Author Topic: Why doesn't Zenit weigh more?  (Read 11122 times)

Offline quanthasaquality

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Why doesn't Zenit weigh more?
« on: 10/25/2012 02:43 am »
The Zenit 3SLB has a takeoff weight of a mere 1,038 klb, but the RD-171 engine provides 1,641,100 lbf of thrust at sea level. http://www.astronautix.com/lvs/zent3slb.htm

The Atlas V has a takeoff weight of ~335 tons, but has an RD-180 engine, with a thrust of ~860 klbs, close to half of the RD-171. It would seem economical to expand the Zenit's first stage fuel tanks to carry more fuel, to carry more payload into orbit. Why hasn't the Zenit been expanded, like the Falcon 9 v1.1 has been? Want to avoid competition with Proton?

I know payload to orbit is not this simplistic, but the Zenit 3SLB seems like it has much untapped potential.

Offline pippin

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Re: Why doesn't Zenit weigh more?
« Reply #1 on: 10/25/2012 03:05 am »
Big enough?

Offline Galactic Penguin SST

Re: Why doesn't Zenit weigh more?
« Reply #2 on: 10/25/2012 03:54 am »
Maybe it was sized for the application of the boosters of Energia?
Astronomy & spaceflight geek penguin. In a relationship w/ Space Shuttle Discovery. Current Priority: Chasing the Chinese Spaceflight Wonder Egg & A Certain Chinese Mars Rover

Offline edkyle99

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Re: Why doesn't Zenit weigh more?
« Reply #3 on: 10/25/2012 04:52 am »
The Zenit 3SLB has a takeoff weight of a mere 1,038 klb, but the RD-171 engine provides 1,641,100 lbf of thrust at sea level. http://www.astronautix.com/lvs/zent3slb.htm

The Atlas V has a takeoff weight of ~335 tons, but has an RD-180 engine, with a thrust of ~860 klbs, close to half of the RD-171. It would seem economical to expand the Zenit's first stage fuel tanks to carry more fuel, to carry more payload into orbit. Why hasn't the Zenit been expanded, like the Falcon 9 v1.1 has been? Want to avoid competition with Proton?

I know payload to orbit is not this simplistic, but the Zenit 3SLB seems like it has much untapped potential.

I can think of a number of reasons.  First, Russian rockets have always tended to have higher liftoff thrust to weight ratios than all-liquid U.S. rockets.  Zenit is nearly 1.6, Proton nearly 1.5, Soyuz nearly 1.4, while the U.S. EELVs can be as low as 1.17 or so.  It is a different philosophy, aimed to reduce gravity losses.  In the U.S., the philosophy seems to be to minimize cost by minimizing thrust when liquid engines are used.  When solids are used, T/W is substantially increased to improve performance.  Delta 7925, for example, had a liftoff thrust to weight ratio of 2.16.

Another reason is that Zenit and RD-170 were originally designed for the Energia booster.  Zenit 2 was probably intended to be only the first of a family of derived rockets.  So yes, there is growth potential.

 - Ed Kyle

Offline Danderman

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Re: Why doesn't Zenit weigh more?
« Reply #4 on: 10/25/2012 04:54 am »
Here's another reason: Yuzhnoye, which designed Zenit, has had a tradition of designing large ICBMs. These ICBMs were designed to get through the atmosphere very quickly, and so all of these have very high thrust to weight ratios. This has something to do with with the lower atmosphere being a very dangerous place for ICBMs during a military engagement.

A few years back, I saw a Dnepr take off, and it flew out of sight in just a few seconds.

Offline baldusi

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Re: Why doesn't Zenit weigh more?
« Reply #5 on: 10/25/2012 05:18 pm »
In the Energiya-Buran book, they do mention a huge discussion between RKK Energia and Yhuzhove about the Zenit-2 as a booster and as a rocket. Basically Energia forced them to use the "same" core, even though it ended up needing different wall widths and vectoring structure. Not the mention the boosters had to be recoverable, with parachutes and air bags.
In any case, the Soviets (and now Russians) are volume limited by their road transport system. 4.1m is the maximum diameter, but the train need exclusive right on the track. To allow two trains (one going and one coming) you need 3.8m. The Zenit-2 is 3.9, but that might be the restriction from Ukraine to Baiknour. The train also limits the length of the stage, with the Zenit-2 being 32.9m at it's maximum (25m to the East, I think). So I seriously doubt they could have increased the weight much due to transport limitations. Going to 4.1m would have added just 10% of propellant and complicated transport due to the need to have exclusive use of the track.
Please remember that originally the Zenit-2 was supposed to replace Soyuz and Proton (with a Heavy version). So logistics was the name of the game. Besides, since the Soviets lacked a high energy US, and transport had them volume limited, high T/W and isp was sort of the only variables available to optimize.
Now a day Zenit flies so little that has very little reason for further investment. In fact, I expect that after Angara takes over the role of intelligence satellite launches, the Zenit will be phased out of Russia and Sea Launch will not be able to support the whole infrastructure.

Offline quanthasaquality

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Re: Why doesn't Zenit weigh more?
« Reply #6 on: 10/26/2012 02:20 am »
The current Zenit served the Soviet purposes well. With the fall of the Soviet Union and Energia, and the increased focus on cost, I would expect a tank stretch, to get increased payload, like the Falcon 9 v1.1.  There have been ~20? Zenit launches since the fall of the Soviet Union. Maybe the launch market is really driven by large nations that want to ensure their access to outer space and pork.

Offline Jim

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Re: Why doesn't Zenit weigh more?
« Reply #7 on: 10/26/2012 02:32 am »
The current Zenit served the Soviet purposes well. With the fall of the Soviet Union and Energia, and the increased focus on cost, I would expect a tank stretch, to get increased payload, like the Falcon 9 v1.1.  There have been ~20? Zenit launches since the fall of the Soviet Union. Maybe the launch market is really driven by large nations that want to ensure their access to outer space and pork.

Huh? How the heck did you come to that conclusion?  Especially in face of the post above.  There can't be a stage stretch due to logistics. 
Also, what says that the market supports a stretch?  Sealaunch doesn't need it.  And Landlaunch doesn't need it.  The other users don't seem to need it.

Offline Salo

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Re: Why doesn't Zenit weigh more?
« Reply #8 on: 10/26/2012 04:47 am »
I can think of a number of reasons.  First, Russian rockets have always tended to have higher liftoff thrust to weight ratios than all-liquid U.S. rockets.  Zenit is nearly 1.6, Proton nearly 1.5, Soyuz nearly 1.4, while the U.S. EELVs can be as low as 1.17 or so.
Proton K  is nearly 1.3.

Offline Salo

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Re: Why doesn't Zenit weigh more?
« Reply #9 on: 10/26/2012 05:05 am »
There can't be a stage stretch due to logistics.

The second stage of Soyuz consist of two parts and bolted together in HIF on launch site.
The second opportunity is additional side mounted tanks on the first stage. 
« Last Edit: 10/26/2012 08:52 am by Salo »

Offline quanthasaquality

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Re: Why doesn't Zenit weigh more?
« Reply #10 on: 10/28/2012 06:59 pm »
Maybe the launch market is really driven by large nations that want to ensure their access to outer space and pork.

Huh? How the heck did you come to that conclusion?  Especially in face of the post above.

Ok. There is not enough evidence in this thread to conclude the launch market being driven by large nations. It is off topic for this thread, and would be a different thread.

Russia's transportation system might not be able to handle a stretched rocket. Heck, America's highways might not be able to handle the Falcon 9 v1.1, and Spacex intends to move the first stage by air or sea.

Zenit also served as a booster for the Energia rocket during Soviet times.

A different second stage would be needed to maximize payload to orbit.  ULA has proposed a dual RD-180 Atlas V. http://www.ulalaunch.com/site/docs/publications/EELVPhase2_2010.pdf
A new upper stage on a stretched Zenit, maybe from a foreign partner, could produce a rocket with 10 mt to GTO. Arianespace is able to find payloads for  10 mt into GTO.

Offline baldusi

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Re: Why doesn't Zenit weigh more?
« Reply #11 on: 10/28/2012 07:22 pm »
Not only is the Zenit first stage the longest core under 4m by far. Not only do I suspect that you can't fit a longer core on a train (you have to consider curves, and curves inside curves, or two trains crossing each other on a curve). The whole rocket had the longest width to height relationship until the Falcon v1.1 came along. So there's really not much margin to enlarge the first stage.
Of course that an hydrogen 2nd stage would greatly increase the GTO and GSO performance. But for GTO Zenit is a third stage rocket. The most logical decision would probably use only an H2 second stage to achieve roughly the same performance, but save one stage. But that's never gonna happen. The Russians only use Zenit because some critical intelligence assets need it. When Angara enters the market Zenit will be phased out and it's only users might be Sea Launch. Which is a grim future, if you ask me.
In fact, they have been able to get a profit because the contract with NPO Energomash for the RD-171M and RD-120 was made during Soviet times in Ukranian rubles. That's why Energomash has been selling their engines under cost. When that program is run out, things are gonna change drastically. Price will change drastically, which is not a good prospect.

Offline Jim

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Re: Why doesn't Zenit weigh more?
« Reply #12 on: 10/28/2012 09:13 pm »
Maybe the launch market is really driven by large nations that want to ensure their access to outer space and pork.

Huh? How the heck did you come to that conclusion?  Especially in face of the post above.

Ok. There is not enough evidence in this thread to conclude the launch market being driven by large nations. It is off topic for this thread, and would be a different thread.

Russia's transportation system might not be able to handle a stretched rocket. Heck, America's highways might not be able to handle the Falcon 9 v1.1, and Spacex intends to move the first stage by air or sea.

Zenit also served as a booster for the Energia rocket during Soviet times.

A different second stage would be needed to maximize payload to orbit.  ULA has proposed a dual RD-180 Atlas V. http://www.ulalaunch.com/site/docs/publications/EELVPhase2_2010.pdf
A new upper stage on a stretched Zenit, maybe from a foreign partner, could produce a rocket with 10 mt to GTO. Arianespace is able to find payloads for  10 mt into GTO.

No, the market is driven by large comsats and they have yet to max out existing vehicles.

Offline Salo

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Re: Why doesn't Zenit weigh more?
« Reply #13 on: 11/14/2012 08:43 pm »
Stretch Zenit on left:
« Last Edit: 11/14/2012 08:46 pm by Salo »

Offline Hyperion5

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Re: Why doesn't Zenit weigh more?
« Reply #14 on: 11/25/2012 05:16 am »
Stretch Zenit on left:

So is this a confirmation then that Yuzhnoye Design Bureau believes they can fit a stretched Zenit onto rail or truck transport then?  If they could do it the payload figures might be worth it if a stretched Zenit could send up to 15 mt into LEO and a Zenit Heavy could do 54-75 mt.  All they need to do to make this a real possibility is improve the Zenit's reliability and they could have a Falcon Heavy competitor ready remarkably quickly. 

I ran into ULA's study of options regarding the Atlas V Phase 2 awhile back.  They had one figure that really caught my eye.  (http://www.ulalaunch.com/site/docs/publications/EELVPhase2_2010.pdf)  On page 5, they listed 79 expected stage failures per 100,000 flights due to engine problems with a single RL-10 engine.  The 4-engine stage in contrast was considered likely to fail at a rate of only 5 in 100,000 flights.  The failure rate in other words was 15.8X as high with only one engine as with four. 

When I look at the Zenit's RD-171 and then at its quarter-size cousin, the new RD-191, it sure seem like using four RD-191 engines would be an easy change.  The question I have is, if the Zenit's major problem has always been reliability and not price, why not consider replacing the RD-171 with its quarter-size cousin?  Is there anything besides price that would stop such a change? 

Offline hkultala

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Re: Why doesn't Zenit weigh more?
« Reply #15 on: 11/25/2012 06:32 am »
Stretch Zenit on left:
I ran into ULA's study of options regarding the Atlas V Phase 2 awhile back.  They had one figure that really caught my eye.  (http://www.ulalaunch.com/site/docs/publications/EELVPhase2_2010.pdf)  On page 5, they listed 79 expected stage failures per 100,000 flights due to engine problems with a single RL-10 engine.  The 4-engine stage in contrast was considered likely to fail at a rate of only 5 in 100,000 flights.  The failure rate in other words was 15.8X as high with only one engine as with four. 

When I look at the Zenit's RD-171 and then at its quarter-size cousin, the new RD-191, it sure seem like using four RD-191 engines would be an easy change.  The question I have is, if the Zenit's major problem has always been reliability and not price, why not consider replacing the RD-171 with its quarter-size cousin?  Is there anything besides price that would stop such a change? 

RL-10 upper stage is completely different thing than RD-171/RD-191 first stage.

On first stage T/W is much more important than second stage so losing one engine would propably be too much unless it happens at quite high altitude.

And those RD-171/RD-191's are very high-pressure staged combustion cycle engines, there is much higher risk of them failing in catastrophical way instead of having nice clean shutoff.

And with 4 RD-191's there are more turbopumps that can fail.

And those 4 RD-191's would also be more expensive than 1 RD-171.

Offline Hyperion5

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Re: Why doesn't Zenit weigh more?
« Reply #16 on: 11/25/2012 05:44 pm »

RL-10 upper stage is completely different thing than RD-171/RD-191 first stage.

True, but you'll notice that it is the RL-10 on the Atlas V that has been the source of failure, not the RD-171's half-size cousin, the RD-180. 

On first stage T/W is much more important than second stage so losing one engine would propably be too much unless it happens at quite high altitude.

Not true if the Zenit's got an already excellent thrust:weight ratio at liftoff. It weighs, depending on version, around 460 metric tons (1.012 million lbs) at liftoff or a few tonnes more.  Even if you were to send up the heaviest version, the single RD-171 engine puts 1,641,000 lbf of thrust, giving the Zenit a robust 1.6:1 thrust:weight ratio at liftoff.  That kind of t:w ratio is good enough to give a four-engine Zenit T+1 engine-out capability. 

It should also be mentioned that four RD-191 engines would be a decent thrust upgrade over their larger cousin.  With each RD-191 putting out 431,600 lbf at liftoff, thrust jumps to 1,724,600 lbf, a 5.2% increase over current levels.  That would give the new Zenit an even better thrust-to-weight ratio at liftoff of approximately 1.7:1.  Even if you lost an engine at T+1, your thrust:weight ratio (1.275) would still be better than an Atlas V 401.  Basically the Zenit is robust enough it could handle an engine loss pretty much anywhere in the SI burn.  If you installed some blast panels like those on the Falcon 9, you could cut chances of an engine failure destroying the rocket with a minimal weight penalty.


And those RD-171/RD-191's are very high-pressure staged combustion cycle engines, there is much higher risk of them failing in catastrophical way instead of having nice clean shutoff.

And with 4 RD-191's there are more turbopumps that can fail.

And those 4 RD-191's would also be more expensive than 1 RD-171.


It's true that with engines like these any failure is more likely to be catastrophic.  It's also true that the Russians will be sending up cosmonauts on the Angara 5, a rocket with not 4 but 5 RD-191 engines.  By your reasoning why would the Russians do any such thing?

I think the answer lies in the inherent unreliability of a quad-chamber engine design compared to single-chamber and even dual-chamber designs.  The Chinese are introducing the Long March 5, with the heaviest variant powered by 2 smaller cores engines and 8 2646 Psi YF-100 staged combustion kerosene engines.  That's a total of 10 engine chambers firing together, with 8 of them having higher pressure than the NK-33. 

There's also the Atlas V, which looks likely to start sending up American astronauts in a few years' time.  Its RD-180 engine, unlike the RD-171, has proven extremely reliable.  It has proven so reliable that ULA floated plans of an Atlas V Phase 2 with the exact same number of engine chambers as the Zenit.  Yet with two RD-180 engines why doesn't ULA even appear to blink at the dangers of "catastrophic failures" you mention?  Why would they even consider an Atlas V Phase 2 Heavy with six of those engines (12 engine chambers and considerably more turbopumps) on three cores?  The answer, I think, is that dual RD-180 engines is a more reliable and safer setup than one RD-171. 

As for catastrophic failures, take a look at the record of high-pressure engines with single or dual chamber design. 

The Space Shuttle had three SSMEs with 2994 Psi in each, yet not once in 135 flights did an SSME cause either of the catastrophic failures, and that was despite engine problems on a few flights. 

The Atlas V's RD-180 has, if anything, proven more reliable than its lower-pressure predecessor.  It has suffered no catastrophic failures in flight. 

Then there's the Zenit and its quad-chamber RD-171.  It has suffered several catastrophic failures, several of them shortly after liftoff that necessitated either a complete pad replacement or renovation. 



« Last Edit: 11/25/2012 05:46 pm by Hyperion5 »

Offline baldusi

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Re: Why doesn't Zenit weigh more?
« Reply #17 on: 11/27/2012 07:01 pm »
Would you care to explain why the four chamber is inherently more unreliable? In particular, compare current 171M to the 180, since the 180 was developed specifically to solve the 170/1 reliability problems (which where on the TNA, btw). Also, do the most basic statistic math, and you'll notice that unless you get to 5 or 6 engine, you don't get better reliability.
The Angara is how it is due to transport and scalability issues, not reliability. In fact, when they wanted a fully customized solution for crew launch, they did chose RD-180V on 3.8m cores, mainly because of transport constraints.
And the RD-180V differs from the RD-180 in that it uses lower pressure to have better tolerance to particle contamination since the cleanness standard of the Russina LV is worse than ULA's. Most RD-171M failures were attributed to contamination.

Offline Jim

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Re: Why doesn't Zenit weigh more?
« Reply #18 on: 11/27/2012 07:12 pm »

There's also the Atlas V, which looks likely to start sending up American astronauts in a few years' time.  Its RD-180 engine, unlike the RD-171, has proven extremely reliable.  It has proven so reliable that ULA floated plans of an Atlas V Phase 2 with the exact same number of engine chambers as the Zenit.  Yet with two RD-180 engines why doesn't ULA even appear to blink at the dangers of "catastrophic failures" you mention?  Why would they even consider an Atlas V Phase 2 Heavy with six of those engines (12 engine chambers and considerably more turbopumps) on three cores?  The answer, I think, is that dual RD-180 engines is a more reliable and safer setup than one RD-171. 


Wrong.  The reason they use RD-180 for advanced designs is because they have a source for engine.

RD-17X problems have nothing to do with the number of chambers.  It has to do with manufacturing quality.

Offline Hyperion5

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Re: Why doesn't Zenit weigh more?
« Reply #19 on: 11/27/2012 07:19 pm »

There's also the Atlas V, which looks likely to start sending up American astronauts in a few years' time.  Its RD-180 engine, unlike the RD-171, has proven extremely reliable.  It has proven so reliable that ULA floated plans of an Atlas V Phase 2 with the exact same number of engine chambers as the Zenit.  Yet with two RD-180 engines why doesn't ULA even appear to blink at the dangers of "catastrophic failures" you mention?  Why would they even consider an Atlas V Phase 2 Heavy with six of those engines (12 engine chambers and considerably more turbopumps) on three cores?  The answer, I think, is that dual RD-180 engines is a more reliable and safer setup than one RD-171. 


Wrong.  The reason they use RD-180 for advanced designs is because they have a source for engine.

RD-17X problems have nothing to do with the number of chambers.  It has to do with manufacturing quality.

I've heard the manufacturer has been investigated for selling each engine under-cost to Lockheed Martin and Yuzhmash (although I've heard the Ukrainian deal is not as enticing).  Why then is the quality so much better with the RD-180? 

I think that even if they took a price hit, buying the same engine from the same assembly line building them for future Angaras would ensure Yuzhmash a more reliable launcher.  Given their relative lack of commercial backlog compared to even more expensive Arianespace, I can't help but feel they might benefit from actually spending more.  At with 4 RD-191 engines, this setup certainly wouldn't hurt the Zenit's overall weight much. 
« Last Edit: 11/27/2012 07:20 pm by Hyperion5 »

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