The Basics

Lt. Snakestrike

Lt. Snakestrike

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#76
#76
Lol, could you maybe remove the period in the title? It's bugging me...
 
Blazer Ayanami

Blazer Ayanami

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#78
#78
8bitCosmonaut said:
Do you need me to speak Spanish? I said the boosters is the first stage, that means the core and boosters are together, and joked that you might as well call the Space Shuttle an SSTO.
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I don't call the Boosters "Stage 1". To me, they are just that: Boosters.

And no. Your English is agressive enough.
 
8BitCosmonaut

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#81
#81
The Blazer said:
If you count the Boosters as Stage 1, the core as Stage 2, and the upper Stage as 3.
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I'm just gonna ignore this part since you completely misunderstood my previous comment.

The Blazer said:
this means you liftoff with stages 1 and 2 on, and get rid of Stage 1 with Stage 2 almost dry.
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The Blazer said:
While in the other configuration, you liftoff with Stage 1, and when you drop it, your stages 2 and 3 are still full, like @Altaïr said.
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Like I said before, both the boosters and the core are considered as the first stage so they burn together. No stage 1 and 2 nonsense.

In your terms "lift off with stage 1" and "stage 2 and 3 still full", I take it that the stage 1 is the booster which is transferring fuel to stage 2 and 3 which is the core and upper stage. This is not how such types of staging works.

What altair was referring to is feeder staging, which means the boosters transfer fuel to the core fast enough for keep the core full and drop the boosters off earlier, these types of configuration require both the booster and core (where in your terms will be both stage 1 and 2) to be ignited, otherwise "stage 2" won't be spending any fuel and no "feeding" can be carried out. Fuel transfer to the upper stage is not needed since it won't start burning until the core is depleted.
 
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Altaïr

Altaïr

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#85
#85
Please guys, keep in mind that it's Horus Lupercal 's thread, he made an amazing work to help newcomers, let's not make it dirty. Let's keep the debate at a reasonable level ok?
 
T

TtTOtW

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#91
#91
Horus Lupercal said:
ISP
Specific Impulse

Sticking with the car theme, ISP is simply how fuel efficient an engine is. In car terms, that’s distance/fuel (miles per gallon for example). In space, that doesn't work because theoretically once you're in deep space with no forces acting on you, then you will continue in motion forever at a constant speed covering all the distance in the universe for free. (I know technically that’s the same for a car but you have drivetrain resistance, rolling resistance, drag etc. all acting on the car, trying to slow it, thus you need to keep applying fuel to maintain a speed).
ISP is essentially a measure of how much of fuel quantity it takes to produce an amount of thrust. The less fuel an engine burns to achieve a specific impulse, the more efficient it is.
Now, 2 things about ISP
  1. It's measured in seconds
  2. It does take gravity into account.
Why seconds, and not lbs, kg, N or whatever? Because NASA, when they were working all this stuff out, was a mix of 'borrowed' German scientists and Americans. One side (Zee Germans) wanted to use Metric. The Americans wanted to use Imperial. After what I'd like to assume was a good natured fight involving a lot of spanners and slide rules, they decided on a unit of measurement everyone knew, but wasn’t in either system. Seconds.
And with gravity, you use Earths gravity 9.8 regardless of where you are. Why? Because its used as a constant, a benchmark for comparison. It allows you to compare an ISP of say a Rocketdyne F1 Engine with a titan without worrying if someone has used...Mars...as a constant for the titan.
How to work out ISP then. Easy, the information pops up when you select an engine will tell you its ISP. And as long as you only use that engine, regardless of how many, the rockets ISP will be that number.
Things get complicated when you're using engine combinations, or boosters with different engines. Even though each engine is using is own ISP, when it comes to working out the Delta V (coming soon...) of that rocket, you need a combined ISP of the engines that are being used at the time.
There are 2 ways of doing this.
Hard Way:
  1. you need the exhaust velocity of an engine. To work this out, you multiply thrust by fuel consumption.
  2. Then, you multiply that, by g (9.8) to get an individual engine ISP
  3. Do that for all engines
  4. Now, for each individual engine, divide its force by its own ISP. Do this for all of the engines and add the totals together. Then take the combined total of all the thrust your engines create and divide that by the all the totals you've just worked out.
Or

Easy Way.
  1. Add up the combined thrust of your engines
  2. Add up the combined fuel consumption of your engines.
  3. Take your gravitational constant (9.8) and multiply it by your total consumption.
  4. Finally, take your combined thrust and divide it by the answer you just got.
  5. BOOM. ISP Baby.
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Altaïr this gives me the wrong answer for ions. 850 instead of 1020. 229.6 for grasshopper. 244.9 for Titan. The rest is right. Check quickly?
 
Altaïr

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#92
#92
TtTOtW said:
Altaïr this gives me the wrong answer for ions. 850 instead of 1020. 229.6 for grasshopper. 244.9 for Titan. The rest is right. Check quickly?
Click to expand...
Ah, that's a rounding problem: thrusts are rounded down. Actually, a Grasshoper at full thrust provides 96 kN, and an ion engine 24 kN. If you use those values you'll get the expected result.
The Titan is right, even if the result is closer to 245 s indeed.
 
Lt. Snakestrike

Lt. Snakestrike

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#94
#94
Horus Lupercal said:
  • titan is 244.8979592 seconds.
  • Hawk is 250.7413222
  • Frontier: 285.7142857 seconds
  • Broadsword: 281.4919071 seconds
  • Grasshopper: 244.8979592 seconds
  • Ion: 1020.408163 seconds
Click to expand...
From what I've seen the ISP and thrust are correct, it's the fuel consumption that's rounded...
 
Horus Lupercal

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#95
#95
'Tis the other way about. The thrusts are listed as tons, but actually equal kN cos he used 10 rather than 9.8 for conversions. The Titan won't lift 300t on Earth but noticed whatever I've worked the burn time out to be will be bang on.
 
Horus Lupercal

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#100
#100
Yeah, the ion and grasshopper thrust values are waaay out
 
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