BFR concepts and ideas thread.

[K^2]

9 hours ago, Cassel said:

Why they did not use diesel? The price difference diesel-kerosene is probably big?

@KSK mentioned a few good points. I'd add that kerosene has significantly lower viscosity than diesel, which makes a difference when you consider how much energy goes into turbopump operation. Especially, in rockets which pump fuel through channels in the nozzle bell for cooling. Diesel also tends to produce a lot more soot in combustion, but I don't know if that's going to be a factor for rockets. Obviously, carbon buildup on the nozzle is bad, since that can cause instabilities in the flow, which will reduce efficiency at best, and lead to destructive vibrations at worst. But whether it can actually happen at typical combustion chamber temperatures found in kerlox rockets, I have no idea.

[mikegarrison]

1 hour ago, K^2 said:

@KSK mentioned a few good points. I'd add that kerosene has significantly lower viscosity than diesel, which makes a difference when you consider how much energy goes into turbopump operation. Especially, in rockets which pump fuel through channels in the nozzle bell for cooling. Diesel also tends to produce a lot more soot in combustion, but I don't know if that's going to be a factor for rockets. Obviously, carbon buildup on the nozzle is bad, since that can cause instabilities in the flow, which will reduce efficiency at best, and lead to destructive vibrations at worst. But whether it can actually happen at typical combustion chamber temperatures found in kerlox rockets, I have no idea.

Diesel producing soot is not really a property of the fuel so much as how a diesel cycle engine works. However, it is known that in gas turbine engines the aromatic content of the fuel is correlated with the amount of particles in the exhaust -- apparently those carbon rings just can't break down quite fast enough to get complete combustion like the chain molecules can.

[MinimumSky5]

I think this is why RP-1 was developed, they needed primarily to get rid of the turpentines and aromatics in the fuel. I do know that petrol (gasoline) was dropped as a fuel early on, because it was much more difficult to get a nice, soot free burn than with kerosene. This may not be a factor anymore, though, as I'm talking about the era of Goddard here! The safety factors of petrol are very significant, as well. Spill kerosene on the ground, and even an ox-acetylene torch won't ignite it, as it needs to be aerosolised before ignition can occur. 

[satnet]

A couple people have alluded to Ignition!, which you can download here: https://archive.org/details/ignition_201612. It is definitely worth reading if you have any interest in rocket propellant chemistry or enjoy explosions. Chapters 3 and 8 are probably the most relevant.

Basically the reason gasoline, diesel, etc. are not used is because rockets are unforgiving beasts with refined tastes and a nasty temper. You can get gasoline to work (probably diesel too) and we probably don't need to be as refined as RP-1, but the expense of the payload and rocket justifies a fuel you can be reasonably sure won't destroy them. As @MinimumSky5 said RP-1 was developed to be remove the impurities, but still have something petroleum based so it was reasonably attainable and affordable after many failed attempts to use gasoline or jet fuel (the military wanted something they already had on hand, but reluctantly had to concede that it wasn't going to happen).

Probably the most relevant passage is in chapter 8 talking about JP-4 (jet fuel) with liquid oxygen:

"But there were troubles. The sloppy specifications for JP-4 arose to
haunt the engineers. It burned all right, and gave the performance it
should —but. In the cooling passages it had a tendency to polymerize
(you will remember that the specifications allowed a high percentage
of olefins) into tarry substances which slowed the fuel flow, whereupon
the motor would cleverly burn itself up. And in the gas generator
it produced soot, coke, and other assorted deposits that completely
fouled up the works. And, of course, no two barrels of it
were alike. (Also, believe it or not, it grows bacteria which produce
sludge!)".

Also in chapter 3 he talks about the issues with both gasoline and JP-4 with acid (generally some nitric acid variant):

"It (JP-4) was acid-gasoline
all over again —a coughing, choking, screaming motor, that usually
managed to reduce itself to fragments, and the engineers to frustrated
blasphemy. Everything was tried to make the stuff burn smoothly,
from catalysts in the acid down —or up —to voodoo."

[Mad Rocket Scientist]

He also calls JP-4 "just about the most permissive specifications to appear since the days of Coal Oil Johnny Rockefeller the First."

Also, the quote on burning JP-4 with nitric acid continues:

"Everything was tried to make the stuff burn smoothly, from catalysts in the acid down — or up — to voodoo. The farthest-out expedient that I heard of was tried at Bell Aeronautic. Somebody had the bright idea that the sonic vibrations of a rocket motor might promote combustion. So he made a tape recording of the sound of a running motor and played it back at the interacting propellants in the hope that they might be shaken — or shamed — into smooth combustion."

Edited September 15, 2018 by Mad Rocket Scientist

[Xd the great]

The point of using methane is that it can be produced on mars. 

[mikegarrison]

On 9/16/2018 at 12:59 AM, Xd the great said:

The point of using methane is that it can be produced on mars. 

Only if it turns out there really is available water. Otherwise you have a little problem getting all those hydrogen atoms.

[Xd the great]

8 minutes ago, mikegarrison said:

Only if it turns out there really is available water. Otherwise you have a little problem getting all those hydrogen atoms.

I thought they found water already.

[mikegarrison]

10 minutes ago, Xd the great said:

I thought they found water already.

I believe they think they have found evidence that water once existed on the surface. Erosion features, etc. But as far as I know, except for the poles nobody has found anything but traces of water. It could still be there, or maybe it's all gone except at the poles.

I guess they have found some places imaged from orbit that look like permafrost or maybe where glaciers are buried close to the surface, but I don't think anyone has proven this.

Edited September 17, 2018 by mikegarrison

[magnemoe]

15 minutes ago, mikegarrison said:

I believe they think they have found evidence that water once existed on the surface. Erosion features, etc. But as far as I know, except for the poles nobody has found anything but traces of water. It could still be there, or maybe it's all gone except at the poles.

I guess they have found some places imaged from orbit that look like permafrost or maybe where glaciers are buried close to the surface, but I don't think anyone has proven this.

Its also strong indications of permafrost being common, if so it should be fairly easy to mine, drill an hole and apply heat. 

On 9/16/2018 at 9:59 AM, Xd the great said:

The point of using methane is that it can be produced on mars. 

its also in an sweet spot, its not as good as hydrogen but it also handles pretty much like LOX so if you use LOX why not also use methane. 

[mikegarrison]

I sure wouldn't land a rocket near what looks like permafrost from orbit without making sure it really was a good source of water. Not if that water was my only way off Mars, anyway.

[tater]

7 minutes ago, mikegarrison said:

I sure wouldn't land a rocket near what looks like permafrost from orbit without making sure it really was a good source of water. Not if that water was my only way off Mars, anyway.

Not to mention what a crappy landing pad something makes when it's X% made of ice, and you're landing a rocket on it.

A tall, tippy rocket.

 

[Terwin]

4 hours ago, mikegarrison said:

Only if it turns out there really is available water. Otherwise you have a little problem getting all those hydrogen atoms.

I thought they had pictures of what appears to be the exposed face of basically a glacier with dirt on top of it, and these mostly pure massive chunks of Ice were identified in multiple locations, including at least one prospective landing site. 

This looks to be related to the article I read:

https://en.wikipedia.org/wiki/Glaciers_on_Mars

Edited September 17, 2018 by Terwin
added wiki link

[mikegarrison]

19 minutes ago, Terwin said:

I thought they had pictures of what appears to be the exposed face of basically a glacier with dirt on top of it, and these mostly pure massive chunks of Ice were identified in multiple locations, including at least one prospective landing site. 

This looks to be related to the article I read:

https://en.wikipedia.org/wiki/Glaciers_on_Mars

From orbit, yeah, they have pictures of what they think might be dirt-covered glaciers. But you need boots (or robot wheels) on the ground to know for sure.

[Terwin]

46 minutes ago, mikegarrison said:

From orbit, yeah, they have pictures of what they think might be dirt-covered glaciers. But you need boots (or robot wheels) on the ground to know for sure.

The viking 2 lander observed nighttime condensation(~ frozen dew) for 200 days in a row, so there is also atmospheric water available.  

(If we are pulling out the CO2, pulling out the water should be trivial)

 So even if it means processing a lot more martian atmosphere than would be needed strictly for the CO2, just processing the atmosphere would eventually provide everything that is needed for fuel.(night-time humidity often gets to 80%-100% in some regions, and pulling the water out of the air should be much less energy-intensive than separating out CO2 would be)

https://www.space.com/29857-mars-humidity-alien-life.html

Edited September 17, 2018 by Terwin
martian not martial

[kerbiloid]

Spoiler

They just must place their landng sites inside the channels.

 

[NSEP]

It would be quite nice for BFR to do some exploration/test missions before they do their cargo and eventually manned missions. It would suck to rely on water and find out there isn't any water present.

Like have 2 Mars Landers and have 1 Phobos/Deimos and orbital mission (all of them unmanned of course). These could be 3 seperate BFS spacecraft or 3 different spacecraft inside one BFS. The Mars landers would have a rover that will explore the surface. These could be small Opportuniy/Spirit like rovers or maybe a giant monster labaratory being powerd by a massive solar panel (maybe even a kilopower reactor), depending on if it landed with a different smaller spacecraft made to fit inside BFS or one BFS as a whole.

Also bring extra food and toilets for future Astronauts.

[mikegarrison]

At least at first, the only thing that would be really safe is if the landing ship contains enough fuel to be self-sufficient and return back to orbit (assuming a Apollo-style orbital rendezvous mission profile). That's the only way to minimize the risk of stranding people on the surface.

A near alternative would be to land a robotic fuel factory first, then make and store all your fuel (and oxidixer) in advance. Assuming you have reasonable confidence in the precision of your landing, you can then know the return fuel is there before you land.

Planning to manufacture your return fuel after you land is just risky AF.

[tater]

The lunar landing talk for BFS was explicit that with orbital refilling, it can sortie to the lunar surface, and back to Earth.

The idea was that it would refill in a highly elliptical orbit as I recall. A possible simplification there is to fill a tanker all the way, then send BFS to the elliptical orbit with the full tanker meeting it (rather than doing 4-5 rendezvous in that eccentric orbit, which is logistically complex).

I'm unsure what the current situation is, because the 2017 version had vacuum engines, and the extra 24 seconds of I_sp matters.

So when it's lunar base building time, then they might well need to sub out some of the cargo pods with vac Raptors.

[sh1pman]

1 hour ago, tater said:

So when it's lunar base building time, then they might well need to sub out some of the cargo pods with vac Raptors.

So, was the payload to LEO decreased from 150 to 100 because of no vac Raptors? Or is it the added dry mass from the fins? 

[ment18]

12 minutes ago, sh1pman said:

So, was the payload to LEO decreased from 150 to 100 because of no vac Raptors? Or is it the added dry mass from the fins? 

The presentation said >100 tons, Which I interpreted as less than 150, but they dont really know specifics. Someone earlier had math suggesting that the lack of vacuum isp reduced payload by about 20tons.  I also think spacex had found their payloads to be less dense than expected, so have focused less on payload mass.  Musk also acted much less sure than he has before, so I think they've started to have difficulties in making the vehicle as light as they want.  So all of the above, probably around 125 tons.

[tater]

16 minutes ago, sh1pman said:

So, was the payload to LEO decreased from 150 to 100 because of no vac Raptors? Or is it the added dry mass from the fins? 

Mostly the single engine type, I bet.

I estimated (based on the 2017 propellant and dry masses) that the difference in dv for the 2017 BFS using all SL engines was ~500 m/s. That was just lofting the 85 ton dry mass. (it was 9.6 vs 10.1 km/s).

For that same vehicle, but using a 150 tonne cargo added to the dry mass, the dv is ~6 km/s with SL engines, and ~6.5 km/s with vac engines. It's closer to 6.8 km/s with just 100 tonnes cargo. Max F9 booster velocity at sep is something like 2.7 km/s which means S2 (BFS) likely needs something similar with the spacecraft providing north of 5 km/s to reach LEO (not counting gravity losses), plus it then needs margin to land.

The fins obviously also come with some penalty.

12 minutes ago, ment18 said:

The presentation said >100 tons, Which I interpreted as less than 150, but they dont really know specifics. Someone earlier had math suggesting that the lack of vacuum isp reduced payload by about 20tons.  I also think spacex had found their payloads to be less dense than expected, so have focused less on payload mass.  Musk also acted much less sure than he has before, so I think they've started to have difficulties in making the vehicle as light as they want.  So all of the above, probably around 125 tons.

Yeah, this would not surprise me at all.

85 tons was supposedly with creep, too, lol.

The mas vs volume issue is even a thing now. The only payload dense enough that you can actually bring it to space to maximize the cargo delivered to orbit by most rockets is propellant. Nothing else is dense enough.

[sh1pman]

16 minutes ago, tater said:

The mas vs volume issue is even a thing now. The only payload dense enough that you can actually bring it to space to maximize the cargo delivered to orbit by most rockets is propellant. Nothing else is dense enough.

I think that the only times when BFS is going to be launched with max payload mass, will be tanker launches, to refuel other ships somewhere. And tankers will definitely need vac Raptors. 150t vs 100t difference means that you need 33% fewer launches to fully refuel a ship. Tankers don’t need aft cargo anyway.

[tater]

That will require a dev cycle to make the vac Raptor, however. Sounds like most of the reason for 1 engine is streamlining dev and production.

[sh1pman]

2 minutes ago, tater said:

That will require a dev cycle to make the vac Raptor, however. Sounds like most of the reason for 1 engine is streamlining dev and production.

For the initial launches, yes. But when (and if) it reaches the point when new BFRs are no longer needed (and existing ones fly a hundred times per year), resources can be diverted to the vac Raptor. Having to make 7 launches instead of 10 to refuel a single Mars ship can make it worth the effort.