Author Topic: MCT Speculation and Discussion Thread 4  (Read 621086 times)

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1160 on: 12/03/2015 10:24 AM »
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

They can take a month or two to launch and refuel both in LEO, just the way they would assemble a Mars fleet to launch into a launch window. What's the problem?

Offline Zed_Noir

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Re: MCT Speculation and Discussion Thread 4
« Reply #1161 on: 12/03/2015 10:38 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.

Since you will need the tankers in whatever form for the Methane & Lox propellants for Mars missions anyway. Using them even inefficiently might be a better trade than developing a separate SEP inspace vehicle with a different propellant (Xenon) that is not readily available in large quantities.

IMO the MCT tanker will not be too different from the MCT. Otherwise you will be developing yet another vehicle that can be reuse. Presuming you version of the MCT tanker is not expendable.

It is cheaper to developed a single general purpose vehicle than 3 different specialized vehicles in parallel.


Offline Zed_Noir

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Re: MCT Speculation and Discussion Thread 4
« Reply #1162 on: 12/03/2015 10:48 AM »
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

They can take a month or two to launch and refuel both in LEO, just the way they would assemble a Mars fleet to launch into a launch window. What's the problem?

Was thinking of a LEO prop depot for the refueling. So if you dual launch, you minimized the time needed to complete the mission. 

You way is also workable.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1163 on: 12/03/2015 11:35 AM »
They can take a month or two to launch and refuel both in LEO, just the way they would assemble a Mars fleet to launch into a launch window. What's the problem?

Was thinking of a LEO prop depot for the refueling. So if you dual launch, you minimized the time needed to complete the mission. 

You way is also workable.

I fully expect some kind of depot to emerge sooner or later. I am just arguing that early missions to Mars or in this case to the moon with two or more MCT can be done without. Especially as the moon missions don't need to interfere with the Mars windows.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1164 on: 12/03/2015 09:01 PM »

Since you will need the tankers in whatever form for the Methane & Lox propellants for Mars missions anyway. Using them even inefficiently might be a better trade than developing a separate SEP inspace vehicle with a different propellant (Xenon) that is not readily available in large quantities.

IMO the MCT tanker will not be too different from the MCT. Otherwise you will be developing yet another vehicle that can be reuse. Presuming you version of the MCT tanker is not expendable.

It is cheaper to developed a single general purpose vehicle than 3 different specialized vehicles in parallel.

Tankers are needed but they are hardly a 3rd vehicle, they are just stretched 2nd stages with a nose cone covering a Xenon/Krypton tank and a hose port.  This can launch and deliver 100 mt of propellants of varying combinations, either all Metho-Lox from residuals or SEP propellants.

The SEP tugs would have an integrated Xenon/Krypton tank for it's own use or around 50 mt and a Metho-Lox tank for offloading to the MCT, around 150 mt would be sufficient.  These tanks should mass 5 and 2.5 mt respectively and the rest of the vehicle would only come out to around 10 mt meaning it can be launched mostly fueled on the BFR.

A vehicle this size is going to be a lot simpler to design and produce then the MCT itself which goes through incredible stresses and flight regimes.  The SEP is basically made of off the shelf satellite parts, Solar panels and Hall thrusters both of which are going to be mass produced by SpaceX in it's Satellite Swarm.

Two tankers refuel 1 SEP with MethoLox and Xenon to make a delivery of Metho-Lox to EML1.  One tanker refuels two SEPs with just Xenon to move two MCT's to EML1.  That's 3 fuel launches and 2 MCT so a 2:5 ratio for cargo delivery.  Nearly doubling the LEO to Mars efficiency at the cost of a small SEP tug that should be a fraction of the cost of the vehicles that are already necessary.

Offline CuddlyRocket

Re: MCT Speculation and Discussion Thread 4
« Reply #1165 on: 12/04/2015 06:39 AM »
With all this talk of tankers for in-orbit refueling of the MCT, I had the following thought concerning the Mars mission it was designed for:

If a two-stage BFR can launch both the second stage and the MCT into LEO with the MCT still attached then both can be refueled - though the MCT most likely will be fueled for the first time. The refueled second stage can then use some of that propellant to provide a substantial fraction of the TMI burn. When the MCT detaches to use its own engines and propellant to provide the remainder of the TMI burn, the second stage uses its remaining propellant to return to LEO orbit (it will obviously need much less propellant to return to LEO without the multi-hundred mt MCT attached!) and/or Earth ground return.

This also works with a one-stage BFR launching an MCT into LEO. Assuming reusable tankers to refuel the MCT, and which will need to securely dock for that purpose, then one such tanker (or more actually) can provide TMI boost assist before themselves detaching and returning to Earth.

My understanding of the minimum size of the MCT is determined by the requirement to carry sufficient propellant to both perform the TMI burn and land 100 mt cargo on Mars, any such craft will then have sufficient performance to launch from Mars directly to Earth given much less cargo and with ISRU refuelling. Any assist to the TMI burn external to the MCT will enable a reduction of the propellant it needs to carry and concomitant reduction in its size and mass with cascading effects on the BFR and tanker designs.

Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #1166 on: 12/04/2015 08:12 AM »
Using SEP for moon missions would save propellant and hence the number of tanker flights. However, that may not make things cheaper.

The SEP stage is likely to be quite expensive, it is basically all electronics, solar panels and thrusters which are all expensive in $/kg, tanks which are cheap make up a small percentage of the total mass.

Using SEP for the MCT transfer to lunar orbit adds costs to the MCT as it would need fully rad-hard electronics. SEP cannot be used for manned transfer to lunar orbit, so there is the overhead of maintaining two systems. The length of time required > 6 months limits the reflight rate for the MCT which adds to the costs.

Finally SEP costs are mainly up-front while refuelling in LEO is pay-as-you-go. This means that near term costs for SEP are higher than all cryogenic. With an increasing flight rate the money spent at any point in time could quite well be more for SEP, even if it seems cheaper on a per flight basis. SEP also has more opportunity cost and more risk (suppose it is superseded before its payback time).

[As an aside, it does not seem possible to use SEP for Mars and maintain a one synod reuse of the MCT. Similar cost arguments mean that SEP is quite likely not cheaper either.]


Offline Zed_Noir

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Re: MCT Speculation and Discussion Thread 4
« Reply #1167 on: 12/04/2015 09:17 AM »
[snip]

IMO the MCT tanker will not be too different from the MCT. Otherwise you will be developing yet another vehicle that can be reuse. Presuming you version of the MCT tanker is not expendable.

It is cheaper to developed a single general purpose vehicle than 3 different specialized vehicles in parallel.

Tankers are needed but they are hardly a 3rd vehicle, they are just stretched 2nd stages with a nose cone covering a Xenon/Krypton tank and a hose port.  This can launch and deliver 100 mt of propellants of varying combinations, either all Metho-Lox from residuals or SEP propellants.

You are describing a new large expendable vehicle that can maneuver and loiter in orbit.

Quote
The SEP tugs would have an integrated Xenon/Krypton tank for it's own use or around 50 mt and a Metho-Lox tank for offloading to the MCT, around 150 mt would be sufficient.  These tanks should mass 5 and 2.5 mt respectively and the rest of the vehicle would only come out to around 10 mt meaning it can be launched mostly fueled on the BFR.

A vehicle this size is going to be a lot simpler to design and produce then the MCT itself which goes through incredible stresses and flight regimes.  The SEP is basically made of off the shelf satellite parts, Solar panels and Hall thrusters both of which are going to be mass produced by SpaceX in it's Satellite Swarm.

AIUI none of those COTS parts are design for BEO radiation and thermal environments. So you are basically doing a partial parallel MCT development. Also the Hall thrusters seems like a mismatch with the impulse required plus developing the solar arrays & radiator arrays infrastructure on the vehicle.

Quote
Two tankers refuel 1 SEP with MethoLox and Xenon to make a delivery of Metho-Lox to EML1.  One tanker refuels two SEPs with just Xenon to move two MCT's to EML1.  That's 3 fuel launches and 2 MCT so a 2:5 ratio for cargo delivery.  Nearly doubling the LEO to Mars efficiency at the cost of a small SEP tug that should be a fraction of the cost of the vehicles that are already necessary.

Count 3 expendable tankers of 2 different types and 3 semi-expendable SEP tugs. As compare to no expendable vehicles using MCT variants.

Plus how do you planned to transfer the propellants and do the orbital rendezvous of the various elements?



Darn @Mike Atkinsom beats me to respond to @Impaler

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1168 on: 12/04/2015 05:58 PM »
Expendable Tankers???  Of course not, this is just the BFR's normal reusable 2nd stage with a mission kit on the nose in the place the MCT would have been.  It would need some loiter time in orbit but is already part of being a reusable stage so you can make controlled re-entry and the mission kit would be used to extend that time.

The SEP mass is tiny compared to the MCT at 15 vs 75 mt, dry mass is the most reliable cost estimate basis and by that metric a SEP tug is far cheaper to develop and build then the MCT itself.  Each round trip from the tug will replace roughly one whole launch of the BFR which is likely to cost around $100 million, so they pay for themselves on the first or second trip and will well into net savings by the end of a Synod.

Offline Zed_Noir

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Re: MCT Speculation and Discussion Thread 4
« Reply #1169 on: 12/04/2015 07:01 PM »
@Impaler

My presumption is that the MCT is the upper stage. You think otherwise?

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1170 on: 12/04/2015 10:06 PM »
Yes, that's something I've been saying for this whole thread.  It's necessary to make BFR a capable launcher for all uses and to allows actual revenue generation before 'tickets to mars' are being sold.

In addition it allows the MCT to be a fraction of the size of the combined concept which reduces propellant needs on the Martian surface AND makes it feasible to actually have a launch abort capability. 

The smaller MCT can actually put it's cargo close to the surface, rather then several floors off the ground making unloading much simpler via a deploy-able ramp.

Lastly such a MCT actually makes sense as a long term vehicle that takes on a surface-2-orbit shuttle role at mars as the whole system evolves, a huge vehicle for direct flight is far over-built for that role.

And finally a smaller vehicle with much less aggressive DeltaV targets makes for a vastly cheaper and faster development at lower risk, particularly if you have already learned from creating a reusable 2nd stage that uses the same engines and built a good flight history with it.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1171 on: 12/05/2015 12:02 AM »
* A single-stage BFR is not likely to be a good cargo or liquid ferry to LEO, much less to GTO.  Payload mass fractions on a single stage are very small.  You can expect a *reusable* single stage rocket-based launch vehicle to be extraordinarily difficult to do with any payload at only 360-380s Isp.

* The optimal designs for transporting large-diameter cargo and liquids start to diverge at scale, with very different needs because of differing densities.  A spherical tank the size of a payload fairing will always be way too heavy when full to transport in a reasonable vehicle for lofting that payload fairing.  As volume dictates reentry vehicle design, this becomes important when you want to reuse all stages.

* MCT is likely to require extensive design compromises and dry mass increases to achieve reusability, which makes it a very poor cargo ferry to LEO, and a moderately bad liquid ferry to LEO.

* Mars missions are always starved for mass, starting with the maximum structurally-integral amount that can be lofted by the largest launch vehicle available;  If you make launch cheap and can spend as many launches as you like in propellant and cargo, you still have this limit on the part of the vehicle that can't be divided.  It makes sense to spend this number on durable vehicular dry mass, and fill the rest of the mission on subsequent launches, because that maximizes the total payload that can be feasibly landed, which has a capped ratio to vehicular dry mass.

* BFR will need to loft many, many liquids payloads for every MCT departure.

* A stage which reaches Earth orbit and lands back at Earth, whatever way that occurs, has fairly high dry mass fraction.

* Their immediate business future, in Falcon Heavy, will be reliant on a stage that lands at the launchpad, another stage that lands far downrange, and another stage that reaches orbit;  This third stage is something which they would like to reuse, but can't square the circle economically, because of dry mass increases or propellant increases.

* The fewer the stages, the worse dry mass increases harm payload capacity.



The simplest, most efficient way to reuse a BFR and to launch Mars missions will be to cut it into two stages, with several modular options for third stages - MCT, a reusable liquids carrier, and a reusable large cargo carrier, with perhaps a fourth expendable boost stage for reaching GTO with smaller payloads inside that large cargo carrier.   The RTLS + barge landing system is preserved, and the reusable third stages reach orbit and then return at their leisure to the launchsite.
« Last Edit: 12/05/2015 02:49 AM by Burninate »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1172 on: 12/05/2015 12:45 AM »
Are you on L2, Burninate?
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1173 on: 12/05/2015 12:58 AM »
A single-stage BFR is not likely to be a good cargo or liquid ferry to LEO, much less to GTO.  Payload mass fractions on a single stage are very small.  You can expect a *reusable* single stage rocket-based launch vehicle to be extraordinarily difficult to do with any payload at only 360-380s Isp.


Are you replying to me?  I've been saying all along that BFR will have 2 stages like Falcon, the MCT would be a 3rd stage on-top of that but the first two stages can be launched on their own just like Falcon to put payloads in LEO or beyond without using the MCT.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1174 on: 12/05/2015 01:07 AM »
MCT may be very stripped down when in cargo mode, essentially a reusable second stage with a long-loiter package and refueling capability. Don't know why you'd insist on yet another stage.
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 #1175 on: 12/05/2015 01:45 AM »
What are you stripping off and what kind of mass estimate do you have this before and after?

If MCT in it's normal state is overbuilt as a Tanker I find it very hard to believe that it could be stripped into
Quote
essentially a reusable second stage with a long-loiter package and refueling capability
for less money then just making exactly THAT from a clean sheet design with a common diameter and tank fabrication process shared with the first stage.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1176 on: 12/05/2015 02:00 AM »
10:1 mass ratio. Maybe 600 tons totally full.

MCT in its normal state IS stripped down.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1177 on: 12/05/2015 02:46 AM »
Are you on L2, Burninate?

Not at present, no.

Offline SolarExploration

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Re: MCT Speculation and Discussion Thread 4
« Reply #1178 on: 12/08/2015 07:01 AM »
Maybe from a more non technical angle, it could be done like this?...
 

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1179 on: 12/09/2015 01:42 AM »
That is actually surprisingly like what I see the LONG term transit solution looking like.

Direct launch of capsule like vehicle with passengers in short term air-plane like accommodations.
Transit vehicle which remains in space and provides for all life-support during transit.
Return from the surface to this transit vehicle in mars orbit for return to Earth.
Re-entry at Earth in the capsule vehicle.

Key differences I see though..

Transit vehicle staging position is EML1 rather then LEO.
Passengers vehicle needs refueling in LEO to get to LEO and this means 3-4 days to the transit vehicle.
Transit vehicle won't stay at mars but will return immediately after picking up return passengers.

On a non-technical note the kind of 'scientific adventure tourism' sounds like promising intermediate between the first humans on mars which will be space agency Astronauts and later full on colonists.

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