capacity it has can be filled with spare parts, or even just stock materials; we're going to need a lot of spare parts for this program  :-)

If a market develops, a modification of the second stage could carry up many different payloads using adapted fairings, without breaking the bank in development costs.

In a similar vein, the shuttle almost never flew fully loaded, and increased capacity by various optimisations with the external tank and the tiles, but it always remained outwardly identical.

Michel Lamontagne

Well, many reasons why I believe there will be an incremental approach.
1. SpaceX always did an incremental approach. The didnt start with Falcon 9 1.1 when they developed their first rocket. Same holds true for the BFR. They need to learn to manufacture, launch, etc. such big rockets and thats easier if they dont jump into it with both feat.
2. There is a business case. They can probably much more cheaply launch large constellations of satellites. Like one or more of the internet constellations. Maybe even modules of a new space station. Maybe even a commercial space station, serviced by Dragon2 for space tourists.
3. All the key technologies for BFR+MCT can be tested on a smaller rocket with a smaller MCT precursor. That includes:
* landing on Mars
* large scale energy production on Mars
* water production on Mars
* methane/oxygen production on Mars
* landing, retanking and MCT (precursor) return to Earth
* flight operations and trajectory execution
* communications
* high bandwidth, high latency internet access on Mars
* booster launch and recovery (using Raptor)
* second stage recovery (if any)
* re-tanking in LEO
* first human on Mars and survivability
* first habitat on Mars
* first plant growth on Mars using Mars soil
* first sustainable food growth on Mars
And that are just the biggies. All this needs to be ready for colonization. And all this is easier with a smaller version of BFR/MCT than the ultimate goal noted by Musk.
Once these technologies are in place and tested and ready, a full size MCT makes sense. I would not be surprised that after the logistical hassles are overcome with the first missions, a bigger version of MCT are produced, one that is closer or even capable of 100 humans to Mars. Even though this particular capability would not be used.
4. I dont think that SpaceX will have the funds to do it alone. They will need the help of governments, NASA might not even be enough. When in cooperation with space agencies, pork needs to be provided. That is far easier with a precursor MCTs that focus on technology development and science rather than direct colonization of a naked planet.

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #537 on: 09/03/2015 09:04 PM »
Well, isn't Falcon Heavy,  fully recoverable, the perfect precursor?  Why another model in between? It can get some nice experimental payloads to Mars.

I indeed hope there is a case! More ships=less cost. I just question the need for an intermediate step between a fully recoverable Heavy and the BFR.

About deltaV, can aerobraking reduce the deltaV required at Earth return or does it have too great a time penalty?
« Last Edit: 09/03/2015 09:18 PM by lamontagne »

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #538 on: 09/03/2015 09:10 PM »
Why only 500 cubic meters storage?  Seems way too small by over a factor of 2.
Just a stretch goal. Total internal volume should be between 1000 to 1500, but some of it is in impractical shapes for storing vehicles, and we might want to keep some of the furnished areas for living space latter, or storing multiple communication antennas, or whatever.  5m3 per lander seemed ample.

ML
Even 1000-1500m^3 is still too small for a volume needed to hold 100 people. It needs to be >2000m^3 or about 2500m^3.  A 15m diameter MCT with a 30m tall payload section has about 2000m^3. This is what I expect this section of the MCT to be like. Plus larger diameters solves some other problems such as Mars entry terminal velocity values. In general it makes the height of the MCT a lot shorter and manageable. Larger diameters also increases prop tank volume for a specified height and decrease the ratio of tank weight to volume.

Nasa papers are available quoting 4 m3 per person as viable for a temporary stay in a rad shelter, 10 m3 per person an the minimum livable and 18 m3 per person as ideal. Google habitability in spacecraft design.  All intermediate values are ok to me.  My question was is 100 rovers an interesting idea, or not viable?  Put them in 2000m if you want ;-)
« Last Edit: 09/03/2015 09:33 PM by lamontagne »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #539 on: 09/03/2015 10:11 PM »
Well, isn't Falcon Heavy,  fully recoverable, the perfect precursor?  Why another model in between? It can get some nice experimental payloads to Mars.

I indeed hope there is a case! More ships=less cost. I just question the need for an intermediate step between a fully recoverable Heavy and the BFR.

About deltaV, can aerobraking reduce the deltaV required at Earth return or does it have too great a time penalty?
Falcon Heavy will do somewhere in the vicinity of 35-40 tons to LEO semi-reusably.  BFR + MCT will allegedly do somewhere in the vicinity of 200 tons to LEO reusably.  FH is a good precursor for unmanned missions that test single subsystems, but some intermediate LV *would* be useful for less ambitious goals and larger subsystems, I suspect.  4.6m of payload fairing isn't a lot to play with.

However, I think that's out of the scope of this conversation.  MCT is "The vehicle that can land 100 tons of payload , to include 100 passengers, on Mars".  BFR is the large-diameter LV that launches MCT.  Other launch / transfer vehicles / experimental platforms of more modest aims may come into existence, but they don't really belong in the MCT thread unless they're involved in MCT's mission architecture.
« Last Edit: 09/03/2015 10:15 PM by Burninate »

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