Author Topic: MCT Speculation and Discussion Thread 4  (Read 504142 times)

Offline CyclerPilot

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Re: MCT Speculation and Discussion Thread 4
« Reply #320 on: 07/07/2015 01:52 AM »


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I don't think it is a very efficient architecture. It means several MCT with all their mass would need to be accelerated a significant part of TMI. Also you mention using half of their fuel. It would not be necessary to reserve half of the fuel for return. Injecting into a highly ellicptic orbit would give the Mars bound MCT much of the needed delta-v and brings the booster MCT back to earth basically free.

Why do you propose to get them back to LEO? More efficient to land them for a new launch with payload.

I think the most efficient way is giving MCT tanks large enough to do TMI burn and Mars EDL by themselves. Use tanker MCT to refuel in LEO either directly fuelling up an MCT or filling depots. They need that tankage and the delta-v to get back to earth from the Mars surface.
You are right, it's not the most fuel efficient. But it might be a method for reaching Mars faster than the least-energy transfer orbit to reach Mars, perhaps in in 3-4 months rather than 6. Also a method that would represent a non-SEP architecture if Elon is serious about it. I am sure this has already been addressed somewhere and lies on someone's spreadsheet.
There have been several MCT designs that had large enough tanks to do the 4 month transfer direct from LEO.  (I think the jump from 4 to 3 months is pretty insane most synods).

If you think those tanks are too large, and want to transfer to a higher energy orbit before TMI, you should just use one extra MCT instead of 4.  That will save hundreds of tons of propellant because you are moving less dry mass.  A reusable SEP tug is another popular option.

Offline Ionmars

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Re: MCT Speculation and Discussion Thread 4
« Reply #321 on: 07/07/2015 10:45 PM »
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There have been several MCT designs that had large enough tanks to do the 4 month transfer direct from LEO.  (I think the jump from 4 to 3 months is pretty insane most synods).

If you think those tanks are too large, and want to transfer to a higher energy orbit before TMI, you should just use one extra MCT instead of 4.  That will save hundreds of tons of propellant because you are moving less dry mass.  A reusable SEP tug is another popular option.
Excellent. I have seen a lot of discussion with the SEP option, but not the propellant only option. Do you know where I could find those designs -- on this forum?
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Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #322 on: 07/07/2015 10:50 PM »
One way to speculate about the BFR launcher for the MCT is to look at mass fraction to LEO efficiency.  The F9 has a mass fraction of ~2.7% to LEO.  Despite full re-usability most of us fanboys/girls here expect that SX will somehow improve on that with the fully re-useable BFR. The tables below 1st assume 180mT to LEO with the dry MCT massing 80mT and the 2nd table assumes a dry MCT at 100mT.  Various optimistic mass fractions yield different BFR takeoff weights.  A T/W ratio of 1.2 is assumed yielding 1st stage thrust and dividing by 500KLBs/Raptor, the # of Raptors needed.

MCT + Payload = 180mT to LEO 

MASS FRACTION   BFR         BFR TAKEOFF      
TO LEO          WEIGHT mT      WEIGHT M LBS   THRUST M LBS     # RAPTORS @ 500KLB
5.0%                3500              7.7                     9.2                       19
                     
4.5%                3889              8.6                    10.3                       21
                     
4.0%                4375              9.6                    11.6                       23
                     
3.5%                5000            11.0                    13.2                       26
                     
3.0%                5833            12.8                    15                         31
                     
MCT + Payload = 200mT to LEO                    
MASS FRACTION                  
TO LEO        BFR                 BFR TAKEOFF      
                WEIGHT mT          WEIGHT M LBS     THRUST M LBS       # RAPTORS
5.0%                4000               8.8                   10.6                      21
                     
4.5%                4444               9.8                   11.7                      24
                     
4.0%                5000             11.0                   13.2                      26
                     
3.5%                5714             12.6                   15.1                      30

Assuming Raptor engine bells are ~1.6m wide, it's likely that 1st stage diameters of over 10m are preferred with 12.5m or even better 13.5m best to allow for max # of engines in case mass fraction drops.  A smaller MCT dry weight really helps reduce BFR mass & # of engines as would be expected.

Are you counting the mass of MCT as payload, or as the stage itself?  If it is it's own 2nd stage, then you should figure 180 or 200mt -gross- to LEO.

It'd be a heavy stage with a bad mass fraction obviously, but a stage nonetheless.

For example, STS could only delivery about 23mt net payload into LEO.  But it could deliver ~90mt gross mass into LEO.   And actually more if you factor in the ~27mt dry ET which was dropped just prior to circular LEO.

Direct's J130 advertised it could put about 70mt into LEO.  But the core went almost to LEO, and it  would have massed about 71mt at burnout (according to Direct's baseball cards).  SO the J130 was pushing upwards of 140mt gross towards LEO (unsure the actual net payload it could have delivered to a fully circular LEO).  And it would have had less than 7Mlbs of thrust at take off.

« Last Edit: 07/08/2015 12:17 AM by Lobo »

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #323 on: 07/08/2015 01:57 AM »
One way to speculate about the BFR launcher for the MCT is to look at mass fraction to LEO efficiency.  The F9 has a mass fraction of ~2.7% to LEO.  Despite full re-usability most of us fanboys/girls here expect that SX will somehow improve on that with the fully re-useable BFR. The tables below 1st assume 180mT to LEO with the dry MCT massing 80mT and the 2nd table assumes a dry MCT at 100mT.  Various optimistic mass fractions yield different BFR takeoff weights.  A T/W ratio of 1.2 is assumed yielding 1st stage thrust and dividing by 500KLBs/Raptor, the # of Raptors needed.

MCT + Payload = 180mT to LEO 

MASS FRACTION   BFR         BFR TAKEOFF      
TO LEO          WEIGHT mT      WEIGHT M LBS   THRUST M LBS     # RAPTORS @ 500KLB
5.0%                3500              7.7                     9.2                       19
                     
4.5%                3889              8.6                    10.3                       21
                     
4.0%                4375              9.6                    11.6                       23
                     
3.5%                5000            11.0                    13.2                       26
                     
3.0%                5833            12.8                    15                         31
                     
MCT + Payload = 200mT to LEO                    
MASS FRACTION                  
TO LEO        BFR                 BFR TAKEOFF      
                WEIGHT mT          WEIGHT M LBS     THRUST M LBS       # RAPTORS
5.0%                4000               8.8                   10.6                      21
                     
4.5%                4444               9.8                   11.7                      24
                     
4.0%                5000             11.0                   13.2                      26
                     
3.5%                5714             12.6                   15.1                      30

Assuming Raptor engine bells are ~1.6m wide, it's likely that 1st stage diameters of over 10m are preferred with 12.5m or even better 13.5m best to allow for max # of engines in case mass fraction drops.  A smaller MCT dry weight really helps reduce BFR mass & # of engines as would be expected.

Are you counting the mass of MCT as payload, or as the stage itself?  If it is it's own 2nd stage, then you should figure 180 or 200mt -gross- to LEO.


Yes the MCT itself plus the 100mT "payload? Elon mentioned has a gross mass to LEO of 180mT or 200mT in my above examples.
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Offline Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #324 on: 07/08/2015 06:25 AM »
F9R 1.1 can put approx 13.1mT to LEO with recovery of the first stage. That might improve for 1.2 but we dont have numbers on that yet.
Second stage weights around 3.9mT while F9R weights about 505mT at launch. That makes a gross mass to orbit of about 3.36%.

But that is with KeroLOX all the way. With MethaLOX, alone that number would increase significantly. So yes, I guess it is safe to assume that the mass to orbit fraction will be around 4 to 5 % as you listed in your table. If SpaceX has enough unicorn hair and ferry dust left from their Dragon production, it might even go higher than 5%, simply because there is no payload adapter, fairings or what have you necessary.

Offline symbios

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Re: MCT Speculation and Discussion Thread 4
« Reply #325 on: 07/08/2015 11:13 AM »
I think this is interesting because everything hangs on the mass "budget" (gross weight to LEO).

I think that Mr Musk is not going for a high mass fraction but rather price and re-usability.

That means that he might be forced to add things that will prolong the life of the booster and MCT.

To compensate he will have to go for a higher thrust on the booster. Maybe that is why he has mentioned 15 mlbs rather then 12 mlbs. He has to have a margin for things always get more heavy in production than originally planed.
« Last Edit: 07/08/2015 11:15 AM by symbios »
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Offline symbios

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Re: MCT Speculation and Discussion Thread 4
« Reply #326 on: 07/08/2015 11:33 AM »
Another interesting discussion is how much of the total delta V budget of the total LV is going into the booster and how much is going into the MCT (second stage)?

I think that there will be a big difference from EELV LV's where most of the fuel is in the booster with a small second stage.

1. You will not be able to land the booster on a barge, it has to come back to a well prepared and solid landing site. This means you cant get to far away (unless you have an island to land on).

2. You will have to have more thrust capacity on the MCT (second stage). But you will need this anyway if you are going to get of Mars.

3. The MCT (second stage) is going to need a high delta-V both for high energy transfer to Mars (3-4 month as stated by Elon) and to get from Mars (delta-v budget of 6-9 km/s). This means you can take advantage of this when staging to LEO.

Is there any flaws in my reasoning or mayor points I have missed?
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Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #327 on: 07/08/2015 02:42 PM »
F9R 1.1 can put approx 13.1mT to LEO with recovery of the first stage. That might improve for 1.2 but we dont have numbers on that yet.
Second stage weights around 3.9mT while F9R weights about 505mT at launch. That makes a gross mass to orbit of about 3.36%.

But that is with KeroLOX all the way. With MethaLOX, alone that number would increase significantly. So yes, I guess it is safe to assume that the mass to orbit fraction will be around 4 to 5 % as you listed in your table. If SpaceX has enough unicorn hair and ferry dust left from their Dragon production, it might even go higher than 5%, simply because there is no payload adapter, fairings or what have you necessary.

Yes.  Thank you for correcting my oversight on computing mass fraction for F9.  It is ~ 3.4% not ~2.7% when you add the mT for stage 2 which reaches LEO.  4% for BFR/MCT may be achievable even with a robust design.
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Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #328 on: 07/08/2015 02:56 PM »
Another interesting discussion is how much of the total delta V budget of the total LV is going into the booster and how much is going into the MCT (second stage)?

I think that there will be a big difference from EELV LV's where most of the fuel is in the booster with a small second stage.

1. You will not be able to land the booster on a barge, it has to come back to a well prepared and solid landing site. This means you cant get to far away (unless you have an island to land on).

2. You will have to have more thrust capacity on the MCT (second stage). But you will need this anyway if you are going to get of Mars.

3. The MCT (second stage) is going to need a high delta-V both for high energy transfer to Mars (3-4 month as stated by Elon) and to get from Mars (delta-v budget of 6-9 km/s). This means you can take advantage of this when staging to LEO.

Is there any flaws in my reasoning or mayor points I have missed?

First, I am not an aerospace engineer (I'm a EE) so I'm not sure I know what I'm doing. 

I have reached the exact same conclusions you cite above. 
Unlike the F9, the BFR/MCT will probably have a 1st stage that stages "low & slow" making boostback to launch site a given.  And yes, the 2nd stage, a.k.a. MCT will after orbital re-fueling need sufficient delta V to escape LEO & transit to Mars in a few months and will also later need sufficient delta v to launch from Mars (having refueled again on the surface) and return to Earth or HEO.  All these requirements dictate a high delta v capability and consequently a larger 2nd stage to 1st stage weight & propellant capacity design point than required for a simple LEO/GEO launcher.  My latest models have a 1st stage with 14-15 million LBS thrust & 7 Raptors (slight overkill) powering the 2nd stage.  The Vacuum Raptors are assumed to have ~610 thousand pounds thrust following the same 1.22 vac/sea level thrust ratios of the Falcon 9.

Think of the MCT as a near SSTO that fell short but gets enough boost from the stage one BFR such that it's good to go.



« Last Edit: 07/08/2015 03:00 PM by philw1776 »
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Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #329 on: 07/08/2015 03:41 PM »
Another interesting discussion is how much of the total delta V budget of the total LV is going into the booster and how much is going into the MCT (second stage)?

I think that there will be a big difference from EELV LV's where most of the fuel is in the booster with a small second stage.

1. You will not be able to land the booster on a barge, it has to come back to a well prepared and solid landing site. This means you cant get to far away (unless you have an island to land on).

2. You will have to have more thrust capacity on the MCT (second stage). But you will need this anyway if you are going to get of Mars.

3. The MCT (second stage) is going to need a high delta-V both for high energy transfer to Mars (3-4 month as stated by Elon) and to get from Mars (delta-v budget of 6-9 km/s). This means you can take advantage of this when staging to LEO.

Is there any flaws in my reasoning or mayor points I have missed?

First, I am not an aerospace engineer (I'm a EE) so I'm not sure I know what I'm doing. 

I have reached the exact same conclusions you cite above. 
Unlike the F9, the BFR/MCT will probably have a 1st stage that stages "low & slow" making boostback to launch site a given.  And yes, the 2nd stage, a.k.a. MCT will after orbital re-fueling need sufficient delta V to escape LEO & transit to Mars in a few months and will also later need sufficient delta v to launch from Mars (having refueled again on the surface) and return to Earth or HEO.  All these requirements dictate a high delta v capability and consequently a larger 2nd stage to 1st stage weight & propellant capacity design point than required for a simple LEO/GEO launcher.  My latest models have a 1st stage with 14-15 million LBS thrust & 7 Raptors (slight overkill) powering the 2nd stage.  The Vacuum Raptors are assumed to have ~610 thousand pounds thrust following the same 1.22 vac/sea level thrust ratios of the Falcon 9.

Think of the MCT as a near SSTO that fell short but gets enough boost from the stage one BFR such that it's good to go.

Yes, this has been my thinking.

A relatively large MCT/upper stage that will take it from "low and slow" booster staging to LEO.  It will refuel there and do a fast transit to Mars.  After refueling on the surface of Mars it will lift off and do a direct return to Earth on a slower transit.  The multiple engines (5 or 7) will give engine out contingency during the various phases of the mission after TMI, as well as during LEO ascent.  (although I'm sure if there were an engine out during LEO ascent, the mission would be scrubbed and the vehicle returned.  But it still should be able to do a safe abort to orbit)

The MCT/upper stage will be large and "fluffy" during EDL, with a large cylindrical cross section, but not overly long so as to not be too tall when landing. 

Not sure it'll need to be quite 14-15Mlbs of thrust on the booster though.  May be possible to get away with less, depending.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #330 on: 07/08/2015 04:00 PM »
If MCT plus 100mT payload is less than 200mT the required 1st stage thrust will drop.  See spreadsheet posted.
If a really innovative design allows 4% mass fraction or better, 1st stage thrust will drop.

Make it so, Elon!

(I just hope that the 2nd stage doesn't have that explode just before staging feature thingy the F9R has)
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Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #331 on: 07/08/2015 04:20 PM »
(I just hope that the 2nd stage doesn't have that explode just before staging feature thingy the F9R has)

Assuming that event has something to do with the helium pressurization, it won't. Both methane and LOX tank will have self pressurization, no helium involved, I am sure.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #332 on: 07/08/2015 04:34 PM »
(I just hope that the 2nd stage doesn't have that explode just before staging feature thingy the F9R has)

Assuming that event has something to do with the helium pressurization, it won't. Both methane and LOX tank will have self pressurization, no helium involved, I am sure.

That would also make a lot of sense so the tanks could be pressurized at all times to provide structural integrity during EDL, landing, etc.   They'd want the tanks pressurized with GCH4 and GOX I'd imagine.  If the pressure got too high, they could collect some and compress into smaller GCH4 and GOX tanks to operate the methalox RCS thrusters. 

Offline CyclerPilot

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Re: MCT Speculation and Discussion Thread 4
« Reply #333 on: 07/08/2015 06:02 PM »


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There have been several MCT designs that had large enough tanks to do the 4 month transfer direct from LEO.  (I think the jump from 4 to 3 months is pretty insane most synods).

If you think those tanks are too large, and want to transfer to a higher energy orbit before TMI, you should just use one extra MCT instead of 4.  That will save hundreds of tons of propellant because you are moving less dry mass.  A reusable SEP tug is another popular option.
Excellent. I have seen a lot of discussion with the SEP option, but not the propellant only option. Do you know where I could find those designs -- on this forum?

In the old MCT threads.

my design

design by Malu from thread 1

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #334 on: 07/22/2015 04:30 AM »
I am not sure where to put this.

Yesterday there was an interview with Hans Koenigsmann in german TV ZDF. He repeated the argument that rockets need to be reusable like airplanes. He added that planes fly for decades and rockets will not fly that much but it should be 100 flights. He did not specify if this would be the Falcon Family or the goal for BFR/MCT.

Offline Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #335 on: 07/23/2015 10:49 AM »
I am not sure where to put this.

Yesterday there was an interview with Hans Koenigsmann in german TV ZDF. He repeated the argument that rockets need to be reusable like airplanes. He added that planes fly for decades and rockets will not fly that much but it should be 100 flights. He did not specify if this would be the Falcon Family or the goal for BFR/MCT.

Do you happen to know the show he said that in? It might be possible to still see it in the zdf mediathek from within Germany.

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Re: MCT Speculation and Discussion Thread 4
« Reply #336 on: 07/23/2015 11:06 AM »
Do you happen to know the show he said that in? It might be possible to still see it in the zdf mediathek from within Germany.

It is available.

http://www.zdf.de/ZDFmediathek#/beitrag/video/2452860/ZDF-heute-journal-vom-21-Juli-2015

The part with Hans Koenigsmann is near the end. Skip through most of it.

Edit: It's at 21:40
« Last Edit: 07/23/2015 11:08 AM by guckyfan »

Offline marcon

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Re: MCT Speculation and Discussion Thread 4
« Reply #337 on: 07/23/2015 11:56 PM »
Do you happen to know the show he said that in? It might be possible to still see it in the zdf mediathek from within Germany.

It is available.

http://www.zdf.de/ZDFmediathek#/beitrag/video/2452860/ZDF-heute-journal-vom-21-Juli-2015

The part with Hans Koenigsmann is near the end. Skip through most of it.

Edit: It's at 21:40

For all non-German speakers:
The segment is really a short introduction of SpaceX to the German news audience and reasonably well done at that. The recent anomaly is shown, with the broken strut in the second stage getting called out. They also show the latest landing attempt and the Merlin engine is mentioned as a core competency, with competitors using Russian rocket engines.   There is a very short interview in German with Koenigsmann as employee No. 4 and now one of the vice presidents. He re-tells the old (for us) comparison with reusable aircraft and mentions the eventual goal of 100 reuses. He also talks about the early decision to do as much in-house and independently from other companies as possible and how they felt as newcomers at Cape  Canaveral.

(just to elaborate on what was mentioned above)
« Last Edit: 07/23/2015 11:58 PM by marcon »

Offline ciscosdad

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Re: MCT Speculation and Discussion Thread 4
« Reply #338 on: 07/24/2015 12:51 AM »
Thank you Marcon

This summary of a foreign language news segment is very welcome. Thank you. Feel free to do this for us at any time.

Offline Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #339 on: 07/24/2015 06:23 AM »
For all non-German speakers:
[...]
(just to elaborate on what was mentioned above)

Thx Marcon. I thought about writing a translated transcript but decided its not worth it since he doesnt mentions anything new. Your summary is way better than either nothing or a transcript. Thx.

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