News Elon Musk wants to put millions of people on Mars.

[RisingFury]

Numbers speak louder. The underlying basis of space travel is a mathematical equation: the rocket equation.
The same mathematics that says you can get a two stage to orbit vehicle, also says you can get a single stage to orbit vehicle.
All it takes is to believe what the mathematics is telling you.


"Free your mind, the rest will follow."

- En Vogue

Math shows a lot of things. Including that traveling nearly at the speed of light is possible. It shows that quantum computing is possible and it shows that lasers that melt a target in a millisecond are possible. That doesn't actually mean we can do it.

 

[C3PO]

Math shows a lot of things. Including that traveling nearly at the speed of light is possible. It shows that quantum computing is possible and it shows that lasers that melt a target in a millisecond are possible. That doesn't actually mean we can do it.

Or if a man digs a hole in one hour, 3600 men can't dig a similar hole in one second. You have the factor in other things to make the mathematical model accurate.

 

[Urwumpe]

The engineering comes from the mathematics.

Engineering is applied physics. Not just mathematics. Mathematics deliver the tools, but physics the relations that you have to work with.

The rocket equation is one important function for spacecraft design, but not the only equation at all. The problem is not the mathematical model, but the engineering reality. On the paper, you can use ideal fuel tanks, that contain the fuel by magic. In reality, it wouldn't work. You have to use real materials, with real qualities, with real errors that are introduced during manufacturing, assembly and flight. And even that isn't the whole truth. You need to get the propellants to the engines with pipes, that have their own physics behind them to work in a way that does not limit your theoretically possible performance too much. Ignore the physics and you get such disasters as the N-1 rocket, that had fuel lines burst by the pressure transients that are introduced when 6 main fuel valves close at the same time, suddenly stopping the fuel flow.

Explanation for those who rarely read physics books: In a pipe, total pressure as the sum of static and dynamic pressure has to be constant. Dynamic pressure is practically the inertia of the flowing medium. When you increase dynamic pressure by increasing the flow speed, the static pressure has to drop. When you stop the flow completely, static pressure is equal to dynamic pressure. When you have a dense kerosene fuel flowing through a pipe every second at 50 m/s, you have a dynamic pressure of about 0.5 * 1000 * 2500 = 1,250,000 Pa or 12 bar. That is quite a lot of pressure, that does usually not act on the walls of the pipe since the flow is parallel to the pipe walls. But when you stop it, the suddenly higher static pressure acts in every direction the same, from the valve to the pipe walls to back into the propellant tank. The flow bounces back from the closed valve and during this short moment in which the flow is slowed, you have a much higher pressure on every part in the system, as during ignition or flight. In case of the N-1, it was too much for the pipes and they burst at their weak spots (eg, welds)

The moral of the long story: Never assume that something works, until you tested it with real materials.

And that is why engineering is more than just a single equation. The equations are not as important for an engineer as knowing which equations he needs to consider in a scenario to get correct predictions.

 

[movieman]

Math shows a lot of things. Including that traveling nearly at the speed of light is possible. It shows that quantum computing is possible and it shows that lasers that melt a target in a millisecond are possible. That doesn't actually mean we can do it.

We've already built SSTOs; Atlas was capable of SSTO flight (though there was no benefit to not dropping the outer engines) and the Saturn S-II stage was light enough to be an SSTO with a reasonable payload. AFAIR there was also a plan to use an SIV-B derivative as an SSTO.

The hard part is not SSTO, it's SSTO-and-back-again-reusably. While an SSTO simplifies launch preparation and eliminates the potential catastrophic failure modes during staging, the big benefit would come from bringing it back to Earth and reusing it. If you're not going to do that then you're almost certainly better off with TSTO because the payload can be significantly larger.

 

[T.Neo]

We've already built SSTOs; Atlas was capable of SSTO flight (though there was no benefit to not dropping the outer engines)

Atlas required dropping the booster engines. Even if it could have made it to orbit with the booster engine attached, I doubt it would have had considerable payload. Therefore, it doesn't count.

the Saturn S-II stage was light enough to be an SSTO with a reasonable payload.

J-2s didn't have enough thrust, liftoff T/W was under 1 and the engines were overexpanded at sea level leading to poor ISP.

An S-II mass ratio combined with several SSMEs could be an expendable SSTO, but this is now a new stage anyway.

AFAIR there was also a plan to use an SIV-B derivative as an SSTO.

I don't know if you could call SASSTO an S-IVB derivative- it was supposed to use different tankage, as well as an all-new plug nozzle engine rather than the S-IVB's J-2.

If you're not going to do that then you're almost certainly better off with TSTO because the payload can be significantly larger.

The same applies to reusable vehicles. TSTO is at a physical advantage.

 

[movieman]

Atlas required dropping the booster engines. Even if it could have made it to orbit with the booster engine attached, I doubt it would have had considerable payload. Therefore, it doesn't count.

Atlas was perfectly capable of reaching orbit with the engines attached, but that would come out of your payload mass so there was no benefit to keeping them; they were being thrown away anyway, so why not throw them away early?

And no SSTO will have a 'considerable payload' relative to its mass. The best case would be a few percent, particularly if it's reusable and needs a few percent for recovery.

The same applies to reusable vehicles. TSTO is at a physical advantage.

For payload per flight, yes. For cost to orbit when compared to a reusable SSTO that flies a couple of times a day (or even a couple of times a week), certainly not.

An expendable 747 might be able to carry twice as many passengers across the Atlantic, but you'd be hard-pressed to find 600 passengers who could afford the ticket price.

 

[Urwumpe]

For payload per flight, yes. For cost to orbit when compared to a reusable SSTO that flies a couple of times a day (or even a couple of times a week), certainly not.

That is now just your wish - but not based on economic qualities. A proper TSTO could for example also fly a few times per day, without looking at the details, there is nothing that makes it worse than your SSTO.

Like in one other thread already: Calculate the specific impulse that would be needed by an TSTO with the same construction mass ratios as an SSTO would use, to haul the same payload as the comparable SSTO.

Not only can you haul more payload into orbit with a TSTO. You can also build a TSTO for the same payload to orbit with much simpler and cheaper parts. A SSTO needs to be at the limit of technology currently, to work at all, and be beyond what can be done today, for being reusable. A reusable TSTO on the other hand, is definitely realistic. The Kistler K1 failed because its market broke away in the .com bubble, but not because of its technology.

 

[T.Neo]

Atlas was perfectly capable of reaching orbit with the engines attached, but that would come out of your payload mass so there was no benefit to keeping them; they were being thrown away anyway, so why not throw them away early?

The payload of the Atlas H was 2255 kg, the mass of the booster engines package was 3646 kg; unless there's something here that I'm missing, it couldn't be carried to orbit even if the vehicle carried no other payload.

And no SSTO will have a 'considerable payload' relative to its mass. The best case would be a few percent, particularly if it's reusable and needs a few percent for recovery.

Modern ELVs have payload fractions of a few percent anyway, but SSTOs would have lower ones.

For payload per flight, yes.

There is no physical limitation to the payload an SSTO can launch, the payload fraction however will be lower than that of a TSTO- i.e. while both an SSTO and TSTO could launch (for example) 20 tons to LEO, the SSTO's GLOW would have to be considerably higher.

For cost to orbit when compared to a reusable SSTO that flies a couple of times a day (or even a couple of times a week), certainly not.

Nobody is seriously proposing such absurd flight rates though. The goal of Skylon is a minimum turnaround time of two days.

There is also nothing preventing a TSTO from flying at least a few times per week as long as recovery and restacking can be done fast enough.

Soyuz achieved a launch rate once of around a launch every couple of weeks- and that included not only stacking the vehicle (with multiple stages, boosters, etc) but constructing it as well.

An expendable 747 might be able to carry twice as many passengers across the Atlantic, but you'd be hard-pressed to find 600 passengers who could afford the ticket price.

There's no reason to limit TSTO to expendability.

 

[RGClark]

Just saw this interview of Elon Musk on NasaSpaceFlight.com:

He mentions manned flights to Mars. He says initially they will be 6 month flights, but eventually they will come down to under a month.

Bob Clark

 

[N_Molson]

He says initially they will be 6 month flights, but eventually they will come down to under a month.

That would be nice, because without being a great mathematician, I can tell you that sending millions of people on Mars that way is going to take a lot of time.

 

[T.Neo]

Hey... it can take only 6 months, if you ship all those millions on a single flight. :rofl:

It's interesting that Musk claims trips to Mars being cut down to "under a month". That would necessitate some extremely impressive propulsion technology. Not the sort of thing that's likely to be built any time soon, but interesting to hear nontheless.

 

[Artlav]

The payload of the Atlas H was 2255 kg, the mass of the booster engines package was 3646 kg; unless there's something here that I'm missing, it couldn't be carried to orbit even if the vehicle carried no other payload.

Engines are not dead weight like the payload, i suppose the math add up somewhere, or so someone think.

On topic, i think that may apply.
Directly or not, the history will judge Musk, not our perceived absurdity of his claims.

 

[Donamy]

Didn't it take the pilgrams 6 months to get to the New World on the Mayflower?

 

[N_Molson]

Didn't it take the pilgrams 6 months to get to the New World on the Mayflower?

They were an hundred. And most of them died of starvation, cold or diseases. Amerindians provided them food (the famous turkeys).

The first crew towards Mars will probably count 5 or 6 members. Nobody will be there to help them, and there is nothing to hunt or harvest.

So I think that such a comparison is out of perspective and very ideological...

 

[Donamy]

Help them ? The pilgrams thought more likely, to be killed by them. My point was that everything is far away at first.

 

[Keatah]

Definitely out of perspective. Mars doesn't have readily available resources, not without bringing tons of materials (and methods of extracting them locally) with you.

There is some big thinking here, I wonder if practical tech can keep up with it?

 

[Codz]

I dislike calling anything impossible, but Musk is essentially promising us Mars on a silver platter. Thus far, he has only delivered on a single test docking to the ISS. I remain skeptical.

 

[fsci123]

Can he put a few thousand people on the moon?

 

[T.Neo]

Directly or not, the history will judge Musk, not our perceived absurdity of his claims.

We must remember that those who criticised Goddard were under the misconception that rockets cannot function in a vacuum. The writer of the New York Times article was uninformed, whereas those criticising Musk are informed, to varying degrees.

but Musk is essentially promising us Mars on a silver platter.

I don't believe he's ever "promised Mars on a silver platter", or indeed, promised anything on a silver platter. At least no more than anyone else in the industry. The fine details, problems and pros and cons of many activities in spaceflight are not often described by people in PR announcements and the like. It's just that it's more noticable in Musk's case because what he's stating is, well... absurd.

 

[Andy44]

Those who can't build rockets criticize the dreams of those who at least try.