BFR concepts and ideas thread.

[wumpus]

17 hours ago, Ultimate Steve said:

I can't imagine liquid hydrogen being used to fuel fighters and bombers any time soon.

Oddly enough, I'd have to assume that the original mission (intercontinental heavy bombing) of the B-52 would have made a lot of sense to use liquid hydrogen.  Same for any air flight expected to go more than 1/4 the way around the planet.  The catch is that using hydrogen would tie each to such a narrow role to make designing and building such aircraft completely unfeasible.  Oddly enough, I mostly point out to hydrogen car fans that the vehicles where hydrogen is a huge advantage is long distance jetliners.  And nobody is talking about using hydrogen in them, and for good reasons.

One thing that the BFR P2P plan is likely to do is kill any funding for a second generation SST.  Any real plans to build a SST would take 20-30 years before the first customer is flow, and do you really want to be that BFR won't be ready to eat your lunch?  Spacex may never even try to develop P2P, but if you prove the market is there, they are likely to have just the thing to swoop in and eat your lunch.

[magnemoe]

56 minutes ago, wumpus said:

Oddly enough, I'd have to assume that the original mission (intercontinental heavy bombing) of the B-52 would have made a lot of sense to use liquid hydrogen.  Same for any air flight expected to go more than 1/4 the way around the planet.  The catch is that using hydrogen would tie each to such a narrow role to make designing and building such aircraft completely unfeasible.  Oddly enough, I mostly point out to hydrogen car fans that the vehicles where hydrogen is a huge advantage is long distance jetliners.  And nobody is talking about using hydrogen in them, and for good reasons.

One thing that the BFR P2P plan is likely to do is kill any funding for a second generation SST.  Any real plans to build a SST would take 20-30 years before the first customer is flow, and do you really want to be that BFR won't be ready to eat your lunch?  Spacex may never even try to develop P2P, but if you prove the market is there, they are likely to have just the thing to swoop in and eat your lunch.

Why is hydrogen an benefit for long range jets, yes for hyper-sonic ones its extremely nice because an much higher ISP but an very low subsonic flight I see none, you have to handle cryogenic fuel and need huge tanks who would increase air resistance. Assume boiloff is not an issue since you burn part of it all the time but it will require insulated tanks simply to avoid ice buildup. 

[kerbiloid]

An experimental cryoplane IRL.
http://translate.google.com.tr/translate?sl=ru&tl=en&js=y&prev=_t&hl=ru&ie=UTF-8&u=https://dron-sd.livejournal.com/26358.html&edit-text=

17.5 m³ cryotank for LH or LNG.
Right engine - cryofuel, others - kerosene,

According to the text: -15% to fuel consumption and ecological bonus to karma.

According to
http://translate.google.com.tr/translate?sl=ru&tl=en&js=y&prev=_t&hl=ru&ie=UTF-8&u=http://www.airwar.ru/enc/xplane/tu155.html&edit-text=
the project has been started in 1970s on global oil crysis and so on (see Mad Max, lol)  when hydrogen energetics was in the trend.

[wumpus]

1 hour ago, magnemoe said:

Why is hydrogen an benefit for long range jets, yes for hyper-sonic ones its extremely nice because an much higher ISP but an very low subsonic flight I see none, you have to handle cryogenic fuel and need huge tanks who would increase air resistance. Assume boiloff is not an issue since you burn part of it all the time but it will require insulated tanks simply to avoid ice buildup. 

Mass,  Mass, and more mass.  If the plane starts being more than half its weight in fuel, expect to be able to change plenty of other parameters.  I'd expect a hydrogen powered plane to fly higher and be longer as a first-order attempt  to reduce air resistance, followed by a wider cabin with the top of the aircraft filled with fuel (possibly something in the bottom allowing a more rectangular cargo space).  If insulation is the issue, it is possible that fore and aft sections of the plane will be entirely filled with hydrogen (pulled together to maintain CoM).  This should allow a longer airframe with equal stress as the areas furthest from the wings (and thus the worst leverage) will have the lightest mass.

This type of design probably winds up looking like an A380 thanks to the need for so much fuel, which is a terrible idea since Dubai was pretty much the main customer of A380s and they aren't interested in direct flights at all.  But even when the A380 was designed, I really don't think they ever considered hydrogen.  The other huge issue for hydrogen is needing special handling at specific airports, and since A380s basically need specifically wide runways, airports could upgrade both at the same time.

The point isn't that it is a good design.  The point is that of all hydrogen's few advantages, airlines are customers who really care a lot about at least one of them (weight).  They just don't care enough about that to design a plane around it.  If airlines won't use hydrogen, and Spacex can originally plan on using hydrogen in BFR (before switching to methane), then you can see that the "hydrogen future" isn't going to happen.  Note that hydrogen still is ideal for all stages from MECO to "Earth Escape Engine Cutoff", although  spacex probably cares more about being able to use the same engine design for both 1st and 2nd stages (like Falcon 9) and also being able to use a single type of fuel (also like Falcon 9).  And they probably want to use the same engine for "Mars escape and return" as "Earth Escape and Hohmann transfer", so "all things are never equal".

[Ultimate Steve]

10 hours ago, Cassel said:

This is because you do not understand the military way of thinking, the range is more important than anything that seems important to you.

https://www.quora.com/What-was-the-thinking-behind-the-external-fuel-tanks-on-the-T-34

That's for tanks, very different beasts then fighters and bombers.

The advantage of Hydrogen would be longer range per kg of fuel, and maybe faster flying, and if using SABRE really fast flying.

But to use liquid hydrogen on a plane you need way bigger tanks (hydrogen is not very dense even in liquid form), cryogenically capable ones, the ability to fuel these (extensive ground equipment), and you need to design the tanks to survive hundreds of cycles.

In addition the fuel would probably be more expensive (Jet fuel just around a dollar per kg, hydrogen 3) depending on the type you're using (high grade jet fuels are probably still more expensive though). And being fighters and bombers, if your fuel tank gets hit you have a bigger problem with it being hydrogen than normal fuel.

[wumpus]

1 hour ago, Ultimate Steve said:

That's for tanks, very different beasts then fighters and bombers.

I'd also expect that the original design was to use the drop tanks to get to battle, then fill up before heading to the front.  Only after realizing that tank fuel doesn't really affect anti-tank combat enough (it can survive a fire next to it, and not enough burns during the hit) that they changed how it was used.  A T-34 example is also pretty weird in that it is often believed (at least in the US) that it proved that a tank needs to be just reliable enough to drive to the front and then drive the few minutes a tank is likely to survive in the brutal battles of the Eastern Front.  I'm not sure the T-34 was that unreliable, just that those manufacturing them quickly learned to just push them out as fast as possible without worrying to much about how long they will last (battle will destroy them first).

As far as bombers go, the entire aim of the Pacific Theater in WWII (after Midway) was the pushing to invade islands that could be used as air bases for bombers.  The longer range the bombers had, the fewer islands Marines would die on.  I'm sure that equation seared itself into a generation of American bomber designers. 

In Europe, bombers certainly would prefer to attack targets within "fighter distance" of home (or within "fighter distance" of friendly fighter airfields as they likely needed longer runways) for defense while doing a bombing run.  Strategic command rarely seemed to go along with this idea as the number of targets increases geometrically with range (pi*r**2) and there was almost always something just a bit further they wanted to hit.

[kerbiloid]

Weren't boranes and fluorine, not hydrogen, the common main idea of the combat aviation of the atompunk epoch.

Hydrogen was always mentioned for spaceplanes, there was a lot of hydrogen spaceplane projects.

Edited September 11, 2018 by kerbiloid

[magnemoe]

3 hours ago, wumpus said:

Mass,  Mass, and more mass.  If the plane starts being more than half its weight in fuel, expect to be able to change plenty of other parameters.  I'd expect a hydrogen powered plane to fly higher and be longer as a first-order attempt  to reduce air resistance, followed by a wider cabin with the top of the aircraft filled with fuel (possibly something in the bottom allowing a more rectangular cargo space).  If insulation is the issue, it is possible that fore and aft sections of the plane will be entirely filled with hydrogen (pulled together to maintain CoM).  This should allow a longer airframe with equal stress as the areas furthest from the wings (and thus the worst leverage) will have the lightest mass.

This type of design probably winds up looking like an A380 thanks to the need for so much fuel, which is a terrible idea since Dubai was pretty much the main customer of A380s and they aren't interested in direct flights at all.  But even when the A380 was designed, I really don't think they ever considered hydrogen.  The other huge issue for hydrogen is needing special handling at specific airports, and since A380s basically need specifically wide runways, airports could upgrade both at the same time.

The point isn't that it is a good design.  The point is that of all hydrogen's few advantages, airlines are customers who really care a lot about at least one of them (weight).  They just don't care enough about that to design a plane around it.  If airlines won't use hydrogen, and Spacex can originally plan on using hydrogen in BFR (before switching to methane), then you can see that the "hydrogen future" isn't going to happen.  Note that hydrogen still is ideal for all stages from MECO to "Earth Escape Engine Cutoff", although  spacex probably cares more about being able to use the same engine design for both 1st and 2nd stages (like Falcon 9) and also being able to use a single type of fuel (also like Falcon 9).  And they probably want to use the same engine for "Mars escape and return" as "Earth Escape and Hohmann transfer", so "all things are never equal".

No you would get an A380 sized flying wing rather than an A380, Dubai is perfect in the Europe, Africa and Asia triangle, lots of runs who don't offer frequent flights. 
Yes one point is that an hub is kind of permanent. Dubai probably think very long term. Quite possible all local flight are electrical. 
Or you get intercepted by an oversize Russian tanker in the Atlantic selling tax free fuel
Yes budget airlines threaten to pull hubs east. 

[mikegarrison]

Hydrogen is a very non-optimal fuel for airplanes. It has terrible energy per volume characteristics.

If you look at all the reasonably common fuels on a map of energy density by weight v. energy density by volume, you see why kerosene is so popular.

You want to be high on both axes. The only conventional fuels higher than kerosene on both axes are gasoline and diesel. But diesel tends to freeze up and gasoline is too dangerously volatile. 

[kerbiloid]

Spoiler

Hm, "Fat Metabolism is as dense as Kerosene"...

 

[mikegarrison]

Did you ever see the mythbusters episode where they made a rocket powered by pepperoni?

[Ultimate Steve]

So, I got bored and did the math while I was on vacation. I forget the exact numbers, but...

Regarding GEO launches, I calculated that if you had a stage with a dry mass of (I forget but its a realistic number) tons that can carry 60-70 tons of methane/lox powered by a subscale raptor engine, with boiloff protection, power systems, and RCS, then you can launch it empty in a BFS, refuel from the BFS to get a full 70t fuel load, and put a 20t commsat direct into GEO and return to the paylaod bay for full reusability.

Not a coincidence, this also has the right amount of Delta-V to visit both Deimos and Phobos in the same mission if it's docked to a stripped down Dragon 2 and a small hab module, provided the BFS aerobrakes into an elliptical orbit.

It can also have cislunar applications, but I didn't do the math that far, it probably has a useful payload of a few tons, but the cost of fueling this thing for a few tons of payload is much less than the cost of the refueling launches needed for the BFS to go to LLO and back.

[qzgy]

On 9/11/2018 at 2:38 PM, kerbiloid said:

Weren't boranes and fluorine, not hydrogen, the common main idea of the combat aviation of the atompunk epoch.

Hydrogen was always mentioned for spaceplanes, there was a lot of hydrogen spaceplane projects.

Yeah, those kinds of fancy exotic fuels were used in the 1950s and 1960s, but I also remember that around the same time, the Lockheed Martin Skunk works did some analysis into building a LH plane for the CIA. It was deemed unfeasible.

[Mad Rocket Scientist]

1 hour ago, qzgy said:

Yeah, those kinds of fancy exotic fuels were used in the 1950s and 1960s, but I also remember that around the same time, the Lockheed Martin Skunk works did some analysis into building a LH plane for the CIA. It was deemed unfeasible.

I recall the SR-71 project investigated hydrogen fuel, but dropped it thanks to a lack of cyro tank technology. Might be possible now with the work done for spaceflight since then.

[hms_warrior]

regarding that table...what is the deal with this lithium borohydride? It look's like pretty much ideal rocket fuel. High energy, high density, boiling point at 380 degree celcius... is it super toxic or someth

never mind, google answered it after adjusting the search words a bit.

Edited September 13, 2018 by hms_warrior

[Cassel]

17 hours ago, kerbiloid said:

  Hide contents

Hm, "Fat Metabolism is as dense as Kerosene"...

 

Now you know what aliens may look on Earth and why they abduct people :-)

18 hours ago, mikegarrison said:

Hydrogen is a very non-optimal fuel for airplanes. It has terrible energy per volume characteristics.

If you look at all the reasonably common fuels on a map of energy density by weight v. energy density by volume, you see why kerosene is so popular.

You want to be high on both axes. The only conventional fuels higher than kerosene on both axes are gasoline and diesel. But diesel tends to freeze up and gasoline is too dangerously volatile. 

So making rocket that burns gasoline or diesel is possible?

 

[K^2]

What looks like a very neat line near the center of that chart took me by surprise, but then I realized that these were all carbohydrates. Neat.

32 minutes ago, Cassel said:

So making rocket that burns gasoline or diesel is possible?

Sure. How is either fundamentally different from kerosene? Now, kerosene does have some nice properties which make it a much better rocket fuel, but the bottom line is that if it burns, you can use it as a rocket fuel. IIRC, Mythbusters used a salami for a hybrid motor once...

[KSK]

4 hours ago, Cassel said:

So making rocket that burns gasoline or diesel is possible?

I can’t see why not. Diesel, gasoline, kerosene - they’re just different oil fractions. They all consist of mixtures of hydrocarbons with varying average chain lengths and different proportions of straight chained vs branched vs cyclic and saturated vs non-saturated and/or aromatic molecules.

Sure, some of those mixtures will make better rocket fuels than others but they’ll burn just fine in LOX.

Edit. Think of gasoline and diesel as being kind of in between methane and kerosene - chemically speaking. Kerosene and methane both work fine as rocket fuels.

3 hours ago, K^2 said:

, Mythbusters used a salami for a hybrid motor once...

Pork barrel funding at its finest.  

Edited September 13, 2018 by KSK

[Cassel]

22 minutes ago, KSK said:

I can’t see why not. Diesel, gasoline, kerosene - they’re just different oil fractions. They all consist of mixtures of hydrocarbons with varying average chain lengths and different proportions of straight chained vs branched vs cyclic and saturated vs non-saturated and/or aromatic molecules.

 

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

[kerbiloid]

20 hours ago, mikegarrison said:

Did you ever see the mythbusters episode where they made a rocket powered by pepperoni?

Any dry organics of course is appropriate, even cows manure.

I just imagine a generation ship or a star destroyer with a farm inside, using manure and fat as propellants.

(Also, I would again leave the link to a cool hot aircraft.)

Spoiler

 

Upd.
Though usually it's better just to split heavy organics into methane or so.

Upd.2.
If you have heavy oil fractions (even heavier than diesel), you can turn them into a fine coke, then convert it into methane.

Edited September 13, 2018 by kerbiloid

[KSK]

2 hours ago, Cassel said:

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

I don’t know for sure but I can offer some speculations.

The propellant costs are a very small fraction of the total cost of building and launching a rocket. That’s still true today despite the progress being made in reusable rocketry.

JP1 (military grade jet fuel) is basically a kerosene prepared to a particular specification. So using the same infrastructure to produce rocket fuel probably makes sense. RP1 or rocket grade kerosene is a more stringently specified version of JP1 if I recall correctly. 

Rocket fuels for the military (read, most early rocket fuels) had to have particular physical properties, especially freezing points, the idea being that they could be stored and deployed in any part of the world. Those requirements probably stuck whether or not they’re still actually required due to bureaucratic inertia and, again, because all the infrastructure needed to make fuels to those standards were already in place.

Rocket motors are designed from the outset to use a particular fuel. New rocket engines aren’t developed very often, so there’s no real impetus to change fuel very often either. 

There may be all sorts of detailed technical reasons why kerosene is better than diesel but that’s way way beyond my level of rocket surgery understanding.

Again - all of the above is just (hopefully) educated speculation. It might be total guff but it’s my best shot at answering the question.

[mikegarrison]

One reason diesel fuel is relatively cheap is because it's a fairly wide cut fuel. Diesel motors are not as picky about what they burn as gasoline motors -- this is why you can run a diesel engine on 100% peanut oil or sunflower oil if you want to. (Not saying the rest of the fuel system would like that, though.) Some gas turbines are designed to be pretty tolerant too. They can run on almost any liquid hydrocarbon. But for aerospace applications they are usually designed with tighter tolerances for the fuel spec.

[tater]

11 minutes ago, mikegarrison said:

One reason diesel fuel is relatively cheap is because it's a fairly wide cut fuel. Diesel motors are not as picky about what they burn as gasoline motors -- this is why you can run a diesel engine on 100% peanut oil or sunflower oil if you want to. (Not saying the rest of the fuel system would like that, though.) Some gas turbines are designed to be pretty tolerant too. They can run on almost any liquid hydrocarbon. But for aerospace applications they are usually designed with tighter tolerances for the fuel spec.

I seem to recall quite a bit about this in the book, Ignition! related to the adoption of RP-1 (which is basically a more narrowly defined jet fuel since they had problems with variant quality (assuming I am remembering the story right)).

Maybe I need to reread that...

[mikegarrison]

Reminds me of one weekend at the racetrack where some people left their truck idling the entire weekend. I asked why, and it turned out they had converted it to run entirely on used cooking oil. They got it for free from a restaurant. But it would freeze up in the fuel system if it got cold, and the ambient temps were well below freezing (of water). So they had to keep the engine running 24/7 to make sure their free fuel didn't freeze up.

(The part of me that is an emissions engineer cringed at running an engine at idle 24/7, but I didn't give them a lecture about it.)

[KSK]

57 minutes ago, tater said:

I seem to recall quite a bit about this in the book, Ignition! related to the adoption of RP-1 (which is basically a more narrowly defined jet fuel since they had problems with variant quality (assuming I am remembering the story right)).

Maybe I need to reread that...

If you’re not remembering it right, that makes two of us.