Constellation program

[Bob B.]

SM, I wrote the following in response to your reply #25 before seeing your reply #28.  Some of this may no longer be applicable.  I’ll read your new data and then comment accordingly.

Combined First stage   
Moi =   5013811
Mfi =   1869192.955

Okay, I see what you did here.  It looks like you used the dry mass of 136,800 kg for the first stage.  I would have used the burnout mass of 146,000 kg.  The difference is the unuseable propellant remaining in the tanks at burnout.  I calculate a slightly different Mfi but the difference is insignificant – only a couple hundred kilograms.

RS-68 stage  (1 1/2 Stage)   
Moi =   1467788.955
Mfi =   571331

Using your initial assumptions, I agree with these numbers.

Ci =   11061.63

Something isn’t right with your value of Ci because it’s way too high.  If we use the data posted in my reply #23 I get,

Ci = (11,561,000*4 + 2,891,000*5) / (4912.2*4 + 816.4*5) = 2,557.8 m/s sea level

Ci = (12,785,000*4 + 3,314,000*5) / (4912.2*4 + 816.4*5) = 2,853.3 m/s vacuum

We’re at sea level for launch but by burnout we’re in essentially a vacuum.  I think you’ll get pretty close by averaging the sea level and vacuum values.
   

Delta-V   10914.39464

This is correct for the Ci you used, but it will of course have to be recalculated for the new Ci.

Ci =   3541.64

This is the correct sea level value for the RS-68, but I think you need to use the vacuum value.  By this point in the flight we’ve risen above the bulk of the atmosphere and are in a near vacuum.
   

Delta-V   3341.69214

Correct for your Ci value, but probably should be adjusted for the vacuum value.

[Bob B.]

Strange, I posted them all in columns and now they are all over the place.

I've recently discovered that if you use "pre" tags (preformatted text) the text will appear exactly how you typed it with all the spacing, etc.  Here's an example (just hit the quote button and you can see how I formatted it):

A     1000    100
B      800     50
C      400     70

[Bob B.]

I did the equations over because I forgot to add in the second set of J-2's

Okay, it looks like you doubled the entire mass of the second stage.  That’s not what I meant when I suggested adding 2 more J-2s.  I meant adding two more engines only to the mass of the second stage to increase the thrust, but not increasing the tanks or propellant mass.  I’m concerned that the second stage may be a little underpowered when trying to lift a 200-tonne payload.

The J-2s used back during the Apollo days were 1,578 kg/each, thus my suggestion is to increase the burnout mass to,

22,063 + 1,578*2 = 25,219 kg

while leaving the propellant mass alone.  This change doubles the thrust and reduces the burn time to 188.5 seconds.

By the way, it looks like the new J-2s will be designated J-2X.  J-2S+ was apparently a temporary designation derived from the J-2S, which was a simplified version of the J-2 developed and tested in the 1970s but never flown.

[Satanic Mechanic]

Bob,
Thanks for the corrections.  I did not know about the burnout mass.  I will go back and fix the spreadsheet.  Good exercise though, I have not done equations in seven years.

SM

[Bob B.]

I will go back and fix the spreadsheet.

Please post your results when you get them; I'm interested in your total delta-v (I've already calculated what I think your answer should be).  Once you do, I'll have some more comments about how to interpret the result.

Good exercise though, I have not done equations in seven years.

I really like doing problems like this, which I guess confirms I'm a bona fide space geek.

[Satanic Mechanic]

Sorry I have not gotten back with you on this Bob.  I have been busy juggling three projects.  I should be freed up in a month to resume our math lessons.

SM

[Satanic Mechanic]

Yeah this topic is old but it was brought up in another forum and they have some number's that look like Bob's:

[ijuin]

I never did quite understand why they were trying to push for 5.5/6 segment long boosters instead of just using four of the reliably-tested four-segment ones. Have they already committed so much to the design of the core stage that it's actually cheaper to try a bigger booster than to re-configure the tank structure to accommodate an additional pair of boosters?

[Satanic Mechanic]

I never did quite understand why they were trying to push for 5.5/6 segment long boosters instead of just using four of the reliably-tested four-segment ones. Have they already committed so much to the design of the core stage that it's actually cheaper to try a bigger booster than to re-configure the tank structure to accommodate an additional pair of boosters?

I don't think anything has been set in stone for the Ares V yet.  I forgot if the extended SRB's were for meeting the 100 tonnes for Ares V or lifting Orion on Ares I. 
Personally, I am having doubts about Ares I.  I do like the Direct concept(s) but I would lean towards using the EELV's since they are already proven.

SM

[evancise]

Pad Abort test #1 (PA1) was completed successfully last week, even though the fate of the Program in general is uncertain (at best).  If nothing else, a great technology demonstration and really fun to watch a 16g acceleration.  Zero to ~600 mph in 2 seconds they say. 

Cool video here - http://vimeo.com/11631855

[LunarOrbit]

Very cool... I love slo-mo videos.

[banjomd]

Can someone please explain the workings of the "Launch Abort System Attitude Control Motor".
THe exhaust plumes "dance" back and forth through the tower. How is that done?

[evancise]

There are at least 3 different sets of motors on the LAS. The first are the large solid motors that propel the tower and the capsule up and away from the booster.  The next are the attitude control rockets near the top that you see "puffing" throughout the flight.  These keep this new rocket (LAS + capsule) precisely oriented to get it up and away from the booster and launch pad, into an orientation for stable flight, and then finally into the jettison attitude.  At the end, at time of jettison, another set of rockets fire to boost the LAS tower away from the capsule once they have separated.

All of these systems, and all the computers underneath, must work together for an abort to be successful.

[ijuin]

The exhaust "dancing" back and forth is due to the peculiar requirements of using a solid-fueled rocket for attitude control. Since solid fuel can not be shut off and restarted like liquid fueled engines can, it must burn continuously. However, continuous thrust is a bad idea for attitude control, since you can not shut it off when you are done with each correction, and instead must redirect the thrust elsewhere. In order to address this problem, the attitude control rocket consists of multiple separate nozzles linked to a single combustion chamber, with each nozzle jutting sideways at a different angle. Thrust is then directed through these nozzles in a cycle (e.g. by a rotating plate that blocks all but one nozzle at any given moment, with the rotation controlled to determine which nozzle gets the thrust). Thus, the single thrust jet can be directed as needed through several nozzles. The jet switching between the different nozzles is what gives the impression of the plumes "dancing".

[banjomd]

Thanks