Author Topic: MCT Speculation and Discussion Thread 4  (Read 621545 times)

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2660 on: 08/27/2016 04:15 AM »
Whilst I agree that Raptors alone would save dry mass, as well as removing the dependency on two successfully serialised miracles, if a Red Dragon could safely land on an unprepared Martian surface, why couldn't a BFS, especially if it resembles a scaled up Dragon?

A Red Dragon's heat shield and motors don't need to survive past touchdown. Safe for it is no destructive motor failures. Gouging up the heatshield and throwing rocks up into motors is fine as long as it can land once. A vehicle that has to take off again and do another re-entry needs an intact heatshield and motors for Earth return.

Sure, the BFS has to take off again, but why are you so sure rocks would be thrown up into the motors? If the Raptors were sufficiently canted, wouldn't they direct ejecta away from the spaceship?
« Last Edit: 08/27/2016 04:16 AM by OneSpeed »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2661 on: 08/27/2016 04:26 AM »
I would hardly consider a set of pressure fed engines firing on touch-down to be a 'miracle', it's a normal and standard part of the Soyuz capsule landing after all.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2662 on: 08/27/2016 04:29 AM »
If the Raptors were sufficiently canted, wouldn't they direct ejecta away from the spaceship?

Launching directly back to Earth is the highest demanding part of the mission¹, those first moments of take-off from the Martian surface will have the highest fuel load and hence require maximum thrust. You can't afford cosine losses of canted engines. Otherwise the stage is overpowered for every other part of the missions, and hence wasting mass.²

¹ Unless Musk has something extra up his sleeve, like Mars orbital refuelling.
² Unless the BFS is built entirely around SuperDraco-like side-mount engines; hence is sized for cosign losses at all stages.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2663 on: 08/27/2016 04:38 AM »
Approaching 30 days from what (we hope) is the big reveal, I thought it a good time to revisit and post revised BFR/MCT speculation before any info leaks out. [...]
Anyone else want to update their speculations?

Shotwell dropped the name Falcon 20 for the BFR. Either a play on Falcon XX (from the early Great-Chart-Of-SpaceX), or signifying the number of Raptor engines. (Or perhaps the first-stage diameter in metres ...what?)

So I'll speculate that we're gonna have to get used to another set of names for both BFR and BFS.

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2664 on: 08/27/2016 05:52 AM »
I would hardly consider a set of pressure fed engines firing on touch-down to be a 'miracle', it's a normal and standard part of the Soyuz capsule landing after all.

Every time a mission critical event in deep space succeeds, it is a (tongue firmly in cheek) 'miracle'. To paraphrase Elon, part of good spaceship design is minimising the number of serialised miracles that need to occur.

If the Raptors were sufficiently canted, wouldn't they direct ejecta away from the spaceship?

Launching directly back to Earth is the highest demanding part of the mission¹, those first moments of take-off from the Martian surface will have the highest fuel load and hence require maximum thrust. You can't afford cosine losses of canted engines. Otherwise the stage is overpowered for every other part of the missions, and hence wasting mass.²

Please bear in mind, I don't expect the engines to be canted for the entire mission. However, it would help optimise SRP if the ability to cant existed, and it would also help for Mars landing and re-launch. If there are five Raptors, as most people seem to believe, then available thrust will greatly exceed the weight of the spaceship in Martian gravity, so you could certainly afford large cosine losses at that point. The real challenge is the ΔV requirement back to Earth. As you suggest, LMO refuelling could help with that.

From the simulations I've done, the point at which maximum thrust is most required is the ascent from Earth to LEO, immediately after BFR MECO, when BFS thrust to weight is less than 1 for a short period. For the rest of the mission, 4 or even 3 Raptors are sufficient, but having 5 provides redundancy.
« Last Edit: 08/27/2016 06:08 AM by OneSpeed »

Online docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2665 on: 08/27/2016 06:21 AM »

Shotwell dropped the name Falcon 20 for the BFR. >

Source?

Or, she could be talking a Dassault Falcon 20 business jet. SpaceX has used Falcon jets in the past.
DM

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #2666 on: 08/27/2016 06:23 AM »
Regarding decelerator devices...
Isn't it somewhat counter-intuitive to put them at the front of the vehicle? Wouldn't such a vehicle normally flip around to have the area of maximum resistance at the back?

With HIAD the center of gravity must be sufficiently low and offset to create lift. It's really no different from a capsule in that respect.

Some of the concepts here which show a long cylinder on a small inflatable aren't realistic IMO.

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2667 on: 08/27/2016 06:45 AM »
Regarding decelerator devices...
Isn't it somewhat counter-intuitive to put them at the front of the vehicle? Wouldn't such a vehicle normally flip around to have the area of maximum resistance at the back?

With HIAD the center of gravity must be sufficiently low and offset to create lift. It's really no different from a capsule in that respect.

Some of the concepts here which show a long cylinder on a small inflatable aren't realistic IMO.

For high velocity Mars EDL, yes, I agree. However, for Earth entry, especially from LEO, lift is less of a requirement. For example IRVE-3, which is quite a long cylinder, has been successfully demonstrated.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2668 on: 08/27/2016 07:19 AM »
I would hardly consider a set of pressure fed engines firing on touch-down to be a 'miracle', it's a normal and standard part of the Soyuz capsule landing after all.

Every time a mission critical event in deep space succeeds, it is a (tongue firmly in cheek) 'miracle'. To paraphrase Elon, part of good spaceship design is minimising the number of serialised miracles that need to occur.


No neither Musk nor any other engineer ever said that every event is a miracle, a real engineer understands the difficulty and likely-hood for success of every event so that an intelligent optimum between cost, safety and performance can be found. 

Pressure fed engines involve valves opening and closing and are orders of magnitude more reliable then a turbo-pump engines like Raptor which we know have to fire several times in mission critical situations.  Protecting thouse big delicate engines is the reason for having touchdown engines, so your trading one risk for another and if you did your homework you reduced total risk. 

Simply saying that less systems and less events is safer is a gross mutilation of the point Musk is trying to make which is about DEVELOPMENT risk not flight risk, but because SpaceX already has an excellent pressure fed engine the development risk is minimal.

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #2669 on: 08/27/2016 07:57 AM »
Regarding decelerator devices...
Isn't it somewhat counter-intuitive to put them at the front of the vehicle? Wouldn't such a vehicle normally flip around to have the area of maximum resistance at the back?

With HIAD the center of gravity must be sufficiently low and offset to create lift. It's really no different from a capsule in that respect.

Some of the concepts here which show a long cylinder on a small inflatable aren't realistic IMO.

For high velocity Mars EDL, yes, I agree. However, for Earth entry, especially from LEO, lift is less of a requirement. For example IRVE-3, which is quite a long cylinder, has been successfully demonstrated.

The CG offset is needed to create lift, but even if you do not need lift the center of gravity must be sufficiently low. The shape of the payload doesn't really matter, as long as the CG is low relative to the cone of the inflatable (to be more precise: There must be sufficient distance between center of gravity and center of pressure for stability).
« Last Edit: 08/27/2016 08:06 AM by Oli »

Offline Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #2670 on: 08/27/2016 08:58 AM »
Ok, I have not posted many speculations in the past, but here it is:

1. Diameter of BFR ~15 m. The shorter and wider it is, the easier it is to land. Also, I assume that the dV of BFR before staging is around the same as for dragon flights to allow for RTLS in all launches.

2. The BFS will be its own second stage and will be designed to maximize vehicle volume rather than utilize HIAD or other deployables for EDL. This has two advantages: Less failure modes and higher Habitat volume once landed. It has the disadvantages of more dry mass and more difficult landing on Mars and Earth. I assume for that reason that the thrust structure of BFS will have 5 to 6 vacuum Raptors. First to minimize gravity losses on Earth assent due to early staging and for redundancy during transit and return.

3. The first version of BFS will be designed to service LEO and GTO missions. Maybe also a trip around the moon for fun (not a serious suggestion). That has many advantages vs. going directly to Mars: Faster turn around time for testing hardware and procedures; no deep space communications; no deep space navigation; no long term propellant storage; no Mars EDL on the first few flights; can generate revenue for GTO and LEO missions. Eventually all above mentioned things have to be solved but this way, some of the near earth problems can be solved before going to Mars.

4. The first BFS/MCT mission to Mars will be a technology demonstrator. It will carry an atmosphere scrabber and maybe able to extract water from the atmosphere for slow fuel production. It might carry some equipment to test the soil for water but no water mining from the ground. I expect that they will concentrate on demonstrate Methane production with the little water from the atmosphere without the goal of filling the tank. LOX production will be on large scale though, with the intent to fill the tank by the time the second BFS/MCT launches from Earth. The energy will come from solar panels. The BFS/MCT will contain enough food and water to supply the crew of the next mission for redundancy. This BFS/MCT will stay on Mars.

5. The second BFS/MCT mission to Mars will have crew of approx 10, one synod after the first successful demonstrator BFS/MCT. The crew BFS/MCT will be the base, no habitat will be brought along. The mission for the first crew will be to return safely one synod later. The task will be to mine the soil for water and execute large scale methane production. The LOX from the first mission will be transferred to the returning BFS/MCT, so no LOX mining equipment will be present. The first landed BFS/MCT will supply any spare parts required for the second BFS, also the food and other consumables transported in the first BFR/MCT are there in case the crew can not leave and has to stay an other synod. The majority of Marstronauts will probably not be SpaceX employees but come from NASA, Roscosmos, ESA, JAXA, India, China, etc. Musk is advertising competition but I think he will not reject the opportunity to sell a seat for ~200 Million a peace to who ever wants to pay.

6. There will be no tanker version of BFR at first. The BFS will be refuelled using a FH Raptor upper stage tanker. The reason for this is, that they dont have rapid re-usability of the BFR just yet. This will be replaced in due time by a tanker BFS but not for the first missions. This has the advantage to supply a Raptor upper stage for FH as the contract with the AF sais and it allows SpaceX to develop a reusable second stage for the Falcon family as well as boost their F9 performance to GTO to allow for RTLS even for GTO F9 missions.

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2671 on: 08/27/2016 09:07 AM »
There must be sufficient distance between center of gravity and center of pressure for stability.

Agreed.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2672 on: 08/27/2016 01:45 PM »
Please bear in mind, I don't expect the engines to be canted for the entire mission.

That was my point. At a time when the vehicle is maximally loaded, needing high thrust, you are deliberately tilting the engines away.

Therefore you need more thrust for this moment, therefore when the engines aren't canted, they are overpowered (oversized) for every other part of the mission.

(As I said in my addendum, it only makes sense if they are perma-canted. I mean, you still get the wasted engine mass, but presumably it is being paid off by some other aspect, such as a simpler re-entry shape.)

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2673 on: 08/27/2016 04:52 PM »
Staging requires more thrust than any other point in the BFS mission. It's in a bigger gravity well, with a full fuel load and a full payload.

A vehicle that can stage during Earth launch with 100t payload can easily liftoff from Mars with a 25t payload.

Offline Chris_Pi

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Re: MCT Speculation and Discussion Thread 4
« Reply #2674 on: 08/27/2016 06:09 PM »
Whilst I agree that Raptors alone would save dry mass, as well as removing the dependency on two successfully serialised miracles, if a Red Dragon could safely land on an unprepared Martian surface, why couldn't a BFS, especially if it resembles a scaled up Dragon?

A Red Dragon's heat shield and motors don't need to survive past touchdown. Safe for it is no destructive motor failures. Gouging up the heatshield and throwing rocks up into motors is fine as long as it can land once. A vehicle that has to take off again and do another re-entry needs an intact heatshield and motors for Earth return.

Sure, the BFS has to take off again, but why are you so sure rocks would be thrown up into the motors? If the Raptors were sufficiently canted, wouldn't they direct ejecta away from the spaceship?

I'm pretty sure there's a link to the article around here somewhere - I do vaguely remember reading it, But I can't find it right now. Very short version is high-thrust motors close to the surface dig really deep holes that throw junk back at the motors. Either have a prepared pad, keep the motors far away, or build something that can take the beating. The Curiosity rover ended up with quite a lot of big gravel all over the top and visible gouges in the ground from the skycrane motors. And those were at the top end of 25 feet of cable and canted somewhat.

I'm wondering about a rover with a solar concentrator to melt a thin layer of surface material in place as a solidified layer.  Presuming this layer didn't crack too badly it should make a good surface for a landing pad or road.
Don't know but intuition says anything less than one m thick under MCT will be flying FOD.

###

ADDED: Your question deserves a better answer so I looked up what NASA scientists have found in their studies. The following statement is a verbatim copy of a summary that addresses the problem. It was a section of the Mars Design Reference Architecture, Addendum A,  published in 2009:

"5.9.1 Summary and recommendations
The predictions and recommendations for a 40-t spacecraft on Mars are described in summary in this section. The next section of the report will then explain in detail how these predictions were obtained.
The engine exhaust plume from a 40-t lander on Mars will blow dust, sand, gravel, and even rocks up to about 7 cm in diameter at high velocity. These ejecta will cause significant damage to any hardware that is already placed on the martian surface within the blast radius. However, the blast radius is modest, extending out to approximately 1 km. The largest debris is accelerated by the plume to lower velocities and, thus, falls closer to the landing site; and the smallest particles are attenuated by the martian atmosphere, also falling closer to the landing. Thus, maintaining the distance of about 1 km between the landing site and any existing surface assets will completely solve this problem for all sizes of debris.
A second concern arises because the exhaust from the large engines will form deep, narrow craters that are directly beneath each of their nozzles, and these craters will redirect the supersonic jet of gas with sand and rocks up toward the landing spacecraft. This has been demonstrated in large-scale engine tests in sand and clay (Alexander, et al, 1966) 25, small-scale experiments (Metzger, 2007) 26, numerical simulations (Liever, et al, 2007) 27, and soil dynamics analysis (see section 5.10.2.3), so there is no question that this will occur. It did not occur in the Apollo and Viking missions because the thrust was lower and/or because the lunar regolith had higher shear strength and less permeability than martian soil. These variables have been taken into consideration in this report. An example of a small-scale test is provided in figure 5-55. The impact of debris striking the lander will be sufficient to cause damage to the lander, possibly resulting in LOM and LOC, and therefore must be prevented. Of special concern is damage to the engine nozzles, because with a multiple-engine lander the debris that is ejected by one engine will be aimed directly at the other engines. One mitigation approach is to add shielding to the spacecraft to block the debris. This will increase the mass of the lander and, therefore, reduce the mass of the payload by approximately 1 t."

Offline the_other_Doug

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Re: MCT Speculation and Discussion Thread 4
« Reply #2675 on: 08/27/2016 07:07 PM »
Someone suggested the possibility that MCT/BFS is just a massively scaled-up Crew Dragon.  While I'm sure other things are happening than that, it did get me to thinking that arranging BFS's Raptors in the same "covered dugout" configuration that Crew Dragon uses for its Super Draco's might be the winning guess.

First, you could just fill the space between the outer and inner hulls with your fuel and oxidizer, inside big bladders.  This might also be one of the boil-off and radiation remediation strategies.  Locating the engines in pods around the circumference also lets you cant them out or swivel them in to the true plus-X direction, as needed.  It lets you have TPS flaps over the Raptor dugouts that let you enter from orbital and super-orbital speeds without damaging the engines, and that can be deployed down to clear the engines for retro-propulsion.  And it lets you have a bottom-end cargo hatch that opens up from the base heat shielding.

It wouldn't surprise me to see such a design.
-Doug  (With my shield, not yet upon it)

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2676 on: 08/28/2016 03:57 AM »
I'm pretty sure there's a link to the article around here somewhere - I do vaguely remember reading it, But I can't find it right now. Very short version is high-thrust motors close to the surface dig really deep holes that throw junk back at the motors. Either have a prepared pad, keep the motors far away, or build something that can take the beating. The Curiosity rover ended up with quite a lot of big gravel all over the top and visible gouges in the ground from the skycrane motors. And those were at the top end of 25 feet of cable and canted somewhat.

Yes, they dig deep holes if the plume is vertical. You can test this for yourself with a high pressure water hose on dirt. As you increase the 'cant' of the hose there comes a point at which a trench rather than a hole is created, and no ejecta comes back towards you. The MSL skycrane was only canted at about 20°, and that was obviously not sufficient to prevent damage to the lander. The plume from a rocket on Mars is surprisingly long and narrow at low altitude, so even though the skycrane engines were at an altitude of 7.5m, four fairly tight craters were formed. So, even if you are prepared to accept the additional mass and complexity, it is not enough to have a secondary set of rocket engines high up on the BFS, the key is how canted they are. For efficient SRP, BFS should be able to cant the engines to at least 45°, which may or may not be sufficient to prevent damage.

An additional concern is the operational risk associated with the creation of a dependency on a second propulsion system. At the point that you might transition to the second system, the Raptors are already running. The chances that they will continue to run until touchdown are extremely good. To shut them down and start a second system at that point sounds to me like the higher risk option. I'd only attempt it if there was no other choice. Anyway, we'll all know what SpaceX have decided in a month, I can't wait.

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #2677 on: 08/28/2016 05:29 AM »
... but why are you so sure rocks would be thrown up into the motors? If the Raptors were sufficiently canted, wouldn't they direct ejecta away from the spaceship?

Fluid dynamics is extremely complex. Gasses swirl in ways that are sometimes impossible to model even on supercomputers. Ever shake one of those lava lamp jars? The variables involved are almost infinite and landing on a surface whose characteristics are not completely known is not something to take lightly.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2678 on: 08/28/2016 05:51 AM »
Someone asked me about my "Shotwell called BFR 'Falcon 20'" comment. I've been trying to find where I read about it, but I didn't keep it, and I can't find it in my browser history. It was a recent, no more than a month ago. But... No sauce means no meat.

Quote
"5.9.1 Summary and recommendations
[...] It did not occur in the Apollo and Viking missions because the thrust was lower and/or because the lunar regolith had higher shear strength and less permeability than martian soil. [...] "

Also the Viking didn't relaunch, and the Apollo LM's had two stages, so the descent stage protected the ascent engine. And neither of them had to protect a heat shield for Earth re-entry.

Staging requires more thrust than any other point in the BFS mission. It's in a bigger gravity well, with a full fuel load and a full payload.

We don't know the final delta-v of the first stage at separation. If it works anything like conventional launches, then the delta-v for a second stage to LEO is much less than the delta-v from Mars surface to direct Earth return.

Of course, the BFS has to land on Earth as well, so who knows what work-around they have for that. Second set of engines? 1atm nozzles? Side-mount canted nozzles?

Someone suggested the possibility that MCT/BFS is just a massively scaled-up Crew Dragon.  While I'm sure other things are happening than that, it did get me to thinking that arranging BFS's Raptors in the same "covered dugout" configuration that Crew Dragon uses for its Super Draco's might be the winning guess.

For example,


(Thirty engine BFR booster, that load-lattice at the top serves as a supersonic brake/stabiliser, while allowing a wider base for the "Roc" heatshield. The "Roc" is shown with cargo hatches open. [Shouldn't the hatches fold down? To become ramps?])

Full image set: https://imgur.com/a/2k10I

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2679 on: 08/28/2016 06:38 AM »
Staging requires more thrust than any other point in the BFS mission. It's in a bigger gravity well, with a full fuel load and a full payload.

We don't know the final delta-v of the first stage at separation. If it works anything like conventional launches, then the delta-v for a second stage to LEO is much less than the delta-v from Mars surface to direct Earth return.

Actually we do. Elon said in an interview that the BFR is just there to compensate for the difference in the two longest poles of the mission. The ΔV to LEO is about 9.5 km/s, including gravity and aerodynamic losses. Mars-Earth is about 7km/s. BFR will provide about 3.5km/s, so if the BFS can generate 7.5 km/s, the remainders are the payloads.

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