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   Why the 5-segments SRB can't work   

The new 5-segments SRB will be the main engine(s) of (both) the Ares-I and Ares-V giving all (in the first) or great part (in the latter) of the thrust to lift their 25 mT and 130 mT payloads.

The 5-segments SRB is made to add "something more" to the booster: MORE THRUST (to lift more tons) or MORE BURNING TIME (to give more acceleration in the first part of the flight) or BOTH.

Unfortunately, the 5-segments SRB can't give more burning time NOR more thrust, as I explain here (of course, this is only my opinion, but I think it's correct).

The SRB looks like a "firecracker" but don't work in the same way; in a firecracker, the "propellent" burns from its top to the end, so, if we double its LENGHT, also the burning time doubles.

But the solid boosters have an "hole" in the solid propellent with a "shape" that, in the standard SRB, is an 11-point star, so, the propellent don't burns from the bottom to the top of the booster but from the center of the booster to the internal part of the booster's rings.

Then, if we add 1+ segments to the standard SRB without change its DIAMETER, the "burning time" remains THE SAME (about 2 minutes) and it CAN'T GIVE more acceleration to the rocket.

Also, the 5-segments SRB can't give more thrust with the same nozzle, since, MORE thrust needs a LARGER nozzle, like ALL rocket engines, or (probably) due to the higher internal pressure, the real thrust may increase about 10%, but NOT the 25% they may expect with a 25% extra-propellent.

The 5th segment don't adds "something GOOD" (more thrust and/or more burning time) but only "something BAD": more INTERNAL PRESSURE (due to 25% more propellent in the same volume) and WEIGHT (about +140 mT) without a sufficient increase of thrust to lift the extra-mass.

The higher internal pressure increase the risk of FLAME LEAKS between rings (like happen in the Challenger accident) with some aborted/failed Ares-I launches and Ares-V tanks' explosions.

However, since the extra-mass of the 5th segment will be SIX TIMES the max payload of the Ares-I (without a sufficient extra-thrust!) the Ares-I will NEVER lift-off because the 140 mT extra-weight will keep the Ares-I smashed to the launch pad!

The Ares-V, instead, CAN fly (if the main tanks don't explode due to a flame leak) but only WITHOUT ANY PAYLOAD because the 5th segment extra-mass (140 mT) will be very close to the (planned) Ares-V max payload (130 mT).  Note: while writing this article, I forgot that Ares-V has TWO boosters with a total extra-mass of 280 mT, then, also the Ares-V can't fly, with or without its 130 mT payload!

The 5-segments SRB can works only after DEEP CHANGES to its height, diameter, nozzle's surface and dimensions, solid propellent's shape, etc. (to match the "magic mix" of the 4-segments SRB).

Unfortunately, the "deep changes" and tests may need over 3 years and over 3 billion$ without any assurance that it will be safe and reliable for manned flights like the 4-segments SRB that have accomplished 229 successful manned flights (two SRB x 115 flights less the leaked SRB of the Challenger) that is an incomparable "field test" no other manned rocket have had or will have.

In some space-forums I've seen the "Stumpy" concept for the Ares-I that uses two 3-segments SRBs.

I don't know if it's a genuine NASA design since no one of that forums give ANY link to an official NASA page/document/press-release about it, however, I think the 3-segments SRB can work but CAN'T BE efficient like the 4-segments version, for the opposite reasons explained here about the 5-segments SRB.

The burning time of the 3sSRB will be the same of the 4sSRB but it can't give 75% of 4sSRB thrust as expected nor can't lift so much payload because a smaller rocket needs a smaller nozzle to avoid losing part of the lower internal pressure, and because the Gross Mass vs. Empty Mass ratio (or SRB case vs. propellent weights) of a 3sSRB is unfavourable compared with a 4sSRB without a 25% reduction of the SRB case and nozzle dimensions and mass.

I think that the real thrust of a simplified 3sSRB (a 4sSRB without a segment) may be around 50/60% of a 4sSRB, also, the 3-segments SRB may need the same amount of time and money of a 5-segments SRB to be modified, built, tested and certified for manned launches.

The ONLY fast, cheap and safe way to build a shuttle-derived rocket with the SRB is to use the standard 4sSRB of the Shuttle WITHOUT ANY (little or big) CHANGES to avoid new tests, re-certification, extra-costs, etc. (with all the navigation system, electronics, attitude controls, energy, etc. built inside the core stage, like with the Shuttle).

I've suggested a FAST (and smart!) solution MONTHS AGO in my article (of May 12 - 2006) about the FAST-SLV and in my article (of May 21 - 2006) about the 3+3 (or 4+4) engines SuperSLV to lift the entire moon-hardware with a single launch.

The Single Launch Vehicle is more reliable for moon missions but needs more time and money to develop and build it, while the FAST-SLV needs two launches for a moon missions (with an bigger total payload) but may be very FAST to develop and very CHEAP to build.

With the 4sSRB the new rocket(s) can be built SOONER, each unit may cost LESS money and, with the time and money saved, NASA can build 4+ TIMES more rockets, to have MORE MOON MISSIONS with the same funds!

Remember that the 4sSRB don't need a DAY nor a CENT of R&D and are ready available NOW, not in 2009 or 2011 or 2013 or 2015... NOW!

July 28 - 2006

Aug. 4 Update: NASA reveals new details about Ares-I - Michael Cabbage's Blog Ares-I article here.

Comparison chart between the standard 4-segments SRB and the new 5-segments version:


Standard SRB

5-segments SRB




  First CEV launch



  First Moon landing



  Moon missions per year



  R&D costs


$3+ billion!

  Unit cost

$40 million

$6o+ million

  Shared R&D costs


$100+ million

  Already man-rated



  Manned launches *



  Successful launches **



  Launch pad changes




Very Good



Very Good







a little bit

The 5-segments' data in this comparison chart is valid also for the
3-segments SRB excluding only the unit price that may be close to the standard version.

* the total manned launches is 230 since the Space Shuttle uses two SRB per launch

** the number of successful launches excludes the SRB failed in the Challenger flight

Aug. 9 Update: I've sent a mail to the NASA Public Affair Office to know if the "Stumpy" concept aka 3-segments SRB Ares-I "backup plan" is a true NASA project (and have more info about it) and this is the NASA PAO answer:

Dear Gaetano,
I am not familiar with a project named Stumpy. The best source of information for Project Constellation and the Ares I and Ares V vehicles is NASA's Exploration web site at:

Paul Foerman
NASA Public Affairs
Stennis Space Center

Then, the only NASA design of the Ares-I is the "stick" in the 5-segmentsSRB/J-2x version.

Aug. 12 Update: I've found a VERY INTERESTING REPORT on the NASA website about the ATK 4+1 segments SRB tests: www.nasa.gov/centers/marshall/news/news/releases/2003/03-186.html

The 4+1 SRB is a standard SRB with a 5th segment, +25% of propellant, +20% of weight (~118 mT) and only small changes to the booster nozzle: +2.25 inch on the nozzle's throat diameter (about +12% of the cone surface) and +8.7 inch on the nozzle's cone diameter (about +8.5% of the throat surface).

The tests was performed in October 3 - 2003 at the ATK test facility of Promontory, Utah and the real data (gathered with 630 sensors) shows a small +9% increase of thrust (not +25%) over the standard SRB peak thrust of 3.3 Mlbs. reaching, in the 4+1 version, a max thrust of 3.6 Mlbs. with a burning time only FIVE seconds longer than a standard SRB (both data very close to my evaluations in this article).

However, after some discussions with rocket experts, the lack of thrust doesn't appear dramatic but only "critic" since all in the Ares-I design must go as planned to have a well working Ares-I. Also, since the Ares-I will be used to launch astronauts, a possible (uncontrollable) excess of thrust in some points of the thrust-curve may give too much G, the 4 or 5 segments SRB works like a firecracker... no way to stop or throttle it when burned! A solid rocket was never used as sole engine for a manned vehicle and the Ares-I work appears more close to a LAS than a SaturnI. It can't contain the acceleration under 3G ( like the Shuttle) and its acceleration is already "estimated maximum of 3.25-3.5 times the force of gravity" (as NASA claims in its Aug. 3 press release), but, this is an "estimated G force", then, only the REAL tests in 2009 will show the true G force in all points of the thrust curve, and, or course, if the REAL launches will evidence too much G for too much seconds in the flight's profile, the 4 or 5 segments SRB will be deleted as Ares-I booster and the ESAS launchers design will restart from zero.

>>>>>>  If you talk/discuss about this argument on space forums/blogs/websites/magazines/articles please refer to the source of the idea and/or put a link to this article. Thank You. <<<<<<<<<<

Copyright © 2006 Gaetano Marano - All rights reserved

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