Thrust From United States of America, joined Sep 2003, 2686 posts, RR: 10 Posted (2 years 5 months 1 week 13 hours ago) and read 3520 times:
Hi there. I wanted to compare single unit rockets to ones that rely on SRBs for assistance in getting them into space. It seems to me like the rockets in Gemini and Apollo, which I believe were all multi-stage liquid rockets, gradually powered their way through the atmosphere with engines that later increased in thrust and efficiency once they were supersonic and through max Q. They used roughly the same type of fuel the whole way through launch. The shuttle on the other hand, seemed to have a system which relied on separate rocket boosters that quickly lifted them out of the atmosphere and used a mixture of fuels (solid fuel for the boosters and liquid fuel for the shuttle). It also seemed unable to climb very high once the SRBs were separated. It reached orbital velocity roughly 30 miles lower than the Saturn V, but 3 minutes sooner. The shuttle system also was burning all of its engines simultaneously and constantly losing weight, whereas the Saturn V gradually lost weight one stage at a time. I guess my question on all this...which of these costs less in terms of fuel and which is a better way to get payload into orbit. Fast and furious, or gradual? And when does it make more sense to build an extremely powerful single unit rocket, or one that utilizes SRB strap-ons? And also, I'm just mainly trying to figure out why the shuttle launches and Saturn V ones operated in the manner they did.
I'm also wondering, on a side note, why the Saturn V's first stage only generated roughly the same total thrust as the entire shuttle system when the weight of the vehicle was much greater. Von Braun when I heard him speak in a video seemed perfectly satisfied with 7.5 million pounds of thrust. The N-1 rocket was designed for 9 million pounds of thrust. I guess my question is, it seems like we were capable of designing a more powerful booster, so why didn't we? Thanks. If I'm repetitive in my past few threads, I don't mean to be. As I find out more, I have even more questions.
spudh From Ireland, joined Jul 2009, 294 posts, RR: 1 Reply 1, posted (2 years 5 months 1 week 11 hours ago) and read 3475 times:
IIRC the single biggest issue with SRB is that you cant throttle or shut them off once ignited. Other than that I think they were more economical than liquid fueled. It would be interesting to find out what the Ussr had planned for their version
kalvado From United States of America, joined Feb 2006, 483 posts, RR: 0 Reply 2, posted (2 years 5 months 1 week 7 hours ago) and read 3438 times:
From general physics perspective it is more efficient to burn fuel as low as possible. Presence of atmosphere changes that quite a bit. Another thing, you want to make sure payload survives launch G-forces (especially live payload). Shuttle tops out at 3G, Apollo and Soyuz hit 4G at launch.
LH2-LOx seem to be preferred fuel for modern heavy rockets due to high specific impulse. However take-off and negotiating atmosphere with LH2-LOx is more challenging due to typically lower engine thrust (fuel pumping problems). Then either boosters (shuttle, energia, ariane V, H-IIs, some delta's) or kerosene first stage (saturn V) are used.
I can only think of Delta 4 heavy for all-LH2 launcher.
Starting both boosters and LH2 engine at the time of launch is probably not required, but I believe starting engine on the ground is easier.
Boosters choice - kerosene vs solid fuel (I don't think hypergolics were ever used in combination with LH2) is probably mostly due to economics. I assume existing technology (read - development costs) at the time of initial design plays a major role in choosing one or the other.
SRBs may be cheaper by now, but technology was not available in USSR, while kerosene engines technology was well developed. Another good thing about SRBs is that military applications paid for initial technology development. Environmental issues are yet another thing with SRBs. Specific impulse is also quite low.
JoeCanuck From Canada, joined Dec 2005, 5281 posts, RR: 30 Reply 3, posted (2 years 5 months 6 days 21 hours ago) and read 3354 times:
SRB's are also significantly more simple...no plumbing or complex storage required. They also seem to be very reliable. Sure, they can't be throttled but predicting their thrust profile is pretty much worked out by now. They are easy to build, store and use and launch aborts don't require defueling...with the costs and dangers involved.
I think we'll continue to see SRB/Liquid fuel combos for space. SRB's to do much of the raw lifting and controllable liquid fueled rockets for the fine tuning.
ZANL188 From United States of America, joined Oct 2006, 3372 posts, RR: 0 Reply 4, posted (2 years 5 months 6 days 9 hours ago) and read 3271 times:
Actually the SRBs are throttleable, after a fashion. The forward section has a star pattern in the core. This provides additional burning area & more thrust right after ignition. As the points of the star burn off the burning area decreases and the thrust goes down. Then as more fuel is burned off the burning area increases again and the thrust goes back up.
All designed to reduce thrust as the stack goes thru Max Q (dynamic pressure).
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Areopagus From United States of America, joined Sep 2001, 1364 posts, RR: 1 Reply 8, posted (2 years 5 months 5 days 10 hours ago) and read 3052 times:
Specific impulse in seconds is a time-honored but dimensionally mixed-up measure of rocket engine efficiency. Specific impulse (Isp) of 269 seconds means that one pound-mass of propellant will produce one pound-force of thrust for 269 seconds -- or, that 1 kilogram of propellant will produce 1 kilopond of force (the weight of 1 kg) for 269 seconds. To get lbf/lbm or kp/kg to really cancel out, you need to multiply by the acceleration due to gravity at the Earth's surface, revealing that it is really a measure of exhaust velocity. A rocket engine with Isp = 269 seconds has an exhaust velocity of 269*9.8 = 2636 meters per second.
So higher numbers are better, is that what I'm reading?
Quoting ZANL188 (Reply 4): Actually the SRBs are throttleable, after a fashion. The forward section has a star pattern in the core. This provides additional burning area & more thrust right after ignition.
Thank you for that little nugget of info! Early in the thread I was wondering if you could vary the 'burnable' surface area to map the thrust produced...
kalvado From United States of America, joined Feb 2006, 483 posts, RR: 0 Reply 10, posted (2 years 5 months 4 days 15 hours ago) and read 2904 times:
Equation describing velocity of a rocket is deltaV=V0*log(Minitial/Mfinal), where V0 is the velocity of exhaust (specific impulse*g) and Minitial and Mfinal - mass of entire stack at beginning and end of a burn.
That does not take into account penalty from atmospheric resistance and gravity, which is somewhere in the range of 1,500-2,000 m/s for an orbital launch plus to 8,000 m/s of orbital velocity. So you need a rocket capable of accelerating by 10 km/s to get to orbit.
So, for a single stage LH2 powered rocket mass has to go down by a factor of exp(10000/(455*9.81))=exp(2.2)=9 between launch and orbital insertion. You would get 11% of launch mass on orbit; that includes rocket hardware (tanks, engines, fuel reserves) and payload.
For a single stage SRB with 2636 m/s exhaust you get 1/44th, or 2.2% delivered to orbit. Once again that includes all hardware plus payload.
Actual values of payload to orbit are generally in 2-4% range for existing launch vehicles.
Then, assuming thrust is constant, your G-force changes with change of rocket mass. If you have 1.1g on takeoff (and you MUST have more than 1 to actually lift off!), G forces would be around 10g and 50g by the end of the burn respectively.
Throttling is good, but I doubt you can get more than a factor of 3 or so in thrust variation for a high thrust engine.
So multistaging, shutdown of some engines etc. are needed.
ADent From United States of America, joined Dec 2006, 1286 posts, RR: 2 Reply 11, posted (2 years 5 months 4 days 5 hours ago) and read 2824 times:
Quoting JoeCanuck (Reply 3): SRB's are also significantly more simple...no plumbing or complex storage required. They also seem to be very reliable. Sure, they can't be throttled but predicting their thrust profile is pretty much worked out by now.
But when they fail they tend to fail catastrophically and the entire pressure vessel of the SRM must be defect free.
For example on the Shuttle there have been failures in one SRM and one SSME, but the results were totally different.
Quote: They are easy to build, store and use and launch aborts don't require defueling...with the costs and dangers involved.
Not super easy to store - they need bunkers. A liquid rocket can be stored in a hanger.
Not easy to ship, especially in large sizes - they are hazardous cargo. SRMs are also heavy to ship - vs an empty liquid rocket.
ADent From United States of America, joined Dec 2006, 1286 posts, RR: 2 Reply 13, posted (2 years 5 months 2 days 2 hours ago) and read 2679 times:
Quoting spudh (Reply 12):
I get what you are saying but LH and LO are not so handy and safe to transport and store in the volume required for a Rocket so that will weigh against the SRB difficulties.
In America the SRBs are made in California or Utah - so the fueled SRMs have to travel thousands of miles to get to Florida. The LO2 tanker just has to drive from the local plant to the launch pad tanks.
I would skip LH2 for a first stage/booster and go with kerosene - but not everyone agrees. LH2 is a pain.