SpaceX SFR: The Small Falcon Rocket

[Rune]

15 minutes ago, sh1pman said:

Only is you assume that the tank wall thickness remains constant. If you scale it down proportionally (e.g. multiply by 0,7937), the mass of an empty tank will also scale with volume, keeping mass ratios unchanged.

That would lower the maximum pressure rating of said tank. An easier attack on my analysis would be to say that I also assumed more-or-less spherical tanks, while skinny cylinders would scale quite differently. But skinny cylinders are much less mass-efficient than BFR's almost-spherical ones anyhow.

 

Rune. Back-of-the-napkin, as I said.

Edited October 19, 2017 by Rune

[FleshJeb]

4 minutes ago, sh1pman said:

Only is you assume that the tank wall thickness remains constant. If you scale it down proportionally (e.g. multiply by 0.7937), the mass of an empty tank will also scale with volume, keeping mass ratios unchanged.

@Rune I DID neglect to mention the structural advantage of scaling down, but like you said, you can only scale down so far. Likewise, the outer skin of the rocket has to withstand a minimum dynamic pressure (aero) as well.

@MatterBeam I don't always agree with your conclusions, but you always force me to THINK. Thank you.

[mikegarrison]

1 minute ago, FleshJeb said:

@Rune I DID neglect to mention the structural advantage of scaling down, but like you said, you can only scale down so far. Likewise, the outer skin of the rocket has to withstand a minimum dynamic pressure (aero) as well.

Yeah, you have worry about minimum gauge issues.

[Rune]

1 minute ago, FleshJeb said:

@Rune I DID neglect to mention the structural advantage of scaling down, but like you said, you can only scale down so far. Likewise, the outer skin of the rocket has to withstand a minimum dynamic pressure (aero) as well.

Wait, what? Advantage? It is very much not an advantage, but a disadvantage! That comment convinces me you have no idea what you are talking about, or your brain just had its biggest fart in a long time.

 

Rune. I was saying the exact opposite thing.

[FleshJeb]

Just now, Rune said:

Wait, what? Advantage? It is very much not an advantage, but a disadvantage! That comment convinces me you have no idea what you are talking about, or your brain just had its biggest fart in a long time.

 

Rune. I was saying the exact opposite thing.

Volumetrically, there's an advantage to scaling up.

Structurally, there's a disadvantage. Per Galileo:

 

[sh1pman]

23 minutes ago, Rune said:

That would lower the maximum pressure rating of said tank. And easier attack on my analysis would be to say that I also assumed more-or-less spherical tanks, while skinny cylinders would scale quite differently. But skinny cylinders are much less mass-efficient than BFR's almost-spherical ones anyhow.

 

Rune. Back-of-the-napkin, as I said.

So if we scale the BFS up instead of down, we can make the mass fraction as high as we want. 

We can even make it an SSTO with actually useful payload mass. It will be a huge flying skyscraper monstrosity though.

[Rune]

21 minutes ago, FleshJeb said:

Volumetrically, there's an advantage to scaling up.

Structurally, there's a disadvantage. Per Galileo:

But you ain't scaling up, you are scaling down. So ignore the structural advantages, because we can safely assume the BFR has already been calculated to be structurally sound, you gain nothing there by being stronger. Tank pressure can also be considered a constant, so you can't thin the walls either. What remains? A drop in volumetric efficiency.

15 minutes ago, sh1pman said:

So if we scale the BFS up instead of down, we can make the mass fraction as high as we want. 

We can even make it an SSTO with actually useful payload mass. It will be a huge flying skyscraper monstrosity though.

As long as we don't run into the structural limits of carbon-fiber composite tanks at BFR's flight pressures, yeah, bigger is always better. And we could go to ballon-like pressure-stabilized sizes like the old Atlas...

 

Rune. As I said, the least believable thing in the IAC talks is seeing the same structural fraction in both versions of the thing, BFR and ITS.

Edited October 19, 2017 by Rune

[MatterBeam]

1 hour ago, sh1pman said:

So if we scale the BFS up instead of down, we can make the mass fraction as high as we want. 

We can even make it an SSTO with actually useful payload mass. It will be a huge flying skyscraper monstrosity though.

Actually, the larger you make your SSTO the closer you get to the mass ratio of your propellant tanks. 

54 minutes ago, Rune said:

But you ain't scaling up, you are scaling down. So ignore the structural advantages, because we can safely assume the BFR has already been calculated to be structurally sound, you gain nothing there by being stronger. Tank pressure can also be considered a constant, so you can't thin the walls either. What remains? A drop in volumetric efficiency.

As long as we don't run into the structural limits of carbon-fiber composite tanks at BFR's flight pressures, yeah, bigger is always better. And we could go to ballon-like pressure-stabilized sizes like the old Atlas...

 

Rune. As I said, the least believable thing in the IAC talks is seeing the same structural fraction in both versions of the thing, BFR and ITS.

Hoop stress is what determines the thickness of the propellant tank walls. Hoop stresses rise linearly with radius, so for the same pressure, you need the same strength, and so the same thickness. However, the smaller tank suffers less stresses due to its own mass; after all, they are not held up by pressure like balloon tanks. 

Either way, I think the point can be ignored. The current Falcon 9 booster manages an overall mass ratio of 19.5, engines and landing legs and everything else included. That mass ratio is dominated by the propellant tank mass, so the aluminum-lithium propellant tanks in use have at worst a mass ratio of 20. Carbon fibre tanks are expected to have about 3.2x better mass ratios, so 1:62 or better. The BFS's mass ratio is 14... the propellant tanks very roughly influence 22% of the dry mass. That's 3.2 tons out of the 14.4 tons of the SFS. How much worse does scaling down make the carbon fibre tank mass ratio? A full cube/square law ruling would make the volume 14.39 times lower for a surface area 5.9 times lower, so the mass ratio could be 2.4 times worse.... this will increase the mass of the propellant tanks to 7.7 tons and reduce the revised cargo capacity of the SFR from 12-14 tons down to 7.5 - 9.5 tons. At worst.

[wumpus]

Any idea if simply loading up the BFR with roughly twice the payload (to LEO) and sending it out as if it were going to GTI?  Do any inclination changes out there (at least for groups of customers with weird inclinations) and then aerobrake down to LEO (and finally bring perigee back up to sanity)?

Designing a rocket is *expensive*.  We saw this with Falcon Heavy (and it still hasn't launched), and scaling down a rocket will be far worse.  Of course, if my design had to replace ITS *and* falcon 9, I'd probably at least work in a way to replace half the engines (and fuel tanks and resulting plumbing) with lightweight dummy loads to avoid a full design.  Replacing falcon 9 with this giant appears odd.

Finally, look at the cost to launch a "proven" rocket.  At least half the cost is either simply the cost to put a rocket on the pad and light the candle (at least $50M) [this might be due to an attempt to stop burning investment money, but that just means they need to keep doing it to remain profitable].  They might just decide to forget the whole "small rocket" and simply launch the biggest they have (when was the last time they fired off a Falcon 1?).

[MatterBeam]

1 hour ago, wumpus said:

Any idea if simply loading up the BFR with roughly twice the payload (to LEO) and sending it out as if it were going to GTI?  Do any inclination changes out there (at least for groups of customers with weird inclinations) and then aerobrake down to LEO (and finally bring perigee back up to sanity)?

Designing a rocket is *expensive*.  We saw this with Falcon Heavy (and it still hasn't launched), and scaling down a rocket will be far worse.  Of course, if my design had to replace ITS *and* falcon 9, I'd probably at least work in a way to replace half the engines (and fuel tanks and resulting plumbing) with lightweight dummy loads to avoid a full design.  Replacing falcon 9 with this giant appears odd.

Finally, look at the cost to launch a "proven" rocket.  At least half the cost is either simply the cost to put a rocket on the pad and light the candle (at least $50M) [this might be due to an attempt to stop burning investment money, but that just means they need to keep doing it to remain profitable].  They might just decide to forget the whole "small rocket" and simply launch the biggest they have (when was the last time they fired off a Falcon 1?).

A BFR with twice the payload cannot make orbit. Even if it gives up trying to land, it is only adding 11 tons to the cargo capacity. 

Designing a small rocket is vastly less expensive than designing two new and massive stages!! Just look at the difference in development cost for a private jet versus a big airliner. Its tens of millions versus billions. So, an SFR will be cheaper. It won't replace the BFR until the market is ready for the BFR. It's far from a 'giant' and building it from the ground up is simpler than trying to redesign the F9 boosters to accept cross-feed and side-mounting, which is the main reason behind the Falcon Heavy delays. 

Abandoning the Falcon 1 was a smart decision. It was aimed at the small satellite market but... that market is utterly tiny. SpaceX at the time, and mostly still is, strapped for cash and has to prioritize where it spends its money. 

[sevenperforce]

16 hours ago, MatterBeam said:

The BFR in orbit would need 1100/150: 7.3 launches to be filled up too, so we're only talking two to three times the launch cadence but with a much smaller vehicle. 

The problem as I mentioned with the BFR is that it is unclear whether it is possible to launch the BFRs mostly empty and still make a profit. I find this possibility quite unlikely because if it were the case, then a smaller rocket that is better filled up would make a killing.

According to Musk, a single launch of the BFR, fully-reusable, amortized over vehicle lifespan, is expected to be cheaper than a single expendable Falcon 1 launch. So...

5 hours ago, sh1pman said:

Quote

And the mass of an empty tank scales with area

Only is you assume that the tank wall thickness remains constant. If you scale it down proportionally (e.g. multiply by 0.7937), the mass of an empty tank will also scale with volume, keeping mass ratios unchanged.

Tank wall thickness has to remain pretty nearly constant.

6 hours ago, Rune said:

As to other sources of error, did you take into consideration a SSTO has a rather humongous TWR at the end of the burn, lowering gravity losses? Just an example of why I never trust calculations about launch dV, there are a lot of things that can't be easily modeled with high school algebra.

Gravity losses are already virtually nil at the end of the burn, because that's how ascents work. SSTO designs actually tend to suffer from more gravity losses overall, because they need a lower TWR at the start in order to carry enough fuel. By the time you're halfway to orbit, gravity drag is virtually negligible.

---------------------

In any case...

I already proposed this last year, after the ITS was originally announced. My suggestion was a 5-meter reusable upper stage mated at the tail to a Falcon 9, with winglets, powered by two development-sized Vacuum Raptors. Rather than packing in Merlins or SuperDracos, it would use an array of the 10-kN hot-gas methane-oxy attitude thrusters to land horizontally, like a sci-fi spaceship. 

I even proposed the same three variants we saw in this year's IAC presentation -- crew, clamshell cargo, and tanker.

With full-reuse using a single Falcon 9 first stage, it could have matched a partially-reusable Falcon Heavy in LEO, and would vastly outperform for BLEO. It could be tested extensively on Falcon 9 and Falcon Heavy, fly regularly, and only then get an upgraded 5-meter methalox first stage to boost it up further.

However, after proposing this in more detail over at NSF, I got a pretty solid statement from some folks from the Cape who said that SpaceX will never plumb a launch pad for both methalox and kerolox. Simply won't happen.

Also, I'd note that the re-entry testing on a smaller version of the BFS isn't so useful. Those things don't always scale easily.

[DerekL1963]

7 hours ago, MatterBeam said:

For spaceships like these, the vast majority of the volume is just a big propellant tank. You can replace the spaceship with a hollow cylinder of the same volume and still be accurate within 10% of the mass by scaling up and down using area instead of volume. 

"Accurate within 10%" is equivalent to not being accurate at all.  You might as well pick numbers out of a hat at that point rather than pretending that your math has any validity.

And it's not just about tank mass, it's also about engine mass, and structural mass (the thrust web and interstages) - the last in particular doesn't scale in any easily predictable way.

Edited October 19, 2017 by DerekL1963

[Rune]

4 hours ago, MatterBeam said:

Hoop stress is what determines the thickness of the propellant tank walls. Hoop stresses rise linearly with radius, so for the same pressure, you need the same strength, and so the same thickness. However, the smaller tank suffers less stresses due to its own mass; after all, they are not held up by pressure like balloon tanks. 

Either way, I think the point can be ignored. The current Falcon 9 booster manages an overall mass ratio of 19.5, engines and landing legs and everything else included. That mass ratio is dominated by the propellant tank mass, so the aluminum-lithium propellant tanks in use have at worst a mass ratio of 20. Carbon fibre tanks are expected to have about 3.2x better mass ratios, so 1:62 or better. The BFS's mass ratio is 14... the propellant tanks very roughly influence 22% of the dry mass. That's 3.2 tons out of the 14.4 tons of the SFS. How much worse does scaling down make the carbon fibre tank mass ratio? A full cube/square law ruling would make the volume 14.39 times lower for a surface area 5.9 times lower, so the mass ratio could be 2.4 times worse.... this will increase the mass of the propellant tanks to 7.7 tons and reduce the revised cargo capacity of the SFR from 12-14 tons down to 7.5 - 9.5 tons. At worst.

Well, a 37.5% decrease in payload is nothing to be sneered at, I think. Certainly not negligible. In any case, I'm glad I made you run the numbers and see for yourself that size does matter.

In any case, the best argument about developing this is, well, development. Every dollar spent on this could be spent on its bigger, more efficient cousin, while F9 and F9h service the current and projected needs of pretty much everyone, at a sufficiently low cost.

53 minutes ago, sevenperforce said:

Gravity losses are already virtually nil at the end of the burn, because that's how ascents work. SSTO designs actually tend to suffer from more gravity losses overall, because they need a lower TWR at the start in order to carry enough fuel. By the time you're halfway to orbit, gravity drag is virtually negligible.

Sure, but the trajectory doesn't look the same, so you can't assume the same dV budget. My point was, basically, that if you have two rockets with the same TWR, one stages, and the other doesn't, then they will see a different dV to orbit. Just like two rockets with the same number of stages will see different dV if they have different TWR. Or, in general, how every design sees a different dV to orbit from every other design, and our '9,400m/s to orbit' is a very rough generalization.

 

Rune. I never pack just 3,500m/s to lift from Kerbin.

[Frozen_Heart]

Really not worth it for a SFR. The rocket is too small to make second stage reuse worth it. 

[mikegarrison]

6 hours ago, MatterBeam said:

Just look at the difference in development cost for a private jet versus a big airliner. Its tens of millions versus billions.

First of all, designing an all-new bizjet isn't three orders of magnitude less expensive, as you claim.  A billion dollars to develop an all-new bizjet like the G650 is a very reasonable estimate. This is why all-new bizjets are introduced very infrequently. It's far more common to refresh the product line with derivatives of the existing products. (The same holds true for commercial airliners, but all-new airliners are still more common than all-new bizjets.)

Also, you aren't really comparing like things, because the rules and regulations for bizjets (CFR Part 23) are different than those for commercial airliners (CFR Part 25).

[Racescort666]

4 hours ago, mikegarrison said:

First of all, designing an all-new bizjet isn't three orders of magnitude less expensive, as you claim.  A billion dollars to develop an all-new bizjet like the G650 is a very reasonable estimate. This is why all-new bizjets are introduced very infrequently. It's far more common to refresh the product line with derivatives of the existing products. (The same holds true for commercial airliners, but all-new airliners are still more common than all-new bizjets.)

Also, you aren't really comparing like things, because the rules and regulations for bizjets (CFR Part 23) are different than those for commercial airliners (CFR Part 25).

I'm pretty sure the G650 is a Part 25 aircraft as it requires 2 pilots. That being said, very little has happened in the bizjet market (part 23 and part 25) since the 60s. Sure, some new technology has come about and avionics and engines have gotten much better but nobody's really paved any new ground. The fanciest thing to have happen in the bizjet (not even a bizet) market was the Beech Starship which was arguably a huge disaster. After Beech broke the ice with the FAA, a few other companies have had limited success with innovation (Piaggio P180, various composite aircraft from Beech, Cirrus, HondaJet, and others) but the fact remains that the bizjet market is stale and any "new aircraft" are really just derivatives of existing aircraft with very few exceptions.

All this to say, @mikegarrison is not wrong about the order of magnitude to develop a new spaceaircraft, I'm merely reinforcing the point that development costs are extremely high in this field. Also reinforced, as evidenced by the lack of innovation, is how risk adverse aerospace is. Why fix it if it ain't broke? The successful manufacturers in the bizjet market are successful most likely because they've taken no risk in the last 40 odd years, everyone else has gone broke.

[mikegarrison]

23 minutes ago, Racescort666 said:

@mikegarrisonI'm pretty sure the G650 is a Part 25 aircraft 

Yes, you are right about that. The TCDS says it is certified under Part 25.

[wumpus]

22 hours ago, MatterBeam said:

A BFR with twice the payload cannot make orbit. Even if it gives up trying to land, it is only adding 11 tons to the cargo capacity.

A BFR should manage twice the payload to GTI as the listed SFR can to LEO.  It would take a lot of launches to amortize a SFR.

22 hours ago, sevenperforce said:

According to Musk, a single launch of the BFR, fully-reusable, amortized over vehicle lifespan, is expected to be cheaper than a single expendable Falcon 1 launch. So...

Musk also claimed the BFR (or maybe an earlier ITS) achieved that via "1000 launches".  There's no reason to believe spacex is anywhere near 99.9% launch probability (hopefully they can get that for the crew via a launch escape system, but the "amortized over lifespan" requires saving the booster as well), let alone 99.9% landing safety.  I'd really want to check *all* the numbers on BFR cost.

11 hours ago, Racescort666 said:

That being said, very little has happened in the bizjet market (part 23 and part 25) since the 60s.

Hondajet is apparently shipping.  It may still be losing money, but I doubt Honda is ready to pull the plug.  While I'm not sure what part 23 and 25 mean, I'd expect having Honda compete will shake things up.

[Racescort666]

47 minutes ago, wumpus said:

Hondajet is apparently shipping.  It may still be losing money, but I doubt Honda is ready to pull the plug.  While I'm not sure what part 23 and 25 mean, I'd expect having Honda compete will shake things up.

CFR Part 23/25 are "Code of Federal Regulations" for airworthiness of aircraft i.e. FAA regulations. Part 23 generally covers smaller aircraft up to 12,500 lbs gross takeoff weight (although takeoff weight isn't the only factor to determine which part if falls under) while part 25 covers larger aircraft usually commercial aircraft but there are a lot of business jets that fall into this category. One of the factors that will determine which part an aircraft falls under is pilot information load. Having a high pilot load will put an aircraft into part 25 even if it's small. Conversely, an aircraft could get an exception if it has low pilot load but is still fairly large keeping it covered by part 23. Among other requirements, aircraft subject to part 25 has to have 2 pilots while part 23 only requires 1. 

[mikegarrison]

7 hours ago, wumpus said:

Hondajet is apparently shipping.  It may still be losing money, but I doubt Honda is ready to pull the plug.  While I'm not sure what part 23 and 25 mean, I'd expect having Honda compete will shake things up.

https://www.law.cornell.edu/cfr/text/14/part-23 Part 23 is for "Normal" airplanes.

https://www.law.cornell.edu/cfr/text/14/part-25 Part 25 is for "Transport Category" airplanes.

Hondajet, for instance, is Part 23.