Author Topic: MCT Speculation and Discussion Thread 4  (Read 506308 times)

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2400 on: 07/11/2016 07:41 PM »
It seems very unlikely that SpaceX would launch horizontally from Mars. Nor does horizontal engines on an upper stage make sense.

You will have to help me on the basis for those assertions. With a thin martian atmosphere there would not be much penalty. Throttling would certainly have to be more responsive than vertical launching but that is not a known show stopper.

You don't have horizontal engines on an upper stage. You have an upper stage that you are launching sideways. You just light the engines on one side of it slightly before the other and it rotates 90 degs. It makes as much sense as a swan dive maneuver on a landing craft.
Except there's still a lot of drag for a second stage, especially one like the BFS (which probably stages fairly early).

The shuttle shape is going to be most aerodynamically stable in the reentry configuration, so launching from Mars (or after staging) horizontally would be like trying to fly a jet airplane backwards. Possible, perhaps, but definitely not ideal. Especially with those winglets.

Offline DanielW

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Re: MCT Speculation and Discussion Thread 4
« Reply #2401 on: 07/11/2016 07:44 PM »
It seems very unlikely that SpaceX would launch horizontally from Mars. Nor does horizontal engines on an upper stage make sense.

You will have to help me on the basis for those assertions. With a thin martian atmosphere there would not be much penalty. Throttling would certainly have to be more responsive than vertical launching but that is not a known show stopper.

You don't have horizontal engines on an upper stage. You have an upper stage that you are launching sideways. You just light the engines on one side of it slightly before the other and it rotates 90 degs. It makes as much sense as a swan dive maneuver on a landing craft.
Except there's still a lot of drag for a second stage, especially one like the BFS (which probably stages fairly early).

The shuttle shape is going to be most aerodynamically stable in the reentry configuration, so launching from Mars (or after staging) horizontally would be like trying to fly a jet airplane backwards. Possible, perhaps, but definitely not ideal. Especially with those winglets.

I don't propose the winglets though. It is also possible to have shapes that are bi-stable.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #2402 on: 07/11/2016 10:29 PM »
Landing horizontally seems like a terrible idea, not at all SpaceX-y.

DC-X demonstrated swan dive maneuver already. We don't need this horizontal landing nonsense.

Agreed. 

Not to mention there's additional issues with horizontal thrusting (which it'd have to do also for TMI, EDL, take off, TEI, and Earth EDL.)  Primarily those being bid doors in the heat shield that need to not just pop open once (like the Space Shuttle or X-37B), but open and close multiple times during a mission.  If one doesn't open or close just right, then you have a problem.

Plus you probably can't use cylindrical tanks, and would have to use spherical tanks, since the propellant will have to drain from the ventral side which would be a problem with a long cylindrical tank, unlike vertical propellant feeding for vertical thrusting.
I think this concept does use spherical, or some odd shape to allow propellant to drain down the ventral side in order to feed the engines.

There's probably a few other issues I'm not thinking of right now. 

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #2403 on: 07/11/2016 10:35 PM »
The booster doesn't go to orbit, and there's no second stage so the shuttle must be it's own 2nd stage (confirmed by the trajectory graphic). At staging it's clearly pointed nose-downrange, but there are apparently no engines in the tail... so does it point it's dorsal side downrange and use belly engines? That seems questionable as there are still considerable aero loads at staging. Or do the engines pivot out and point back enough to continue vertically? There's no mechanism described for that.

The booster clearly lands vertically, but I don't see any indication that the shuttle does.

Yes, at booster staging, the ship would need to pitch 90 degrees, open it's belly engine doors (which would need to be closed during launch for aerodynamics) and then begin thrusting up to LEO.  Would probably need to be high enough so that any aerodynamic issues would be minimal for that.  At that point it can keep the doors open for TMI.  It would need to close them for Mars atmospheric entry, and then open them up again at terminal velocity to propulsively brake and then land.  .  They could stay open on the surface until launch, and stay open for TEI.  They'd need to close again for Earth atmospheric entry, and then open again at terminal velocity for propulsive landing and braking.

It also provides a pretty un-aerodynamic surface for Mars Launch like that.  But maybe that's not too much of a problem in the thin Mars atmosphere?  Not sure.


Online lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #2404 on: 07/11/2016 11:54 PM »
The booster doesn't go to orbit, and there's no second stage so the shuttle must be it's own 2nd stage (confirmed by the trajectory graphic). At staging it's clearly pointed nose-downrange, but there are apparently no engines in the tail... so does it point it's dorsal side downrange and use belly engines? That seems questionable as there are still considerable aero loads at staging. Or do the engines pivot out and point back enough to continue vertically? There's no mechanism described for that.

The booster clearly lands vertically, but I don't see any indication that the shuttle does.

Yes, at booster staging, the ship would need to pitch 90 degrees, open it's belly engine doors (which would need to be closed during launch for aerodynamics) and then begin thrusting up to LEO.  Would probably need to be high enough so that any aerodynamic issues would be minimal for that.  At that point it can keep the doors open for TMI.  It would need to close them for Mars atmospheric entry, and then open them up again at terminal velocity to propulsively brake and then land.  .  They could stay open on the surface until launch, and stay open for TEI.  They'd need to close again for Earth atmospheric entry, and then open again at terminal velocity for propulsive landing and braking.

It also provides a pretty un-aerodynamic surface for Mars Launch like that.  But maybe that's not too much of a problem in the thin Mars atmosphere?  Not sure.
I've looked up the original article of version II of Heidmann's design.  The ship flies up after separation in the horizontal position using the belly landing nozzles.  The back doors stay closed.  There are no additional thrusters.
The author proposes that aerodynamic drag at separation altitude is low enough to allow this, as is the drag at Take-off from Mars.
So it also takes off from Mars with the ship in the horizontal position, as per the picture.
The authors admits the additional doors required for the engine bays might be a serious design problem, but sees this as a details to be worked out in time.

« Last Edit: 07/12/2016 12:00 AM by lamontagne »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2405 on: 07/12/2016 12:19 AM »
Vertical landing after a ventral entry certainly seems feasible, that's not what I was questioning. I don't see any indication that Heidmann considered a vertical landing at all for this spaceship: all his articles show horizontal landings.
...
Do we need a new thread for Heidmann's design? Because this thread is supposed to be about MCT, and it may get confusing if we're talking about a /specific/ (and fairly fleshed-out) speculation of someone's own imagining in a thread that's supposed to be about SpaceX's actual MCT.

???  Have you read the title of the thread, it is indeed a SPECULATION thread, their will not be an 'actual' MCT thread until Musk spills the beans publicly and we can comment on the plan, even then most comments will be speculation as to the unknown details and which details will be changed in the course of production.  Commenting on the Heidmann speculation is perfectly on topic.

That said I find this Richard Heidmann speculation line very weak in it's use of horizontal landing, vertical is almost certainly the path SpaceX will choose given their growing expertise in the area and the mechanical efficiency of cylinders under axial compression.

Their are some elements I like, notably the recognition that large cargo-holds are necessary and they need to be close to the ground to be unloaded safely and efficiently (and such cargo-holds are compatible with vertical landing).  Also the use of a reasonable 12.5 m diameter second stage, though I foresee the first stage being the same diameter rather then the N-1 colossus he foresees.
« Last Edit: 07/12/2016 12:26 AM by Impaler »

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2406 on: 07/12/2016 07:18 AM »
Have you read the title of the thread, it is indeed a SPECULATION thread,

It is supposed to be a MCT speculation thread. Not a generic speculation thread on just any Mars architecture. We have the Mars section for that. Things here should be based on what was said by SpaceX about MCT.

Offline JamesH65

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Re: MCT Speculation and Discussion Thread 4
« Reply #2407 on: 07/12/2016 08:19 AM »
Have you read the title of the thread, it is indeed a SPECULATION thread,

It is supposed to be a MCT speculation thread. Not a generic speculation thread on just any Mars architecture. We have the Mars section for that. Things here should be based on what was said by SpaceX about MCT.

Which is practically nothing. So, generic speculation would appear to be a valid approach.

Offline kch

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Re: MCT Speculation and Discussion Thread 4
« Reply #2408 on: 07/12/2016 08:20 AM »
Vertical landing after a ventral entry certainly seems feasible, that's not what I was questioning. I don't see any indication that Heidmann considered a vertical landing at all for this spaceship: all his articles show horizontal landings.
...

Do we need a new thread for Heidmann's design? Because this thread is supposed to be about MCT, and it may get confusing if we're talking about a /specific/ (and fairly fleshed-out) speculation of someone's own imagining in a thread that's supposed to be about SpaceX's actual MCT.

Yes, please!  :)

Offline mfck

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Re: MCT Speculation and Discussion Thread 4
« Reply #2409 on: 07/12/2016 09:37 AM »
ISTM that accommodating two main load axis (for a 100 mT to Mars surface vehicle) that are perpendicular one to another is in severe conflict with SX known optimizations for PMF and cost. That alone seems to make the Heidmann (HCT) design off-topic for any MCT discussion, before even counting L2 info available (which further invalidates HCT as possible SX design). YMMV

Offline DanielW

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Re: MCT Speculation and Discussion Thread 4
« Reply #2410 on: 07/12/2016 01:13 PM »
ISTM that accommodating two main load axis (for a 100 mT to Mars surface vehicle) that are perpendicular one to another is in severe conflict with SX known optimizations for PMF and cost. That alone seems to make the Heidmann (HCT) design off-topic for any MCT discussion, before even counting L2 info available (which further invalidates HCT as possible SX design). YMMV

Heidmann aside, There is nothing off-topic about an architecture that lands and flies "horizontally." Unless you are proposing basically a massive dragon capsule with the heatshield on the bottom you need to support side load paths for 4-5gs anyway. I don't see anyway that a super-dragon style capsule will get enough drag to slow down enough. It almost has to be side entry. Thus your assertion that there is some SX way of doing things that precludes any discussion of a two load path structure is not based on physics.

I am also in L2, you are welcome to pm me any L2 info that contradicts such an architecture. There simply isn't any that I have seen.

I am not saying that a side flying / landing craft is my preferred architecture. I think a big pancake of a capsule would be better for all phases of the mission other than earth ascent. Just not sure how you could ever get such a thing into space.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2411 on: 07/12/2016 01:36 PM »
I was simply suggesting that if we're starting to talk for several pages about one person's specific vision for what MCT is like, and if it's highly fleshed out, then it's probably best to have a dedicated thread. That doesn't mean it's off-topic here, but that it might be better in its own thread. Geez y'all.
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Offline DanielW

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Re: MCT Speculation and Discussion Thread 4
« Reply #2412 on: 07/12/2016 02:12 PM »
I was simply suggesting that if we're starting to talk for several pages about one person's specific vision for what MCT is like, and if it's highly fleshed out, then it's probably best to have a dedicated thread. That doesn't mean it's off-topic here, but that it might be better in its own thread. Geez y'all.

Agreed. I just want to clarify that some aspects of his architecture are things I have been thinking independently without any knowledge that someone else proposed launching into LEO sideways. If we are going to discuss specifics of his plan then yes another thread is appropriate. But I am not terribly interested in his over-all plan. We need to divorce the idea of a horizontally laid-out craft from any specific person or proposal. I honestly think it addresses many issues, not without draw-backs of its own. But I do think we have collectively let the earth ascent part of the mission drive what we think a proper space-ship should look like.
« Last Edit: 07/12/2016 02:13 PM by DanielW »

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2413 on: 07/12/2016 02:30 PM »
...
I don't see anyway that a super-dragon style capsule will get enough drag to slow down enough.
...

If Red Dragon works, a larger vehicle with the same entry velocity, lift/drag ratio, ballistic coefficient, propellant mass fraction, and engine efficiency should work just as well. Because of the cube-square law it will have to be less dense, but since it's largely made of nearly empty propellant tanks that shouldn't be a major issue.

Trading propellant mass for engine efficiency (Raptor 380s vs. SuperDraco 240s) also helps hit the required density reduction.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2414 on: 07/12/2016 03:23 PM »
I was simply suggesting that if we're starting to talk for several pages about one person's specific vision for what MCT is like, and if it's highly fleshed out, then it's probably best to have a dedicated thread. That doesn't mean it's off-topic here, but that it might be better in its own thread.

This thread has quietened down lately, so a few pages of renewed activity will hardly kill us. (And judging by the last time people discussed Heidmann's articles, it won't go much beyond a few pages.)

OTOH, a separate thread will die out within a couple of weeks and drop off the first page within a month, then it won't be here with the rest of the MCT speculation - where it belongs.

OTGH, discussion of Heidmann's design has already peaked and is moving onto a generic discussion about landing strategies. So a new thread would be redundant unless you can convince a admin-privileged mod to move the last couple of pages to the new thread; at which point that thread would go off-topic as people spin off into generic discussion as they are doing here.

Online the_other_Doug

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Re: MCT Speculation and Discussion Thread 4
« Reply #2415 on: 07/12/2016 03:29 PM »
...
I don't see anyway that a super-dragon style capsule will get enough drag to slow down enough.
...

If Red Dragon works, a larger vehicle with the same entry velocity, lift/drag ratio, ballistic coefficient, propellant mass fraction, and engine efficiency should work just as well. Because of the cube-square law it will have to be less dense, but since it's largely made of nearly empty propellant tanks that shouldn't be a major issue.

Trading propellant mass for engine efficiency (Raptor 380s vs. SuperDraco 240s) also helps hit the required density reduction.

Yes, but -- how wide would such a scaled-up Dragon capsule be at the base, if it's got to be designed to carry at least 100 people, along with all the stuff (like food, water and life support) they will need for three to six months?

I think simply scaling up the dimensions of a Dragon to where it has the interior space available for the actual stated mission requirements would make the base diameter in the hundreds of meters.  How ya gonna fit that on any booster, much less one with the most-commonly-speculated (see L2) width of the BFR?

And if you change the basic shape, you inevitably have to go through a lot of design work to achieve similar lift/drag characteristics, etc.  In other words, BFS is gonna have to be designed from scratch, it won't take advantage of Dragon's shape or flying characteristics as a starting point.
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Online Lar

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Re: MCT Speculation and Discussion Thread 4
« Reply #2416 on: 07/12/2016 03:34 PM »
I was simply suggesting that if we're starting to talk for several pages about one person's specific vision for what MCT is like, and if it's highly fleshed out, then it's probably best to have a dedicated thread. That doesn't mean it's off-topic here, but that it might be better in its own thread. Geez y'all.
Yes.

If there are enough posts any mod would be happy to fork it off for you. Not sure there were or if it has legs but... PM any mod if you think so.  In case you can't tell I tend to agree with Paul451 regarding forking... not warranted. And all this meta discussion is meta.
« Last Edit: 07/12/2016 03:40 PM by Lar »
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2417 on: 07/12/2016 04:24 PM »
Yes, but -- how wide would such a scaled-up Dragon capsule be at the base, if it's got to be designed to carry at least 100 people, along with all the stuff (like food, water and life support) they will need for three to six months?

I think simply scaling up the dimensions of a Dragon to where it has the interior space available for the actual stated mission requirements would make the base diameter in the hundreds of meters.  How ya gonna fit that on any booster, much less one with the most-commonly-speculated (see L2) width of the BFR?

And if you change the basic shape, you inevitably have to go through a lot of design work to achieve similar lift/drag characteristics, etc.  In other words, BFS is gonna have to be designed from scratch, it won't take advantage of Dragon's shape or flying characteristics as a starting point.

Let's do at least a first order pass at actually calculating this. If BFR's diameter is 15m, BFS can easily be at least 15 to 16m.  Dragon's OML including the nose but not the trunk encloses about 25 m^3 with a major radius of 1.85 meters. Volume increases with size as r^3, so a 15 meter Dragon would enclose about 1,875 m^3 and a 16m version about 2,050 m^2

Estimates of necessary volume per person and propellant mass vary, but usually range somewhere from 5 to 15 m^2 per person and 300-1200 tonnes of propellant. Averaging those gets 10 m^3 per person and 750t propellant, or 1000 m^2 habitable volume and 915 m^3 tank volume (100 people, 820 kg/m^3 methalox). That totals 1915 m^3, or pretty close to what a 15 to 16m Dragon would enclose.

Obviously, that's only enclosed volume and not all the space can be utilized efficiently, and engines etc. all take up volume. But volume shouldn't be a show-stopper considering that it's probably feasible to launch up to a 20m diameter (4,500 m^3 volume) object on top of BFR.

Online AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2418 on: 07/12/2016 04:34 PM »
I believe this is first available translation of recent French article:

Quote
Mars Colonisation Transportation: Project Revelation Before Rumours

https://medium.com/@YawLife/mars-colonisation-transportation-project-revelation-before-rumours-ac7c33fcd40a#.d8140tfsh

Translation isn't great...

One interesting tidbit, though:
Quote
But a noise is that SpaceX would work on a Raptor 700 Tf !

Not sure of rumor source for an F-1 class engine, but certainly would simplify first stage design if nine 1.5-1.6Mlbf engines were used on booster.
"If we shared everything [we are working on] people would think we are insane!"
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Offline DanielW

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Re: MCT Speculation and Discussion Thread 4
« Reply #2419 on: 07/12/2016 04:46 PM »
One interesting tidbit, though:
Quote
But a noise is that SpaceX would work on a Raptor 700 Tf !

Not sure of rumor source for an F-1 class engine, but certainly would simplify first stage design if nine 1.5-1.6Mlbf engines were used on booster.

Not sure how this jives with the news that the AirForce is paying for a raptor based upper for F9 and FH. 2 engines or rumor wrong?

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