Flight Question Spacecraft parameters for a booster stage

[Grover]

im planning on making a simple booster stage, which i can use with UCD to launch a stack consisting of a CTV and an LTV (phoenix from a similar package) totalling just over 40 tonnes to lunar orbit.

i will model and texture it myself, then use a spacecraft3 INI to give it its functionality, no docks, just UCD to attach the stack

can somebody help we by suggesting values i can use to give the engine enough power to accelerate the stack at around 1G (full fuel) and give enough delta-v to perform TLI, assuming the booster mass is about 10 tonnes empty

thanks to anyone that can help!

---------- Post added at 09:55 PM ---------- Previous post was at 09:09 PM ----------

or a way to calculate them would be nice, im a little stuck here when it comes to ISP and stuff, is ISP like that maximum fuel flow rate, since i believe its a measure of the exhaust gas ejected per second?

 

[N_Molson]

The Apollo+LEM stack was just over 40 tons.

So the SIVB parameters should be good :

Mass : 118,800 kg
Empty Mass : 12,900 kg
Specific impulse: 421 s (*9.81 = 4130 m/s)
Thrust : 1,001 kN
Burn time: 475 s

This way you have a realistic Lox/LH2 booster

 

[Grover]

i found those values as well, can i just plug THOSE values into a spacecraft3 INI file and it comes out good?

---------- Post added at 10:21 PM ---------- Previous post was at 10:19 PM ----------

and if i add a few more tonnes of fuel could increase the range so it can perform the LOI as well? (my current stack is just short of DV)

 

[N_Molson]

Of course, the more fuel you add, the more Dv you gain. Add another engine to have a better acceleration (2,002 kN total). For the empty mass, the "10%" thumb rule give realistic results : 100 tons -> 10 tons of empty mass...

For more precise calculation, use the [ame="http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation"]Tsiolkovsky rocket equation[/ame].

Exemple for the S-IVB booster stage, with a 40 tons payload :

Dv = ISP*9.81 * ln(118,800+40,000/12,900+40,000)
Dv = 421*9.81 * ln(158,800/52,900)
Dv = 4539.87

You need 3200 m/s for TLI, more than 4100 m/s for LOI.

In the Apollo missions, the S-IVB was fired a first time to complete the LEO insertion burn, and restarted for the TLI.

Yes, you can modify the values in the spacecraft3 ini files... That's the point of spacecraft.dll ! :lol:

 

[Grover]

4100m/s for LOI? i thought it was closer to 800.

could you suggest some figures for empty and fuel masses? assuming a single engine of 1100kN and isp of 475?

thanks

 

[N_Molson]

4100m/s for LOI? i thought it was closer to 800.

3200+900 = 4100 (3200 for TLI THEN 900 more for LOI, we have 2 burns that are "cumulative" in terms of Dv)

I think that a total mass of 100,000 tons and an empty mass of 10,000 tons should be well enough.

However, be aware that an ISP of 475 s can't be achieved with the Lox/LH2 technology, you'll probably need something more exotic (and awfully dangerous) like LF2/LH2 (liquid fluorine), which give you an ISP of 470 s. To go beyond that, consider nuclear thermal propulsion.

 

[Grover]

ok, so if i stick to ISP 425, prop mass of 90T and dry mass of 10T i should be fine?

 

[N_Molson]

The dry mass could be reduced to 9 tons, then.

Dv = 425*9.81 * ln(90,000+40,000/9,000+40,000)
Dv = 4169.25 * ln(2.65)
Dv = 4068

I'd say, try a full mass of 95,000 tons and a dry mass of 9,5 tons to be safe. If your payload is a little over 40 tons, stick with a full mass of 100,000 tons and a dry mass of 10,000 tons. That should make it.

 

[Grover]

nice one!

i just worked out that my own stack (CTV + LTV) has enough DV on its own to perform LOI and TEI, but i would prefer my 1G acceleration for LOI

thanks for the help man

---------- Post added 02-07-11 at 10:48 PM ---------- Previous post was 02-06-11 at 11:09 PM ----------

well, i have the basis of this thing done, but it can hardly reach TLI velocity itself, ive set ISP to 425, thrust to 100100, dry mass to 10,000 and fuel mass to 100,000 as you said i should, but theres no way this will reach the moon at the moment

---------- Post added at 10:50 PM ---------- Previous post was at 10:48 PM ----------

wait, never mind, units were in kg/m/s, not s, fixed now