Author Topic: MCT Speculation and Discussion Thread 4  (Read 610762 times)

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2420 on: 07/12/2016 05:00 PM »
This is just a guy's opinion, not actual new information. In case that wasn't clear already (the article seems a bit misleading that way).
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Online AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2421 on: 07/12/2016 05:10 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?

Don't think 'rumor' applies to USAF paying for F9/FH upper stage raptor -- that one is fact, at least for a prototype Raptor.  When the upper stage development effort hit the press, there were many speculations about more than one size of Raptor.  Don't think a definitive answer that there will only be one size is in hand yet.

Some of the number-of-engine dots we're trying to connect:
F9/FH upper stage engine might call for smaller engine than BFR booster.
SpaceX has declared the engine quite scaleable. 
Optimum efficiency (maybe thrust-to-weight ratio?) was pronounced by EM to be 550klbf, and result would be 'lots of them.'  (15Mlbf BFR requires 27 at this size.)
Landing engines on Mars would require deep throttling of 550klbf engine.
Nine engines has some heritage (my personal dot).

So, it wouldn't surprise me if September reveal shows more than one engine size.  (Three is my guess.)
« Last Edit: 07/12/2016 05:11 PM by AncientU »
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Offline DanielW

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Re: MCT Speculation and Discussion Thread 4
« Reply #2422 on: 07/12/2016 05:18 PM »
So, it wouldn't surprise me if September reveal shows more than one engine size.  (Three is my guess.)

Booster, upper / MCT, and Orbital maneuvering?

I would personally like to see an electrically pumped methalox orbital maneuvering / attitude engine.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2423 on: 07/12/2016 05:25 PM »
...
Landing engines on Mars would require deep throttling of 550klbf engine.
...

Just to address this point: Merlin throttles to 40%, and MVac to 30%. That's more than deep enough to land 100t payload on Mars even using two 550 klbf engines.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2424 on: 07/12/2016 06:44 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.

Nit: I get ~1,000 Kg/m3 for methalox at a 3.7:1 oxygen ratio.  Maybe ~7% higher with superchilling.
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Online AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2425 on: 07/12/2016 09:30 PM »
...
Landing engines on Mars would require deep throttling of 550klbf engine.
...

Just to address this point: Merlin throttles to 40%, and MVac to 30%. That's more than deep enough to land 100t payload on Mars even using two 550 klbf engines.

How's that?
100tonne payload would  'weigh' 37tonnes on Mars.
550,000lbf (250tf) at 40% is 100tonnes... T/W=2.7 (major hoverslam required)
Two 550klbf engines at 40%... T/W=5.4
Even if spacecraft dry weight is same as payload, T/W is still greater than 1 with a single engine.

Single engine would also have to be directed straight downward, digging a crater and ejecting debris back up toward the descending stage.

To answer DanielW's question above, my three would be booster engines at 1.5-1.6Mlbf, an MCT/BFS engines at 550klbf each, and landing (or second stage F9/FH) at 150-200klbf.  Note: Purely speculating -- we don't have a definitive answer yet...  But this is a speculation thread.
« Last Edit: 07/12/2016 09:31 PM by AncientU »
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Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2426 on: 07/12/2016 09:41 PM »
Even if spacecraft dry weight is same as payload, T/W is still greater than 1 with a single engine.

There is the all important difference between weight and mass. It is the weight that would need to be countered for a hover. But they don't want to hover. It is the mass that needs to be decelerated. That's mostly the same effort as on earth for landing, just with less gravity losses.


Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2427 on: 07/13/2016 12:38 AM »
My expectation is that the main propulsion will 'hover slam' to a full stop at a height of a few dozen meters above the ground and then cut out.  Then the vehicle will touch down on orbital maneuvering engines located higher on the vehicle and canted outward, where they will not impinge on the ground, this will only require the equivalent of a dozen Super Draco engines.

Offline Adriano

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Re: MCT Speculation and Discussion Thread 4
« Reply #2428 on: 07/13/2016 03:35 AM »
It is clear Spacex is mastering first stage usability, but they indicated they are not finding practical bringing the second stages down to earth. ULA on their part are toying with ACES, a second stage spending extended time in orbit, but they have not been explaining how they plan to bring supplies to orbit without creating a glut of second stages up there... If it is not practical to bring back to earth the second stage, the only remaining solution for a reusable second stage is to park it in orbit and perform a suborbital load transfer from the first stage carrying the load and a second stage descending from orbit to pick up the load. Unquestionable the maneuver is tricky and must be performed quickly... Have you seen discussions on this topic somewhere?

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2429 on: 07/13/2016 03:51 AM »
It is clear Spacex is mastering first stage usability, but they indicated they are not finding practical bringing the second stages down to earth. ULA on their part are toying with ACES, a second stage spending extended time in orbit, but they have not been explaining how they plan to bring supplies to orbit without creating a glut of second stages up there... If it is not practical to bring back to earth the second stage, the only remaining solution for a reusable second stage is to park it in orbit and perform a suborbital load transfer from the first stage carrying the load and a second stage descending from orbit to pick up the load. Unquestionable the maneuver is tricky and must be performed quickly... Have you seen discussions on this topic somewhere?
SpaceX didn't think reuse of the second stage was practical when both these conditions are true:
1) high energy, GTO style missions (majority of commercial payloads)
2) kerolox propellants

But other than that, they are fully supportive of 2nd stage reuse, provided at least one of the two above conditions are not met.

They are developing Raptor, which is a methane/LOx engine (higher performance) and thus would potentially allow reuse of a Falcon 9 or Falcon Heavy upper stage built around Raptor. Or, BFR/BFS will have a reusable upper stage as well (BFS).


SpaceX doesn't think 2nd stage reuse is impractical, they just haven't developed a vehicle where it makes sense to attempt it.
« Last Edit: 07/13/2016 03:27 PM by Chris Bergin »
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Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2430 on: 07/13/2016 04:37 PM »
... If it is not practical to bring back to earth the second stage, the only remaining solution for a reusable second stage is to park it in orbit and perform a suborbital load transfer from the first stage carrying the load and a second stage descending from orbit to pick up the load. Unquestionable the maneuver is tricky and must be performed quickly... Have you seen discussions on this topic somewhere?

This is off-topic in this thread, try the General Q&A thread:
http://forum.nasaspaceflight.com/index.php?topic=13543.0
It's also not possible with current technology as the orbiting second stage is travelling about 15,000 miles per hour (24,000 km/hr) faster than the first stage ever goes. It's a problem of speed, not altitude.

Online AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2431 on: 07/13/2016 09:07 PM »
Even if spacecraft dry weight is same as payload, T/W is still greater than 1 with a single engine.

There is the all important difference between weight and mass. It is the weight that would need to be countered for a hover. But they don't want to hover. It is the mass that needs to be decelerated. That's mostly the same effort as on earth for landing, just with less gravity losses.

Good point.  High-G deceleration can be accomplished with two engines or more reasonable decelaration with one for 100 tonnes (mass).  I was considering the terminal phase of landing where they won't be able to handle the payload (crew, for instance) as roughly as they do an inanimate booster during a hover slam -- 10-ish Gs on three engine ASDS landings from GTO launches.   Landing the BFS, as they are practicing with Dragon 2, will require hover or near hovering and gentle touchdown, requiring T/W<1 on a 37tonne (weight) payload.  There may also be a need to do last minute obstacle avoidance which would require lateral translation near the surface.
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #2432 on: 07/13/2016 09:20 PM »
My expectation is that the main propulsion will 'hover slam' to a full stop at a height of a few dozen meters above the ground and then cut out.  Then the vehicle will touch down on orbital maneuvering engines located higher on the vehicle and canted outward, where they will not impinge on the ground, this will only require the equivalent of a dozen Super Draco engines.

This was my conjectured solution to the landing pad problem, at least for the many missions required before a hard-pad is constructed.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2433 on: 07/13/2016 10:51 PM »
BFS will likely mass some 50 to 100 tonnes dry, so reasonable acceleration on two Raptors burnout is certainly possible. At 30% throttle that's 80t thrust per engine and an acceleration of about 1g (for 80t ship+100t payload). Even at WOT two Raptors would only get 3 gees at burnout, which is nowhere near the limit for transient acceleration even with crew.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2434 on: 07/14/2016 06:17 AM »
I like 4 raptors on BFS to allow an engine out capability.  With two pairings of engines on the sides of the vehicle you can run on either diagonal pair of engines in case of failure.  With the engines not in the vehicles center their is room for an extensive cargo-bay at ground level, Propellant is in the nose above the cargo.  Deceleration drag area can be increased with some form of mechanical expandable shield on the nose which folds down umbrella like over the vehicle and forms its conical outer mold line when not in use, entry would be nose first followed by a flip over for landing burn.  The bottom of the vehicle would have landing gear and systems used in orbit such as radiators and solar panels which likewise fold when not in use.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #2435 on: 07/14/2016 06:50 PM »
My expectation is that the main propulsion will 'hover slam' to a full stop at a height of a few dozen meters above the ground and then cut out.  Then the vehicle will touch down on orbital maneuvering engines located higher on the vehicle and canted outward, where they will not impinge on the ground, this will only require the equivalent of a dozen Super Draco engines.

This is certainly a possibility, and something that has been speculated on before.  To have "landing thrusters", likely pressure fed, for more precise throttle response as well as engine out capability, like superdracos and D2.

However, if Raptor has adequate throttle response, and if there are 5 engines in a cross pattern under the BFS, and if Raptor can throttle down to where two Raptors are not over powered to hover and land, and if you plan to land on an area that's level and rocky rather than with deep regolith that would be excavated during a landing....then they could potentially still land on Raptors, and still have engine-out capability and not have issues with the engines kicking up too much debris.
That advantage of this is then you don't need to have a separate propulsion system in addition to your main propulsion, which simplifies things. 
But there are some technical hurdles to overcome.
If MCT masses 100mt, and has 100mt of cargo on it, and maybe 20mt (guess) of residual propellants at touchdown, then that's 220mt total mass.  On Mars that would weigh about 184,300lbs.  So one 550klbs enigne would need to throttle down to about 33.5%...which is very plausible given Merlin-Vac throttle and RD-180 is about that.  But with 5 engines, you want redundancy if you loose your center engine.  So you really need to be able to land on either of the two outer pairs of engines.  That means about 17% throttle.  That's pretty low.  So I think that'd be the primary hurdle to land on the primary engines.
If you can do that, then you have not one, but two engine-out during EDL backup scenarios.  During EDL, once BFS is at terminal velocity and ready to do retro propulsion, it lights all 5 engines briefly.  That way all 5 can be tested to see if there is a failure after the long cruise to Mars.  If all are nominal, then the 4 outer engines are shut down and landing is done on the central engine like the F9 booster.  If there is a failure in the center engine, then all but one of the outer pairs of engines are shut down, and landing is done on that.  If the center engine plus one outer engine fails, then landing is done on the other outer pair. 

Ascent should also be possible with one engine failure, as depending on the weight of the fueled MCT, it should be able to lift off on 3 engines. 

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2436 on: 07/14/2016 07:06 PM »
I like Impaler's idea. I arrived at the same thing independently.

There's an opportunity to allow some abort capability, too, if the terminal landing thrusters are located on a relatively small, separable module on top that the crew is smushed into for launches and/or landings. Could allow aborts for launches on either planet (requires propulsive landing on Mars, of course, not parachute) and landings on either planet.

Of course, there's no actual evidence for such an idea from SpaceX, that's just something I think needn't add much mass and could significantly reduce loss of crew probability.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2437 on: 07/14/2016 11:32 PM »
Some form of orbital maneuvering engine is a must, you can't use the main propulsion system for thouse kinds of maneuvers.  Draco (not super) might do the very fine maneuvers for docking and such but you will still want something stronger for a de-orbit burn.

Lets compare to shuttle, it massed around 75 mt on orbit and had OMS with 53 kN thrust.  Scale up to a likely 200+ mt mass in orbit and your looking at the thrust of 2 Super Draco engines for a de-orbit burn.  A Raptor engine would need to throttle down to 6% to give that thrust. 

Still we would be looking at about a 6 fold increase over a reasonable orbital maneuvering system so it's not free.  Their may be additional benefits to these engines, their position high on the vehicle gives them huge leverage on the vehicle useful in EDL if a flip over is needed.

Online matthewkantar

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Re: MCT Speculation and Discussion Thread 4
« Reply #2438 on: 07/14/2016 11:42 PM »
I agree there will need to be some sort of maneuvering system, but if what Elon has said holds up, there will be no need for a deorbit burn, MCT will be designed to hit Earth's or Mars' atmosphere at interplanetary velocity. A number of distributed Draco sized engines may suffice.

Matthew

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2439 on: 07/15/2016 12:01 AM »
Apollo used a 90 kN engine for orbital maneuvers of a 30 tonne CSM  ::) that would work pretty well at hovering on Mars.

And yes, BFS will need deorbit maneuvers. That how it gets out of orbit... not every mission will be to escape or free-return.

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