tag:blogger.com,1999:blog-192883022973901942.post4386034570415966243..comments2017-07-19T03:03:43.985-07:00Comments on Bootstrapping Space: Colonize Mars - part 1, a really big rocket (updated 12 July with corrections)Chris Wolfehttp://www.blogger.com/profile/11247630943891521469noreply@blogger.comBlogger7125tag:blogger.com,1999:blog-192883022973901942.post-56675904795848374652016-07-15T00:58:17.972-07:002016-07-15T00:58:17.972-07:00I believe that this is a good way of presenting th...I believe that this is a good way of presenting the data. Much clearer, to me at least.<br /><br />https://goo.gl/hvzG72<br /><br />And the dry mass of a stage is just that stage itself, without any of the next stages. That way its easier to work with if you change the next stages out. For example when you stretch the second stage for more fuel.<br /><br />The burnout mass for a stage is unnecessary, as staging will get rid of the stage dry mass before the ignition of the next stage. The payload of a stage is only of interest if you want to turn that stage into an SSTO.<br /><br />As for the 'Fuel Ratio', usually its the Mass Ratio that gets noted down, meaning that the fuel weights x times the dry mass.<br /><br />From Atomic Rockets:<br /><br />R = M / Me // M = Total (wet) Mass, Me = Empty (dry) Mass<br /><br />R = (Mpt / Me) + 1 // Mpt = Total Propellant Mass (propellant in stage)Warringerhttps://www.blogger.com/profile/16942950119067535104noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-36950232936935846612016-07-14T16:33:50.400-07:002016-07-14T16:33:50.400-07:00The value for stage 1 dry mass is the mass remaini...The value for stage 1 dry mass is the mass remaining after stage 1 engine cutout which is then detached from the second stage. (seen as the difference between stage 1 burnout mass and stage 2 ignition mass.) That includes engines, tanks, the interstage fairing and other hardware as well as unburned fuel. This would be useful information if you wanted to calculate the fuel required for first stage return.<br />Likewise the value for stage 2 dry mass is the mass remaining after stage 2 engine cutout which is then detached from the payload. (Stage 2 burnout mass minus payload.) That includes engines, tanks, unburned fuel, an allowance for fairing (even though it would have been discarded earlier) and payload adapter plus other components. This would be useful information if you were considering repurposing the second stage in orbit since you know its mass, thrust and propellant capacity from the values in the table.<br /><br />The fuel ratio given is the fraction of the vehicle at ignition which is fuel. For the BFR-15 first stage that's 72.4% fuel and 27.6% everything else (stage 1 dry mass, stage 2 dry mass, stage 2 propellant and payload). For a given fuel ratio Rf the final mass Mf equals the initial mass M0 * (1 - Rf).<br />There seem to be varying conventions on what fraction to specify and what name to use for it; I've seen mass fraction, fuel fraction, fuel ratio and payload fraction (in that case taken to mean both vehicle mass and payload). Each of those has a specific technical meaning but I've found people rarely use them as precisely as their definitions warrant.<br /><br /><br />That said, it's certainly not the clearest table I've put together and it went through a few iterations. Can you suggest a better way to show this information?Chris Wolfehttps://www.blogger.com/profile/11247630943891521469noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-38952084201587813182016-07-14T14:46:49.665-07:002016-07-14T14:46:49.665-07:00I also think that the your table of dry mass vs fu...I also think that the your table of dry mass vs full mass is a little misleading.<br /><br />And your Mass Ratio calculation is off. Its M/Me for the Mass Ratio, for a single stage without any additional stages.Warringerhttps://www.blogger.com/profile/16942950119067535104noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-20718712258809105382016-07-14T14:38:29.446-07:002016-07-14T14:38:29.446-07:00Always happy to help.
And I have noticed the same...Always happy to help.<br /><br />And I have noticed the same for a few rockets I've developed for my SciFi universe myself.Warringerhttps://www.blogger.com/profile/16942950119067535104noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-34243438829862856242016-07-12T10:56:11.875-07:002016-07-12T10:56:11.875-07:00It was a pretty big mistake; I was using a tempora...It was a pretty big mistake; I was using a temporary table to explore masses and get a few dependent variables to converge and ended up using the wrong manually-entered payload for the dV calculations. Fixed, and the post has been updated.<br />I was able to get good results from the calculator you referenced, but only after scaling everything down. I'm guessing the absurdly high values are causing an overflow somewhere. One interesting result is that increased first stage thrust (more than the 1.258 to 1 shown here) would generate a higher net payload; the methodology paper explains why this is so.<br /><br />Thanks again for the reference; this is a very useful tool for estimating losses during ascent.Chris Wolfehttps://www.blogger.com/profile/11247630943891521469noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-28660593804095244062016-07-12T07:53:55.689-07:002016-07-12T07:53:55.689-07:00It seems to work nicely for existing rockets, than...It seems to work nicely for existing rockets, thanks for the tip. My numbers make it angry. I'm not sure if that is due to an error on my part or if it is because I am entering values far above the tool's valid range. No error is returned, so I can't tell where the problem lies. I tried scaling things up and down without success. It does return 18.9 tons for the F9 (my numbers) / Cape Canaveral / 185x185x28.5° which sounds reasonable.<br /><br />As a quick gut check, I can use the rocket equation to get the expected dV for each stage.<br />For stage 1 of BFR-15 I get:<br />dV = Ve ln(Minitial / Mfinal)<br />dV = 2948.5m/s * ln(29,897,400kg / 8,673,700kg)<br />dV = 2948.5m/s * ln(3.4469)<br />dV = 2948.5m/s * 1.2375<br />dV = 3,648.7m/s<br />For stage 2 I get:<br />dV = 3484.3m/s * ln(7,535,400kg / 1,914,800kg)<br />dV = 3484.3m/s * ln(3.9353)<br />dV = 3484.3m/s * 1.3700<br />dV = 4773.5m/s<br /><br />The stage 2 number doesn't agree with my table, so I've made a mistake in my spreadsheet somewhere.Chris Wolfehttps://www.blogger.com/profile/11247630943891521469noreply@blogger.comtag:blogger.com,1999:blog-192883022973901942.post-56940019460685660272016-07-12T02:05:37.249-07:002016-07-12T02:05:37.249-07:00I would like to point towards the Launch Vehicle P...I would like to point towards the Launch Vehicle Performance Calculator for any calculations for existing and not-existing launch vehicles.<br /><br />http://www.silverbirdastronautics.com/LVperform.html<br /><br />It's a good web-program to help with getting a payload out of a rocket.Warringerhttps://www.blogger.com/profile/16942950119067535104noreply@blogger.com