One big engine vs multiple smaller engines

[Crossover_Maniac]

The great one, Robert Truax, in an article for Aerospace America which is highlighted on the following website, http://www.suite101.com/article.cfm/13813/99510, was critical of the aerospace industry for over-designing rockets with complex turbopumps and lofty plans for reusuable launch vehicles that were more buck than bang.&nbsp; One of his main points was to simplify rockets by using only one engine per stage rather than multiple engines.&nbsp; But I want to take issue with that.&nbsp; It's due to safety and abort options.&nbsp; If a rocket has only one engine, then there's a risk of that engine failing causing the lost of the mission.&nbsp; But with multiple rocket engines, it's possible to have an abort option or even having a successful mission if the multiple engines are designed to be revved up in case of a lost of one or two of the engines.&nbsp; This is very important because reliability is a factor in the cost of launching a payload into space.&nbsp; A disastrous launch cost in destroyed payload and launch vehicle, downtown to investigate what went wrong, lost in confidence of customers, and a high insurance rate.&nbsp; What say you? <div class="Discussion_UserSignature"> Feel the Hope-nosis </div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The great one, Robert Truax, in an article for Aerospace America which is highlighted on www.suite101.com/article.cfm/13813/99510]this website[/url], was critical of the aerospace industry for over-designing rockets with complex turbopumps and lofty plans for reusuable launch vehicles that were more buck than bang.&nbsp; One of his main points was to simplify rockets by using only one engine per stage rather than multiple engines.&nbsp; But I want to take issue with that.&nbsp; It's due to safety and abort options.&nbsp; If a rocket has only one engine, then there's a risk of that engine failing causing the lost of the mission.&nbsp; But with multiple rocket engines, it's possible to have an abort option or even having a successful mission if the multiple engines are designed to be revved up in case of a lost of one or two of the engines.&nbsp; This is very important because reliability is a factor in the cost of launching a payload into space.&nbsp; A disastrous launch cost in destroyed payload and launch vehicle, downtown to investigate what went wrong, lost in confidence of customers, and a high insurance rate.&nbsp; What say you? <br />Posted by Crossover_Maniac</DIV></p><p>That is nice in theory, but I am not aware of any vehicles with an abort to orbit option.&nbsp; Such an option was one reason for considering the 5-segment SRBs for the shuttle when that design was developed about 5 years ago, but was never implemented.</p><p>The plain fact is that rocket performance is so critical and so difficult to come by that designs do not accomodate the redundancy needed to abort to orbit.&nbsp; If one had that kind of capability the likely response would be to launch heavier payloads.&nbsp; Many liquids are operated beyond 100% of rated capability as it is.<br /></p> <div class="Discussion_UserSignature"> </div>

 

[Crossover_Maniac]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>That is nice in theory, but I am not aware of any vehicles with an abort to orbit option.&nbsp; Such an option was one reason for considering the 5-segment SRBs for the shuttle when that design was developed about 5 years ago, but was never implemented.The plain fact is that rocket performance is so critical and so difficult to come by that designs do not accomodate the redundancy needed to abort to orbit.&nbsp; If one had that kind of capability the likely response would be to launch heavier payloads.&nbsp; Many liquids are operated beyond 100% of rated capability as it is. <br /> Posted by DrRocket</DIV></p><p>&nbsp;</p><p>My idea is that, even if there are six engines running at 75% capacity in case of them goes out, there wouldn't be enough fuel to warrent a boost in payload barring larger fuel tanks.&nbsp; The only difference in revving up the engines to +100% is that the launch vehicles has higher g forces.&nbsp; And it doesn't have to be an abort-to-orbit.&nbsp; If it's a reusuable first stage, it could just be getting off the launch pad and reaching the ocean where it could be recovered for later use. </p> <div class="Discussion_UserSignature"> Feel the Hope-nosis </div>

 

[qso1]

<p>The idea of simple, cheap rockets has been touted for decades but funny how most of the critics of current rocket design have yet to actually demonstrated their claims on the giant engines or rockets they advocated. Truax was one of the prominent figures in the "Sea Dragon" proposal which called for a giant LV with a single giant first and second stage engine. A rocket built at a shipyard out of steel rather than lightweight materials such as aluminum.</p><p>Sea launched also and tho seemingly sound in design, it never came close to actual concept demonstration on Sea Dragon scale AFAIK.</p><p>The proposal was canceled and never got beyond the paper studies as a result. Truax had done some testing of his proposals on rockets far smaller than the Sea Dragon was to have been. I'm no expert and by no means would I simply criticize the man for his concepts but...small rockets are relatively easy for individuals or small groups to develop and test. Giant rockets, a whole nother ball of wax.</p><p>Then Truax went on to propose a concept for cheap human spaceflight in the form of the Volksrocket. I recall this as far back as the mid 1980s or possibly even further back. The VR has yet to fly some two decades later and its nowhere near the size of Sea Dragon or BDB (Big Dumb Booster) concepts that were being touted post Challenger as the "Be all bend all" answer to cheap access to space.</p><p>Truax has to be 80 or 90 by now.</p><p>He wasnt the only one who claimed to be able to do better than NASA. There have been others and all have one thing in common. They have never been able to demonstrate their claims. Their supporters will tell you that its because the government is keeping them from doing it. But I have to ask and wonder, if these rockets were so cheap to build and operate, why have the government involved to begin with?</p><p>If a private sector company really thinks something will work...they simply develop what is known as proprietary technology and unveil it when there is little or no chance of government interferance. The "X" prize is the best example. Burt Rutan and his company Scaled Composites unveiled their plans to claim the "X" prize pretty much when they were about ready to fly...and fly it did.</p><p>And though Rutan is an ardent critic of NASA himself, he didn't let that be too well known till he at least had a provable concept. A concept that eventually claimed the "X" prize. Personally, I'm still waiting to see what Truax can do but at this point...I think I'll continue to breath.&nbsp;</p> <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;My idea is that, even if there are six engines running at 75% capacity in case of them goes out, there wouldn't be enough fuel to warrent a boost in payload barring larger fuel tanks.&nbsp; The only difference in revving up the engines to +100% is that the launch vehicles has higher g forces.&nbsp; And it doesn't have to be an abort-to-orbit.&nbsp; If it's a reusuable first stage, it could just be getting off the launch pad and reaching the ocean where it could be recovered for later use. <br />Posted by Crossover_Maniac</DIV></p><p>If you are still on the launch pad, why not just shut down ?&nbsp; If you are not, I doubt seriously that you are running at 75% since that implies a rather enormous intert weight penalty compared with running at 100+% of rated thrust.&nbsp; You are carrying the weight of 1 1/2 engines that are acting only to provide redundancy and add weight.&nbsp; Generally that is not a good idea.</p><p>Besides, just falling into the ocean doesn't buy you much.&nbsp; You will probably ruin the payload, which is worth more than the rocket by quite a large margin in most cases.&nbsp; If you want to save a crew in a manned flight you are better off taking the Ares 1 approach and designing a crew escape system that separates the crew capsule from the main rocket.<br /></p> <div class="Discussion_UserSignature"> </div>

 

[Crossover_Maniac]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>If you are still on the launch pad, why not just shut down ?&nbsp; If you are not, I doubt seriously that you are running at 75% since that implies a rather enormous intert weight penalty compared with running at 100+% of rated thrust.&nbsp; You are carrying the weight of 1 1/2 engines that are acting only to provide redundancy and add weight.&nbsp; Generally that is not a good idea.Besides, just falling into the ocean doesn't buy you much.&nbsp; You will probably ruin the payload, which is worth more than the rocket by quite a large margin in most cases.&nbsp; If you want to save a crew in a manned flight you are better off taking the Ares 1 approach and designing a crew escape system that separates the crew capsule from the main rocket. <br /> Posted by DrRocket</DIV><br /><br />I watch footage of a rocket engine shutting down seconds after firing, just enough for the rocket to fall back to earth and explode.&nbsp; Had it been multiple engines, it could have at least cleared the pad and at least the launch pad and tower could have been saved.&nbsp; And something could happen midflight.&nbsp; I know the redunancy takes up valuable weight but isn't the big problem with launch vehicle is that one thing goes wrong and everything is blown to bits?&nbsp; Giving abort options for launch vehicles, could that at least save some of the launch vehicle and payload and reduce the catastrophic failure rate?&nbsp;</p> <div class="Discussion_UserSignature"> Feel the Hope-nosis </div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I watch footage of a rocket engine shutting down seconds after firing, just enough for the rocket to fall back to earth and explode.&nbsp; Had it been multiple engines, it could have at least cleared the pad and at least the launch pad and tower could have been saved.&nbsp; And something could happen midflight.&nbsp; I know the redunancy takes up valuable weight but isn't the big problem with launch vehicle is that one thing goes wrong and everything is blown to bits?&nbsp; Giving abort options for launch vehicles, could that at least save some of the launch vehicle and payload and reduce the catastrophic failure rate?&nbsp; <br />Posted by Crossover_Maniac</DIV></p><p>Sounds like Ariane.&nbsp; A better plan is to design your rockets so that they don't do that. If I recall that failure correctly, it was a software problem.&nbsp; Generally speaking if you start shutting down engines you don't go very far, because you do not get enough thrust from what remains.&nbsp; Again, if you had that kind of redundancy the most likey course of action would be larger payloads.&nbsp; Clearing the pad may or may not be a good thing if a crash is going to follow.&nbsp; There currently are no abort options&nbsp;for unmanned launch vehicles.&nbsp; Ares 1 is being designed to provide for a crew abort.</p><p>I don't know if what you descrive is the "big problem with launch vehicles" but it is a fact of life and a major reason for all of the creat care taken in quality control for rocket components.</p><p>The bottom line is that gravity is really quite a worthy foe and it takes a very high performance and relatively costly rocket to overcome it and put a payload into orbit.&nbsp; You simply do not have the luxury of adding in redundancy that adds weight but is not used in the main launch event.&nbsp; Rocket are much closer to Ferrari's than to Peterbilt trucks, and they need every bit of their potential performance to get the job done.</p><p>To add the reduncancy that you seek will require a major advance in propulsion.&nbsp; Something relatively cheap but still with high performance is needed before rockets can be designed to have the normally unused performance&nbsp;that would allow for non-catastrophic aborts.&nbsp; <br /></p> <div class="Discussion_UserSignature"> </div>

 

[scottb50]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> Something relatively cheap but still with high performance is needed before rockets can be designed to have the normally unused performance&nbsp;that would allow for non-catastrophic aborts.&nbsp; <br /> Posted by DrRocket</DIV></p><p>The Russians have used the more is better idea from the beginning and had good luck with it. That they have had few failures makes it even safer. It also depends on where a failure happens, with the Shuttle one engine quitting would allow longer burn time for the other two, or even one, before things have to happen.&nbsp;</p><p>The same holds true for aircraft. I posted, as long time ago, my idea of the ultimate personal airplane. Five seats, with Corvette or Ferrari type accomodations, transcontinental non-stop range, Seattle to Miami, with reserve, from a 5,000 foot runway. Mach .96 cruise and better then ETOPS redundancy. Key to the idea is two engines and a single nozzle combined with mechanical inlet and nozzle ramps to manage airflow.</p><p>The same could be used for a rocket, multiple motors using a single nozzle, loss an engine and you lose flow but don't create yaw. It also makes more sense to trade the weight of turbopumps for structural weight and use nozzle cooling to heat an inert gas for a pressurized tank. Helium liquid could cool the nozzle and the heated cases would pressurize the tanks, two or maybe three combustion chambers would feed each nozzle. One fails the thrust reduces but the output is still there and can be more easily compensated for. Beyond that with two or more combusters sharing a nozzle and propellant supply the remaining units could be operated at higher power to compensate for a lose.</p><p>Not one big engine or multiple smaller engines but multiple combusters combined into a single engine, similar to the cannular design of older jets, multiple combustion chambers driving the turbine section.&nbsp; </p> <div class="Discussion_UserSignature"> </div>