Return To The Moon, Everything Old is New again Options

[Guest_BruceMoomaw_*]

The main thing that makes me nervous about using an SRB -- although they really have totally solved the Challenger leak problem -- is that solid motors have an unpleasant habit of blowing up suddenly, without any telemetry warning that would trigger a launch escape system.

[MiniTES]

The main thing that makes me nervous about using an SRB -- although they really have totally solved the Challenger leak problem -- is that solid motors have an unpleasant habit of blowing up suddenly, without any telemetry warning that would trigger a launch escape system.

But how many times has that happened to the Shuttle SRBs inflight?

[Ames]

14.7MN on an LV of 589 ton SRB + 80 ton 2nd stage+module = 21 m/s^2 or  2.2G

The real carefull issue is how to design the SRB to throttle down at the end of the burn - otherwise with 14.7 MN on a nearly empty SRB of 86.1 ton, you would have 9G

Doug

It's a little worse than that
F = ma
Takeoff (590ton full SRB)
a = 14700/(590 + 80) = 22 (m/s^2)
then add 1G!
3.2G

Orbit (87ton empty SRB)
a = 14700/(87 + 80) = 88 (m/s^2)
don't add 1G we are in orbit!
9G

The effects of drag will reduce the G experienced whilst punching through the atmosphere, but at takeoff and orbit this is zero.

[Guest_Analyst_*]

I'm pretty sure liftoff trust of the whole STS is about 28 MN, 1.7 MN per SSME gives about 11.5 MN for one SRB. This NASA page:

http://www.nasa.gov/returntoflight/system/system_SRB.html

gives 2,650,000 lbs per SRB. I'm with the metric system and can't translate this into MN exactly, but it should be right. 14.7 MN is to high.

Ames, why do you ADD 1G? The forces are in different directions (up and down). IMO you must substract it, but only at liftoff. Just before burnout, you fly almost parallel to the earth and the gravity effect ist pretty small.

Btw, I believe the SRB trust is reduced just before burnout right now. You do this by reducing the exposed fuel area in the booster.

Analyst

[djellison]

It's not fair to add the 1G on. I'm sat on a chair, and I dont really feel any acceleration going on, do you

Doug

[djellison]

2,650,000 lbs = http://www.google.co.uk/search?q=2650000lb...:en-US:official

11 787 787.3 newtons

or 11.7 MN

Doug

[chris]

It's not fair to add the 1G on. I'm sat on a chair, and I dont really feel any acceleration going on, do you

Doug

Fall off the chair. You might notice soething then.

Chris

[Ames]

http://science.ksc.nasa.gov/shuttle/techno...ewsref/srb.html

"Each booster has a thrust (sea level) of approximately 3,300,000 pounds at launch"

Have they uprated them?

[djellison]

3.3 M Lbs = 14.6 MN

Doug

[Ames]

Morton Thiokol (ATK)
http://www.atk.com/newsreleases2005/2005-07-26-discovery.asp

"Alliant Techsystems (NYSE: ATK) reusable solid rocket motors (RSRM) provided more than 6 million pounds of thrust in the first two minutes of flight to help send the Space Shuttle Discovery on its historic return to flight mission."

Must have been uprated since the nasa figures were posted.

Ah! All is clear...

http://www.atk.com/RocketMotors/rocketmotors_rsrm.asp

"...each RSRM generates an *average* thrust of 2,600,000 pounds and burns for approximately 123 seconds..."

[MiniTES]

Even so I don't think it presents a problem. You can a) cut the fuel such that you don't get more than maybe 4 or 5 sustained Gs, which is uncomfortable but bearable, and b) even if you do have 9 Gs it wouldn't be for very long, and it certainly can be dealt with by tranied astronauts.

[Ames]

It's not fair to add the 1G on. I'm sat on a chair, and I dont really feel any acceleration going on, do you

Doug

I work at a PC all day
At the end of it I am glad to take the weight of my butt and stand up a while.
People ask me if I would like to sit down, but I say "no thanks, I have sat down all day and would rather stand" - makes people nervous!

I definitely *feel* the acceleration!

F = ma

[ilbasso]

In the postings that someone put up this week for about the CEV and the Shuttle-derived launchers, the maximum G-force that was listed for any of the SRB derivative launchers was 4G. And I think that max was for the SRB 1st stage/liquid 2nd stage combo, with the greatest acceleration in the 2nd stage. The max for any of the SRB stages was 3.8G.

[MiniTES]

In the postings that someone put up this week for about the CEV and the Shuttle-derived launchers, the maximum G-force that was listed for any of the SRB derivative launchers was 4G.  And I think that max was for the SRB 1st stage/liquid 2nd stage combo, with the greatest acceleration in the 2nd stage.  The max for any of the SRB stages was 3.8G.

Sounds perfectly reasonable to me - where did you see this?

Honestly I'm getting really tired of people making fun of the idea and calling it the "Boomstick" and whatnot. It's safe, proven, cheap, and capable, and 3.8G is not at all unreasonable. Should be cheaper with greater lift capacity than the EELVs. And also man-rated and flown much more than the EELVs.

[tty]

Ames, why do you ADD 1G? The forces are in different directions (up and down). IMO you must substract it, but only at liftoff. Just before burnout, you fly almost parallel to the earth and the gravity effect ist pretty small.
Analyst

You neither add nor subtract one g. Think of what happens if the thrust is equal to the weight of the rocket. The rocket will sit still on the pad and You will feel one g as usual. It is only the net acceleration that is added. Those 2,2 g consists of 1 + 1.2 g. The "1" is needed just to not fall back to Earth. If the engine cuts out you will experience zero g but be accelerating 9.81 ms^2 downwards.

That the gravity effect is essentially zero at burnout is because at that point you are in a free-fall orbit, or nearly so.

tty