SpaceX News

Point out news stories, on the net or in mainstream media, related to polywell fusion.

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Skipjack
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Joined: Sun Sep 28, 2008 2:29 pm

Re: SpaceX News

Post by Skipjack »

happyjack27 wrote:
Skipjack wrote:
happyjack27 wrote:
however, as you mention, a transport, in it's current form can only carry itself to LEO - no payload. and that's after modifying its thrusters to high-thrust sea-level optimized, which is going to reduce their isp substantially. (from 382 to 334), thus reducing your total delta-v to 334/382 or about 87%.
Actually, it would be able to launch to orbit just fine with the current engine configuration (but less fuel). Modifying the engines to be closer to SL would further improve the delta- V, not make it worse. That is because the vac engines are not just dead weight until the air is thin enough for them to work and you can take off with more fuel in the tanks.
The vac Isp of the SL engines is 361 not 334. So you are off there already. Average Isp would be closer to 350.
Delta- v requirement for the typical reference LEO (200 km x 28.5 degrees) is 9300 m/sec, including the average gravity and drag losses.
If we enter the following parameters here:
http://www.strout.net/info/science/delta-v/
Dry mass: 90t
Fuel 2500t
Assumed cargo: 20t
Total spaceship GLOW: 2610t
Average Isp with all engines SL engines: 350
Resulting delta-v: 10842.49
That leaves plenty of margin for the deorbit and landing burns.
So you get an upper class medium lift SSTO RLV with at least 20 t of cargo to LEO.

And then there is still room for optimization here:
1. The smaller SL engines will definitely weight a lot less than the much bigger vacuum engines.
2. You could increase the expansion ratio on some of the engines giving a tad more vac Isp but slightly less thrust. Though some people have indicated that the SL engines may be over expanded for size constraints. One would have to calculate the benefits of that over the flight profile. It might get a few m/sec Isp from that. Though the reduced weight from 1. might gain you more. For SSTOs lightweight structures are generally more important than Isp.
3. You can probably scrap the legs and land in a cradle on the launch pad like they want to do for the booster. That would save at least 5 tonnes (current F9 legs are 2.4 tonnes and this thing is much bigger which would mean much heavier legs).
All this means additional delta-v for more demanding orbits or more payload to LEO.

You can't improve delta-v by lowering in total fuel. Higher initial fuel means it will eventually each the point of lower initial fuel, only presumably at a higher altitude and speed.
That is not what I meant. I was talking about the T/W ratio for the tanker which is a problem since the vac engines cant fire at SL. But you can reduce the fuel so it could take off with its current engine config. delta-v would be relatively bad, though. Musk mused in his presentation that it could make orbit anyway. I haven't done the calculaton for that one since it is more complex but I take his word for it.
Either way, it does not change the rest of my post. The calculation resulting in 10842m/s delta-v was for a version with all SL engines,which is much easier to calculate.

happyjack27
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Joined: Wed Jul 14, 2010 5:27 pm

Re: SpaceX News

Post by happyjack27 »

Skipjack wrote:
happyjack27 wrote: You can't improve delta-v by lowering in total fuel. Higher initial fuel means it will eventually each the point of lower initial fuel, only presumably at a higher altitude and speed.
That is not what I meant. I was talking about the T/W ratio for the tanker which is a problem since the vac engines cant fire at SL. But you can reduce the fuel so it could take off with its current engine config. delta-v would be relatively bad, though. Musk mused in his presentation that it could make orbit anyway. I haven't done the calculaton for that one since it is more complex but I take his word for it.
Either way, it does not change the rest of my post. The calculation resulting in 10842m/s delta-v was for a version with all SL engines,which is much easier to calculate.
...and you estimated about 20 tons per launch.

so to get 450 tons, that'd take 23 launches.

do you think that's more expensive or less expensive than a single 450 ton launch with a booster stage?

happyjack27
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Joined: Wed Jul 14, 2010 5:27 pm

Re: SpaceX News

Post by happyjack27 »

Skipjack wrote: For SSTOs lightweight structures are generally more important than Isp.
the rocket equation is:

deltav = isp * ln(full mass / empty mass).

now algebraically this can be rewritten:

deltav = isp * ln(full mass) - isp * ln(empty mass).

so while deltav grows linearly with isp, it only grows logarithmically with mass reduction. meaning to get the same amount of improvement as raising the isp, you'd have to decrease the mass -- the majority of which is your payload -- exponentially.

happyjack27
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Joined: Wed Jul 14, 2010 5:27 pm

Re: SpaceX News

Post by happyjack27 »

using the same masses in your example, and the sea level isp of a sabre engine (3600), you'd get 111.7 km/s delta-v.

to get the same delta-v with a 350 isp engine, you'd need a mass ratio of 1E14. that is, for every 1 ton that your craft+cargo weighted, you'd need 100,000,000,000,000 tons of fuel. this is of course disregarding any constraints on thrust-mass ratio. presumably that 1 ton would have to give you >100,000,000,000,000 tons of thrust to get you off the ground.

so tell me again how "For SSTOs lightweight structures are generally more important than Isp."

happyjack27
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Joined: Wed Jul 14, 2010 5:27 pm

Re: SpaceX News

Post by happyjack27 »

ran some more numbers: https://docs.google.com/spreadsheets/d/ ... sp=sharing
(second sheet: "SSTO")

* calculated the average isp of the skyon to be about 600, given that it's max payload to orbit is 13 tons -- 600 gives it about the right dv to leo.

* using that as a figure, a skylon space plane with the same fuel mass to dry mass ratio as a falcon 9, could carry 6 times the payload per fuel mass than a falcon 9.

presumably if spacex used a comparable jet for its first stage, it could have a comparable improvement.

cost of designing and making the massive jet, however, might be prohibitive.

paperburn1
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Location: Third rock from the sun.

Re: SpaceX News

Post by paperburn1 »

the FAA cost most certainly would.
I am not a nuclear physicist, but play one on the internet.

Skipjack
Posts: 6805
Joined: Sun Sep 28, 2008 2:29 pm

Re: SpaceX News

Post by Skipjack »

happyjack27 wrote:
Skipjack wrote:
happyjack27 wrote: You can't improve delta-v by lowering in total fuel. Higher initial fuel means it will eventually each the point of lower initial fuel, only presumably at a higher altitude and speed.
That is not what I meant. I was talking about the T/W ratio for the tanker which is a problem since the vac engines cant fire at SL. But you can reduce the fuel so it could take off with its current engine config. delta-v would be relatively bad, though. Musk mused in his presentation that it could make orbit anyway. I haven't done the calculaton for that one since it is more complex but I take his word for it.
Either way, it does not change the rest of my post. The calculation resulting in 10842m/s delta-v was for a version with all SL engines,which is much easier to calculate.
...and you estimated about 20 tons per launch.

so to get 450 tons, that'd take 23 launches.

do you think that's more expensive or less expensive than a single 450 ton launch with a booster stage?
What payload has 450 tons?!!!

Skipjack
Posts: 6805
Joined: Sun Sep 28, 2008 2:29 pm

Re: SpaceX News

Post by Skipjack »

happyjack27 wrote:
Skipjack wrote: For SSTOs lightweight structures are generally more important than Isp.
the rocket equation is:

deltav = isp * ln(full mass / empty mass).

now algebraically this can be rewritten:

deltav = isp * ln(full mass) - isp * ln(empty mass).

so while deltav grows linearly with isp, it only grows logarithmically with mass reduction. meaning to get the same amount of improvement as raising the isp, you'd have to decrease the mass -- the majority of which is your payload -- exponentially.
A reduction in dry mass by a ton means a direct increase of payload by a ton. Increasing the Isp is much harder than cutting a ton of weight from the LV somewhere. And people like Gary Hudson and Philip Bono agree with me on that one, actually.

Skipjack
Posts: 6805
Joined: Sun Sep 28, 2008 2:29 pm

Re: SpaceX News

Post by Skipjack »

happyjack27 wrote:ran some more numbers: https://docs.google.com/spreadsheets/d/ ... sp=sharing
(second sheet: "SSTO")

* calculated the average isp of the skyon to be about 600, given that it's max payload to orbit is 13 tons -- 600 gives it about the right dv to leo.

* using that as a figure, a skylon space plane with the same fuel mass to dry mass ratio as a falcon 9, could carry 6 times the payload per fuel mass than a falcon 9.

presumably if spacex used a comparable jet for its first stage, it could have a comparable improvement.

cost of designing and making the massive jet, however, might be prohibitive.
What are you talking about? We are not talking about Falcon9. We are talking about an SSTO based on the ITS. The ITS has a much higher mass fraction. The Skylon has a comparably bad mass fraction because of the heavy engines. It makes up for much of that with the higher Isp.
And again, the numbers dont lie. An SSTO based on the ITS Tanker with 20t of cargo gets a delta- v of 10842.49. That is plenty and certainly enough to launch most commercial payloads.

happyjack27
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Joined: Wed Jul 14, 2010 5:27 pm

Re: SpaceX News

Post by happyjack27 »

Skipjack wrote:
happyjack27 wrote: ...and you estimated about 20 tons per launch.

so to get 450 tons, that'd take 23 launches.

do you think that's more expensive or less expensive than a single 450 ton launch with a booster stage?
What payload has 450 tons?!!!
you mean what rocket can carry such a payload?

spacex's proposed ITS.
i thought that's what we were all talking about.

happyjack27
Posts: 1439
Joined: Wed Jul 14, 2010 5:27 pm

Re: SpaceX News

Post by happyjack27 »

Skipjack wrote:
happyjack27 wrote:
Skipjack wrote: For SSTOs lightweight structures are generally more important than Isp.
the rocket equation is:

deltav = isp * ln(full mass / empty mass).

now algebraically this can be rewritten:

deltav = isp * ln(full mass) - isp * ln(empty mass).

so while deltav grows linearly with isp, it only grows logarithmically with mass reduction. meaning to get the same amount of improvement as raising the isp, you'd have to decrease the mass -- the majority of which is your payload -- exponentially.
A reduction in dry mass by a ton means a direct increase of payload by a ton. Increasing the Isp is much harder than cutting a ton of weight from the LV somewhere. And people like Gary Hudson and Philip Bono agree with me on that one, actually.
i don't dispute any of that. but increasing isp is exponentially better than reducing empty mass, as you can see clearly from the equation.

happyjack27
Posts: 1439
Joined: Wed Jul 14, 2010 5:27 pm

Re: SpaceX News

Post by happyjack27 »

Skipjack wrote:
happyjack27 wrote:ran some more numbers: https://docs.google.com/spreadsheets/d/ ... sp=sharing
(second sheet: "SSTO")

* calculated the average isp of the skyon to be about 600, given that it's max payload to orbit is 13 tons -- 600 gives it about the right dv to leo.

* using that as a figure, a skylon space plane with the same fuel mass to dry mass ratio as a falcon 9, could carry 6 times the payload per fuel mass than a falcon 9.

presumably if spacex used a comparable jet for its first stage, it could have a comparable improvement.

cost of designing and making the massive jet, however, might be prohibitive.
What are you talking about? We are not talking about Falcon9. We are talking about an SSTO based on the ITS. The ITS has a much higher mass fraction. The Skylon has a comparably bad mass fraction because of the heavy engines. It makes up for much of that with the higher Isp.
And again, the numbers dont lie. An SSTO based on the ITS Tanker with 20t of cargo gets a delta- v of 10842.49. That is plenty and certainly enough to launch most commercial payloads.
we're talking about minimizing the cost per ton of putting a payload in LEO, and optionally increasing the rate.

As you've demonstrated by your own calculations, using the ITS tanker as an SSTO isn't economically viable.

Skipjack
Posts: 6805
Joined: Sun Sep 28, 2008 2:29 pm

Re: SpaceX News

Post by Skipjack »

happyjack27 wrote:
Skipjack wrote:
happyjack27 wrote: ...and you estimated about 20 tons per launch.

so to get 450 tons, that'd take 23 launches.

do you think that's more expensive or less expensive than a single 450 ton launch with a booster stage?
What payload has 450 tons?!!!
you mean what rocket can carry such a payload?

spacex's proposed ITS.
i thought that's what we were all talking about.
No, I mean who needs such a huge payload?! The cost per kg to orbit is one thing, but the cost per launch is another. My proposed ITS- Tanker based SSTO RLV beats the full ITS stack on cost per launch by quite a large margin.
The only time you need 450 tons into orbit at one time is if you want to launch a space station in one go, or if you want to conduct large scale colonization of a planetary body the way Musk wants to do it. Any other time, you will fly mostly empty and your cost/kg will go through the roof.

Skipjack
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Joined: Sun Sep 28, 2008 2:29 pm

Re: SpaceX News

Post by Skipjack »

happyjack27 wrote: As you've demonstrated by your own calculations, using the ITS tanker as an SSTO isn't economically viable.
2.4 million for 20 tons to orbit is not economically viable? Even a falcon 9 launch costs almost 30 times more!

happyjack27
Posts: 1439
Joined: Wed Jul 14, 2010 5:27 pm

Re: SpaceX News

Post by happyjack27 »

Skipjack wrote:
happyjack27 wrote: As you've demonstrated by your own calculations, using the ITS tanker as an SSTO isn't economically viable.
2.4 million for 20 tons to orbit is not economically viable? Even a falcon 9 launch costs almost 30 times more!
its tanker + booster can deliver 23 times that for less than twice the cost.

if you had your pick between paying x units and 10 times that for the same thing, which would you pick?

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