Dual propellent tank?!

[Arugela]

Is it possible to make a huge orb tank for hydrogen/liquid hydrogen/maybe slush or gel hydrogen then have an inner lining that shrinks on use to be filled with collected liquid air or lox? Would this be a good idea. It seems ideal from geometric sense. Is there sufficient ways to do with pressure/embrittlement/whatever other issues like cost. This is assuming you are less worried about perfect optimization and just if it can be done.

[Shpaget]

What collected air / lox?

[kerbiloid]

LOx is much hotter than LH.

[magnemoe]

1 hour ago, kerbiloid said:

LOx is much hotter than LH.

Yes, this is the problem, its an risk that the LOX freezes in contact with hydrogen with common bulkheads I assume they use some insulation here to keep this from happening. 

[kerbiloid]

Melting point: O = 91 K,  H = 14 K.
Boiling point: H = 20 K.

This equilibrium temperature corresponds to 18.7 AU,  792 AU, and 387 AU respectively, as I can compute sqrt(3.86e26/(4*pi*5.67e-8)/91^4)/1.5e11.

So, the cryotank should be actively cooled everywhere in the Solar System.

Unless the flight is short.

Also, this means that the oxygen tank should encase the inner hydrogen tank, and there should be a vacuum gap between them.

Edited April 25, 2023 by kerbiloid

[steve9728]

But if you consider adding another kind of fuel, it means adding a corresponding weight. I think it would be better to work on the engine and orbit design a bit more.

[DDE]

I know helium tanks are often placed inside lOx tanks...

@Arugela's proposed layout resembles the elastic baldders in Vostok's retromotor or the LM's fuel tank. But these wouldn't take kindly to cryogens.

[Arugela]

On 4/25/2023 at 4:13 AM, magnemoe said:

Yes, this is the problem, its an risk that the LOX freezes in contact with hydrogen with common bulkheads I assume they use some insulation here to keep this from happening. 

This might be stupid, but could you use the frozen collected lox/air as the insulation and just build it up?

The orb I'm thinking of are around 80ft and or 120 ft diameter for a larger ship variant. Although a much smaller version could be made. The ship(ssto spaceplane btw.) is a giant diamond shaped ship where the width of the body is divided by the sqaure root of 3 for the length, and that is divided by the sqaure root of 3 again for the height. The sphere is approximately the height divided by the square root of 4/3 subtracting a bit. Not sure what other ship shape would be good for this.

Any such orb would have humongous volume to use for any collected oxidizer as the main fuel depleted. You could probably even use plain air given the volume for hydrogen based fuels. Unless you put in a main fuel with a much larger density. For instance liquid methane.

Actually, with a ratio of around 2-3 Liquid methane might be ideal fuel for such a shaped vehicle given the geometry. But I think it would force a liquid oxidizer.

I'm guessing engines built around methane or hydrogen might be useful along with air/oxidizer collection for getting off earth.

Edited May 3, 2023 by Arugela

[Shpaget]

It makes zero sense to collect air/oxygen. You still need to accelerate all of it, but now need to carry all the mass of scoop, compressors and chillers, plus all the extra drag of the scoop.

If you need it, take it from home.

Also why sphere?

[Terwin]

Hydrogen leaks.  Through anything.  While doing so, Hydrogen also make the things it leaks through more brittle.

Leaking that hydrogen into a LOX tank seems like a good way to get a big explosion.

 

[Arugela]

I'm thinking of making it liquid methane and then do pure lox collection. Not sure if that is possible. 120ft diameter sphere does approximately 12000 tons of fuel. For a 3.8 ratio using raptor 2 and raptor vacuum engine you could get 57600-60,000 tons of thrust using a lot of engines... But using existing tech. Then potentially doing a collapsing bladder tank.

[Shpaget]

And again, why would you collect oxygen?

[JadeOfMaar]

@ArugelaYou need to provide a use case for this question, this idea. When and why do you need your tank to be able to hold one propellant then another?

I know at least that this tank is not going to be used for the launch vehicle. Everything is wrong with that.

[Arugela]

3 hours ago, Shpaget said:

And again, why would you collect oxygen?

To lower the takeoff weight. And possibly spare a runways destruction. Although I don't know the specific values enough. I'm looking at a vehicle weighting different amounts depending on the fuel used and which way to use it. In this case air breathing at takeoff and then fills up with a lot of oxidizer. I was hoping to spare cargo space by using a single tank for fuel.

The original design is supposed to be a giant diamond shape based on dividing by the sqaure root of 3 for each dimensions to get the other parameters.(with the goal of getting 120 and 60 degrees for more optimal flight.) I'm trying to find the limits of the vehicle and the fuel tank is what naturally fits in the vehicle in it's center. But I'm wonder if a pure sphere vehicle would be better as I think it reduces the structural cost and weight of the vehicle. Unless I'm doing the math wrong. Which is very likely.

The current design is 432x250x144 w/l/h. But I found out that the carbon fiber at 1 ft thick is over 7000tons...(If I did the math right.) I originally accidentally did the calculation in cubic inches and thought it was clear for my desired weights. The sphere on the other hand is only like 21 tons for 1/6th of the total volume. And I assume better structural strength.

The engines are on a partial sphere plates on the top and bottom of the craft that go with the spherical part of the craft. The vehicle is supposed to go at takeoff/low altitude flight aiming into the 120/60 degree front then swing the body sideways or rotate the engines 90 degrees and fly into the 60/36 degree edge for hypersonic flight. I'm assuming a complete sphere would reduce this need but add drag overall and potentially lower mass for the body. Then I could also stack sphere in sphere and potentially move the engines to one location.

It's for an SSTO design. So, it might be a little odd.

Edited May 8, 2023 by Arugela

[Terwin]

6 hours ago, Arugela said:

It's for an SSTO design. So, it might be a little odd.

A SSTO will be out of the usefully dense atmosphere before its condensing equipment could even produce it's own mass in liquid oxygen.

As such, you will have a lower launch mass by just starting with all the lox you will need.

[kerbiloid]

In 1950s..1970s the air collection was a popular idea for SSTO spaceplanes.

On launch the plane has full fuel tanks (hydrogen and maybe kerosene for turbojets.
Though, some turbojets can use hydrogen, too.

The oxygen tanks are empty.

The hydrogen is used as both fuel and coolant.

On takeoff the turbojets accelerate the plane up to scramjet ignition speed, using fuel and air.

Then it switches to scramjets which keep accelerating the plane. They consume hydrogen and external air.
At the same time, some part of the air from the scoops is redirected into turboexpander, gets liquified, and fills the oxygen tanks.

Once the plane had reached ~25 km altitude and ~2 (?) km/s speed, and its oxytanks are full, it closes the are intakes, and starts feeding the scramjet engines with oxygen from tanks.

In such manner it reaches the LEO and either starts cruising to an orbital station, or releases the cargo and lets it go on its own, then returns.

The advantage is that the propellant component ratio is 8 LOx : 1 LH, so by keeping the oxytanks empty on start, you decrease the launch mass by several times.
The plane mass is first decreasing due to spent fuel, but then starts growing due to collected oxygen.
Thus, you can use smaller wings (because on launch they carry a lighter plane, while at high speed you don't need big wings).

The main problem was the mass of liquifying equipment and complexity in whole.

[sevenperforce]

17 hours ago, Arugela said:

17 hours ago, Shpaget said:

And again, why would you collect oxygen?

To lower the takeoff weight. And possibly spare a runways destruction. Although I don't know the specific values enough. I'm looking at a vehicle weighting different amounts depending on the fuel used and which way to use it. In this case air breathing at takeoff and then fills up with a lot of oxidizer. I was hoping to spare cargo space by using a single tank for fuel.

What you're describing is LACE, the Liquid Air Cycle Engine.

The problem with LACE is that you have to carry a coolant to liquefy the air as you collect it.

Basic physics should tell you that the mass of coolant you need to liquefy a given mass of air is going to be on the order of mass of air you're liquefying, so you might as well start off with liquid oxygen in the first place.

[Arugela]

Is there any way to run lace with methane rockets. Potentially using the raptor engines space x runs? I don't know if you can simply take the air and bypass it into the exit of the rocket or afterwords as a way of mixing it? One idea was to set all the engines midline into another object that would encompass the entire thrust of the rockets and then funnel it to another location farther in the rear and in essence be a second giant vector exit consisting of all other thrust. If the air was bypassed to this area and ignited could you split the lox addition between in engine and out of engine for different mixture ratios to reduce the total lox carried. Then do closed mode or bypass some of the lox to the bypass similar afterwords if it's needed to keep the same ratio. (IE air/lox/lch4, or lox/lox/lch4.)

I'm assuming this would produce more thrust, which would be nice on takeoff, and might allow more weight to be carried overall. That seems to be a limit for the craft. It needs a lot of thrust. I'm assuming the limit might be heat for the engine vector surface. I'm assuming the air/methane could also be passed into this to try to cool this also.

Could a massive exit vectoring design like that work to remove the weakness of engine failures making you shut off more engines. This design would have a potential problem of having to shut of 4 engines for every failure if it has to maintain synchronized/balanced geometry for the thrust. If you funnel all the thrust would you be able to minimize or eliminate the problems of asynchronous thrust sources? IE maintain only 1 engine thrust loss instead of multiples. Or in worse case scenario maybe only 2 from the engine in the opposite corner to the lost engine. Let alone be able to do something like try to artificially allow both the vacuum engines and regular raptor 2 engines fire at the same time by artificially adjusting the realities in the second larger chamber to extend their usability potentially. Or allow a different single engine to be used that works in this artificial atmosphere made up of the engine exhaust.

 

Edited May 9, 2023 by Arugela

[magnemoe]

On 5/8/2023 at 4:18 PM, sevenperforce said:

What you're describing is LACE, the Liquid Air Cycle Engine.

The problem with LACE is that you have to carry a coolant to liquefy the air as you collect it.

Basic physics should tell you that the mass of coolant you need to liquefy a given mass of air is going to be on the order of mass of air you're liquefying, so you might as well start off with liquid oxygen in the first place.

This, with the added downside that only 23% of the air is oxygen. Now its two much simpler tricks to keep takeoff weight low, this let you get away with much lighter landing gear as you only use it to land your empty spaceplane. 
First is stage zero or an trolley launch, put spaceplane on top of an car powered by jet or even rocket engines. Then you are up to takeoff speed push front up and then you get enough pull on all the hold-downs release. 
Second is put the LOX in an tanker who take off first and pump over LOX while flying at decent attitude, This is probably more efficient than trolley. 

[Arugela]

The geometric limits of the craft are that if you take it's volume and divide it by 6 you get the remainder of the volume for cargo/etc. so 5/6's of the craft is fuel if fully taken with you.

432*250*144/6 = 2592000 ft3 max *5/6 = 2160000 max fuel space with lch4/lox.

A 432ft wide model is 2,160,000 ft3 of space for fuel. This is 12000 tons Lch4 and 45600 tons lox. The problem is that this allows only something like 2400 tons for cargo and body mass(Although 7200 would still give near 7200delta v at worst case scenario.).

9.81*337*ln(25) = 10641.50691073952563083774 (60000/2400)

9.81*337*ln(9) = 7263.95853593622119292388 (64800/7200)

The body must be super light and super strong. The other problem is to get 8 minutes of travel time you can only do 25tons a second of lch4 fuel use. This is approximately 108 raptor 2 engines at around 32,000 tons thrust. For a absolutely minimal 60,000 ton ship. So, ideally you want a ship that never weighs more than that 32,000 tons(maybe 28,800 tons) of thurst(assuming that is enough thrust to fight drag.). and you need a super sexy sleek craft. Which might be possible with only 1-6 inch of carbon fiber as a surface. Assuming that is enough. I'm assuming not as the plane needs 2200+lbs(literally a ton) of wing loading at 60,000 tons of body weight.

The wing surface is approximately 54000ft2.

60000*2000/54000  = 2222.222222222222222222222 lbs.

I'm not sure if the pure or nearly pure diamond nature of the craft modifies the normal wing loading formula.

So, you need to literally or effectively collect around 28,000-30,000 tons of fuel assuming you can't go super heavy. Basically the geometry doesn't allow the full storage of enough fuel and enough engine thrust to carry it. I don't know but it might have enough potential room with the sphere cap idea to potentially carry cooling equipment if you can chill and collect /air/lox/lair as a fuel or additive. Not sure if that is possible with lch4. It would allow some extensive cooling which it might need for extra thrust.

I'm not sure if it can carry enough hydrogen to get to orbit. It might though. Something I read said you can hypothetically get 4400dv from pre close cycle saber stuff. And then just need around 3500dv for the other. This help with the geometry problem. But it still suffers from thrust potentially. Especially if the body ends up being heavier than this. It might be a fragile plane.

http://www.sworld.com.au/steven/space/lace.txt

The body surface has approximately 129,600ft2 of surface just purely as geometric diamond/rhomboid shape.

I'm assuming the logic after this goes from the sphere that a sphere is more versatile weight wise. But then aerodynamics/volume/something else then lead to a cylindrical rocket shape. Haven't gone over spheres yet.

But, if you could do some spiffy(extreme) stuff with it you might be able to get it to orbit...

 

Edited May 10, 2023 by Arugela

[kerbiloid]

A side note: one should not use all decimal digits after point, if he desires to be taken seriously.
Actually,  "60000*2000/54000 "  is  "~=2200" or at least "2222.2"

Edited May 10, 2023 by kerbiloid

[sevenperforce]

21 hours ago, magnemoe said:

Second is put the LOX in an tanker who take off first and pump over LOX while flying at decent attitude, This is probably more efficient than trolley. 

Or, better yet, HTP!

Here we go Black Horse!

[Arugela]

I'm stupid. The size of the tank increase proportionally, but the amount of fuel in a sphere is 8x per doubling... So, the solution is to increase the size until it has enough fuel. Hopefully without a problem with engine thrust still. Still haven't gotten that down mentally. I did a 72 foot one the size of a modern aircraft, but it only got 2 minutes of burn time at best. Even refueling couldn't help it. So, this giant geometry ship has to be scaled to the fuel type in essence. With denser fuel needing a bigger ship and hydrogen potentially fitting a smaller on. Especially with lace concepts. But it probably has worse thrust. I'm assuming an RP1 version would have horrific thrust unless the thrust to geometry is better. That seems to be important for this type of design. I'm looking at a 576ft version to see how it pans out. Not sure what the ideal size for a methane version with raptor engines would work out. I know it would take hundreds or engines. And any such ship would literally bankrupt spacex itself. 8)

I think the 432 version is in the 10's of billions to make or more.

Spoke too soon. The relevant fuel flow to get 8 minutes stays the same and the max engine thrust is stuck at 1.8 time less than the a moderate maximum tonnage of fuel and body weight. Although I think the flight time still increases. I'm thinking that doesn't actually sense.

So, the only solution is intake of lox or refueling. If the max flight time increases as it scale up a super large version might be able to refuel. But nothing could physically refuel it but another version of it. So the only solution for a smaller ship is to go hydrogen and lace potentially.

The other solution is to decrease the body surface and shape. But I'm not sure how simple that would be as that might even increase the amount of material used and surface area. Without abandoning the diamond ship design that is.

I'm still not sure on the aerodynamics. Could an ssto plane with 0.5 thrust to weight make it to orbit?

Edited May 10, 2023 by Arugela

[Terwin]

1 hour ago, Arugela said:

I'm still not sure on the aerodynamics. Could an ssto plane with 0.5 thrust to weight make it to orbit?

I expect, that in-theory, with infinite dV and arbitrarily good aerodynamics, you could get just about anything that is able to get off the ground into orbit.  Thus far, in the real world, we have not been able to demonstrate any vessel getting into orbit without at least one stage with TWR > 1.

 

Hydrolox is great in space for its specific impulse, but not so good for a first stage due to it's lower thrust.

While it can work(see delta IV heavy), it is not common to have a pure hydrolox first stage for this reason.

[magnemoe]

9 hours ago, sevenperforce said:

Or, better yet, HTP!

Here we go Black Horse!

HTP would be easier to handle but LOX has better performance. And nobody has build an tanker plane able to carry LOX but that sounds some order of magnitude easier than an SSTO.

Remember that in KSP the X-15 is almost an SSTO and falcon 9 first stage can put second stage and payload into LKO, so 100 ton. 
Now its not set up for reentry but the fact that orbital velosity is just 3 times the SR-71 top speed makes spaceplanes very tempting.