Discussion Elon Musk: the F9 first stage can reach orbit as an SSTO.

[Urwumpe]

No, because how to do altitude compensation has been known since the 60's.

Sorry, but if you are not knowing what altitude compensation means, that is really another topic. I thought you are trying to make a joke.

A magically perfect altitude compensation means: You have the same specific impulse at an altitude as an nozzle optimized for this altitude, which means you have a static exit pressure at the nozzle that is equal to the ambient pressure. It does not mean you have vacuum specific impulse, because there is always an ambient pressure to work against.

In reality, altitude compensation does not even result in that ideal, but stays slightly below the optimal, but then has this peak performance for a larger region of ambient pressures than a single ambient pressure value. This is what makes it attractive - without even getting close to perfect vacuum performance. More so, an altitude compensating nozzle is never a perfect vacuum nozzle. Much less than real vacuum nozzles (which have an finite length, while in theory, an infinite length would be needed)

All that has to be gotten rid of is the mental block that SSTO's can not carry significant payload.

Don't you agree that religion belongs to the basement?

 

[RGClark]

Don't you agree that religion belongs to the basement?

I mean it is literally a mental block. In order to get beyond it, that SSTO's can't be useful, people would need to do the calculation. But they won't do the calculation because of the mental block that SSTO's can't be useful. It's a vicious circle.

Bob Clark

 

[PhantomCruiser]

If it were as easy as you say, wouldn't some group of bean-counters have forced the issue by now?

 

[Urwumpe]

I mean it is literally a mental block. In order to get beyond it that SSTO's can't be useful, people would need to do the calculation. But they won't do the calculation because of the mental block that SSTO's can't be useful. It's a vicious circle.

Bob Clark

No it is not - everytime the calculation ends up with homoeopathic or negative payload mass ratios already in first order estimates, it is useless to go from the hundredth to the thousandth and look if a few kg payload might be hiding somewhere if we use magic engines.

If the first order estimate already says less than 2% payload mass ratio for a SSTO, you can stop right there. Feel free to design a 500 ton SSTO for launching a cansat, if you like scientific jokes.


The mental blockage is not on our side - it is on yours. You are refusing to accept that at least feasible SSTOs are not working like some magic starwars ship. You have a price for everything and that "It is possible to build one, my calculation proves that you only need zero density structures, engines with infinite mass flow and portable vacuum fields around the engines." is a rather poor argumentation in favour of a realistic SSTO, scientifically you only reinforce the opposite, that it is unrealistic to build a SSTO. A realistic SSTO feasibility study requires that you use as few assumptions in your favour as possible - get over this religious zeal and we can talk business.

See my calculation program above - one part of the deal was a fitness function that prefers high structural mass. Absolutely not in support of SpaceX, but it means in the end that the numbers also have to work without being too optimistic. Maybe SpaceX uses better structural mass values, maybe not. But we know that there is an upper limit around 3.8% for a Falcon 9 1.2-like rocket and that exceeding this limit significant means you won't find a solution that satisfies the other constraints.

 

[PhantomCruiser]

-snip- Suppose then you were able to get the first stage engines to have the vacuum Isp of the Merlin Vacuum at 342 s, and with the proportional increase of the vacuum thrust.
Bob Clark

OK, I'm only a layman. But I was under the assumption that the isp value of the vaccum engine was due in part to the shape of the engine bell and tuning; and that it was quite a different setup from the same engines that make up the first stage.

Wouldn't the isp value of the "vacuum" Merlin be lost at sea level (when you need the most power begin with)?

Having a stage that can reach orbit all on it's own is quite useless if it can only carry a handful of microsats. If it burns all the fuel available in the effort and is stranded in LEO, where is the value add when the energy taken to get it going that fast has to be expended to recover the stage if it can't carry the fuel to do so.

I've no doubt that the F9 could be made SSTO; the math may add up, but it's an industry also driven by the almighty dollar.

Sorry if some of this is hard to follow, I've been at work all night and really need to go home and get some sleep.

 

[RGClark]

If it were as easy as you say, wouldn't some group of bean-counters have forced the issue by now?

The scientists and engineers around 1900, among some of the greatest intellects alive, all said heavier than air flight couldn't be done. It took amateur bicycle builders to go beyond the mental block to see it could.

Bob Clark

 

[Urwumpe]

The scientists and engineers around 1900, among some of the greatest intellects alive, all said heavier than air flight couldn't be done. It took amateur bicycle builders to go beyond the mental block to see it could.

Legends. Ever heard of Otto Lilienthal? Or Sir George Cayley? The Wright bros had been standing on the shoulders of giants and they even paid respect to this themselves.

When Lord Kelvin dismissed heavier-than-air flight as impossible, Lilienthal was already doing controlled glides. When he said it was impossible to create a useful flying machine, heavier or lighter than air, the Zeppelin company was already large and many people had been researching ways to combine Lilienthal gliders with engines.

Lord Kelvin was no good example of a scientific consensus or "all scientists".

 

[orbitingpluto]

The scientists and engineers around 1900, among some of the greatest intellects alive, all said heavier than air flight couldn't be done. It took amateur bicycle builders to go beyond the mental block to see it could.

Bob Clark

Like Urwumpe said, that's a legend. Look up Samuel Pierpont Langley, who got funding from the US military and also the Smithsonian in 1898 to produce a piloted airplane. That Langley's plane was ultimately inferior to the Wright's and also spawned a bad case of institutional revisionism is noteworthy in other ways, but it's just one of many efforts of that time.

Octave Chanute also comes to my mind, particularly because as a retired civil engineer with a background in railroads, he has a background almost fitting for a old-guard stick-in-the-mud doubter. Chanute wasn't what you might expect though, writing up information on the state of aviation at the time(collected into the book Progress in Flying Machines(1893)), and organizing a conference at Chicago's World's Columbian Exposition about flight. Far from being an old man shutting down dreams of flight, Chanute spent a great deal of is time supporting younger people get into aviation, one thing being exchanging letters with people like Alberto Santos Dumont, Louis Bleriot, and especially important to note, the Wrights themselves.

Noting the amount if people Chanute corresponded with, the fact he wasn't a lone voice in a wilderness, and came from a sober engineering background, really puts the lie to the legend your pushing about there being some sort of mental block.

 

[PhantomCruiser]

The scientists and engineers around 1900, among some of the greatest intellects alive, all said heavier than air flight couldn't be done. It took amateur bicycle builders to go beyond the mental block to see it could.

Bob Clark

The political scientists and social engineers around 1900, among some of the greatest intellects alive, all said eugenics was a brilliant idea. It took a few psychopaths and a world war to prove otherwise.

The physics of a helicopter are questionable enough to doubt some of the great intellects; having worked on them, I'm convinced that the gravity field repells them due to their large size and overall uglyness. (it's just a large collection of parts moving in formation)

 

[Urwumpe]

The physics of a helicopter are questionable enough to doubt some of the great intellects; having worked on them, I'm convinced that the gravity field repells them due to their large size and overall uglyness. (it's just a large collection of parts moving in formation)

Depends. Hueys beat the air into submission. :lol:

 

[RGClark]

Like Urwumpe said, that's a legend. Look up Samuel Pierpont Langley, who got funding from the US military and also the Smithsonian in 1898 to produce a piloted airplane. That Langley's plane was ultimately inferior to the Wright's and also spawned a bad case of institutional revisionism is noteworthy in other ways, but it's just one of many efforts of that time.

Octave Chanute also comes to my mind, particularly because as a retired civil engineer with a background in railroads, he has a background almost fitting for a old-guard stick-in-the-mud doubter. Chanute wasn't what you might expect though, writing up information on the state of aviation at the time(collected into the book Progress in Flying Machines(1893)), and organizing a conference at Chicago's World's Columbian Exposition about flight. Far from being an old man shutting down dreams of flight, Chanute spent a great deal of is time supporting younger people get into aviation, one thing being exchanging letters with people like Alberto Santos Dumont, Louis Bleriot, and especially important to note, the Wrights themselves.

Noting the amount if people Chanute corresponded with, the fact he wasn't a lone voice in a wilderness, and came from a sober engineering background, really puts the lie to the legend your pushing about there being some sort of mental block.

It wasn't literally ALL. It was only, say, 99.9%. And among them included some of the greatest intellects of the time. That is why these far more experienced even more brilliant scientists couldn't see how to do it. Because they already believed it couldn't be done.

Now, we have the heads of billion dollar aerospace companies with billion dollar launch contracts having to be dragged kicking and screaming to the idea reusable vehicles can cut launch costs. And they are being led there by a newcomer to the industry, not someone with decades of experience in the field.

As Arthur C. Clarke famously said, "When a distinguished but elderly scientist says something is possible, he is very probably right. When a distinguished but elderly scientist says something is impossible, he is very probably wrong."

Bob Clark

---------- Post added at 04:47 AM ---------- Previous post was at 04:37 AM ----------

No it is not - everytime the calculation ends up with homoeopathic or negative payload mass ratios already in first order estimates, it is useless to go from the hundredth to the thousandth and look if a few kg payload might be hiding somewhere if we use magic engines.

If the first order estimate already says less than 2% payload mass ratio for a SSTO, you can stop right there. Feel free to design a 500 ton SSTO for launching a cansat, if you like scientific jokes.

You STILL haven't shown the calculation for the altitude compensation case. That's the entire point! People continue to say SSTO's can't carry significant payload because they refuse to do the calculation for that case.

Bob Clark

 

[Urwumpe]

You STILL haven't shown the calculation for the altitude compensation case. That's the entire point! People continue to say SSTO's can't carry significant payload because they refuse to do the calculation for that case.

Pay me or do it yourself! I am not your math monkey!

 

[PhantomCruiser]

You STILL haven't shown the calculation for the altitude compensation case. That's the entire point! People continue to say SSTO's can't carry significant payload because they refuse to do the calculation for that case.

Bob Clark

I have yet to see your calculations either. :shifty:
Nor have you explained/addressed any potential losses of using vacuum Merlins at sea level.

Like I said, layman, but I'm following along to expand my knowledge base.

 

[RGClark]

Pay me or do it yourself! I am not your math monkey!

I've done the calculation, multiple times. And it shows surprisingly high payload by using altitude compensation. To argue otherwise would require first actually doing the calculation to see what the answer should be.


Bob Clark

 

[Urwumpe]

I've done the calculation, multiple times. And it shows surprisingly high payload by using altitude compensation. To argue otherwise would require first actually doing the calculation to see what the answer should be.


Bob Clark

You have? How? You have not even yet understood how altitude compensation works at the most abstract level, without any mathematical models. How did you calculate any performance value that is not worse than rolling dices? Do you even know the basic formula how to estimate the specific impulse of an ideal rocket engine right at the altitude it was optimized for?

 

[RGClark]

I have yet to see your calculations either. :shifty:
Nor have you explained/addressed any potential losses of using vacuum Merlins at sea level.

Like I said, layman, but I'm following along to expand my knowledge base.

Do a google search on "SSTO" and "Bob Clark".

The idea behind altitude compensation is to use variable size nozzles, or nozzles that can emulate this such as the aerospike, so that your engine gets optimized performance both at sea level and at vacuum. For instance, based on the size of the nozzle, it could look like the Merlin 1D at sea level and look like the Merlin Vacuum at vacuum.

Bob Clark

 

[Urwumpe]

Do a google search on "SSTO" and "Bob Clark".

The idea behind altitude compensation is to use variable size nozzles, or nozzles that can emulate this such as the aerospike, so that your engine gets optimized performance both at sea level and at vacuum. For instance, based on the size of the nozzle, it could look like the Merlin 1D at sea level and like the Merlin Vacuum at vacuum.

Bob Clark

And... does already a theoretical aerospike reach the performance of a magic shape-shifting ideal nozzle? Or is the theory only slightly better than a vacuum optimized nozzle?

 

[Fabri91]

I have yet to see your calculations either. :shifty:
Nor have you explained/addressed any potential losses of using vacuum Merlins at sea level.

Like I said, layman, but I'm following along to expand my knowledge base.

Let's see if I remember things correctly: the whole point of having a "corrected/optimized" engine (or rather, a nozzle) is to ensure that the pressure at the inlet matches the ambient pressure at the altitude that the rocket is operating at.

This is done for two reasons:

1. We need to expand the gas flow coming from the combustion chamber as much as possible to get every last ounce of energy out of it
2. We do not want to expand the flow too much or a normal shock wave forms in the nozzle and stuff hits the fan

This happens because if you reach too low a pressure in the divergent, i.e. expand too much like it would happen by using a vacuum-optimized nozzle a normal shockwave is the only thing that would satisfy the condition of having ambient pressure at the nozzle exit.

But this is bad:

Fast supersonic flow || slow subsonic flow
                     ^
                     |
                     |
           Normal shockwave

In addition to having something quite violent happening in the nozzle and possibly causing a rapid unscheduled disassembly of the whole contraption, we now have slowed down our exhaust gases, which is exactly the opposite of what we set out to do: specific impulse is essentially a way to measure exhaust velocity, and the higher the better.

From what I understand it is possible to get away with a slightly overexpanded nozzle without a shockwave forming, but in general it is undesirable to push your luck too much in this regard.

TL;DR:

• underexpanded nozzle (e.g. sea-level nozzle used in vacuum): safe, but leaves some performance on the table
• overexpanded nozzle (e.g. vacuum engine at sea-level): unsafe

Hope this helped!

 

[Urwumpe]

From what I understand it is possible to get away with a slightly overexpanded nozzle without a shockwave forming, but in general it is undesirable to push your luck too much in this regard.

generally saying, any change of the velocity vector in a supersonic flow happens by shockwaves, and they are all good and bad at the same time.

An normal shock always means the flow is subsonic afterwards.
An oblique shock only reduces the Mach number more or less slightly.

For example, in a jet engine, having a normal shock is absolute necessary - supersonic combustion is still something we are fighting with.

And no, you want no normal shock INSIDE your nozzle. You want expansion fans, which accelerate the flow to higher Mach numbers. And for optimum performance, you want to do that at the maximum possible curvature before the flow separates from the nozzle walls.

 

[PhantomCruiser]

Do a google search on "SSTO" and "Bob Clark".

The idea behind altitude compensation is to use variable size nozzles, or nozzles that can emulate this such as the aerospike, so that your engine gets optimized performance both at sea level and at vacuum. For instance, based on the size of the nozzle, it could look like the Merlin 1D at sea level and look like the Merlin Vacuum at vacuum.

Bob Clark

I've seen the many, many, many links to your blog. Where you seem to make a habit out of quoting (and linking back to) yourself. It reminds me of the dictionary joke where you look up "redundant" and find "redundant" as the entry (think it was either Robin Williams or Steven Wright).

So you want a "Turkey Feathers" mechanism for an engine bell? How much is that going to weigh? X how many engines? Those things take up more space than you may realize. Particularly when it's adding complications to an engine that is already performing as desired, for it's desired application.

It has been argued that SpaceX didn't exactly do any ground breaking work on a "new" rocket motor, it's essentially old tech on a modern production line. There's a whole thread somewhere on the forum offering plenty of criticism over the whole thing.

Anyway, while I'm a rocket layman & semi-pro Orbinaut, I've spent a good portion of my life in, around and on jet engines. Variable geometry convergent/divergent nozzles (aka turkey feathers) are something that I'm pretty familiar with. Exit velocity and pressures of a rocket motor are on an order of magnitude (to use one of your favorite terms) higher than on a turbo-fan (aka jet engine).

An aerospike motor is a totally different animal, and the guys with the Firefly rocket might be able to make it work, but last I saw they only had a powerpoint rocket. http://www.fireflyspace.com/