kerbalkicker Posted September 4, 2011 Share Posted September 4, 2011 Okay, maybe this has been covered before but I can't find it using search.So I've been using the java orbital calculator someone made to help get me into orbit, and I've tried a couple Hohmann transfers without much luck. I've noticed some pretty large variations in what the calculator tells me, and what my actual orbit is. If I input altitude and velocity at apogee, errors of 500+ m in altitude between what the calculator thinks my perigee should be and what it actually is are not uncommon (this is for a pretty nearly circular orbit around 45-50k). Has anyone else experienced this? Is there a known reason why I would be consistently getting 'off' results?Using the calculator, I tried to do a Hohmann transfer from 60k to about 40k. I plugged in the values and got my deltas, did the first burn at apogee... did my second burn at 180 degrees around the planet... and watched my goal altitude go screaming past as my kerbonauts plummeted into the atmosphere. When you guys do Hohmann transfers, do you do your second burn when you reach your goal altitude or before it? It would seem like you'd have to burn before you got there, because the ship is going to have some inertia from the descent it has to bleed off.tl;dr: inconsistencies between the calculator output and my results. Wondering if other people see the same, and what I can do to compensate. Link to comment Share on other sites More sharing options...
foamyesque Posted September 4, 2011 Share Posted September 4, 2011 The calculator consistently outputs orbital velocities that are too low; I've noticed this as well. I believe HarvesteR has also upped the size of the planet to 660km in .10Xs. Link to comment Share on other sites More sharing options...
kerbalkicker Posted September 4, 2011 Author Share Posted September 4, 2011 I believe HarvesteR has also upped the size of the planet to 660km in .10Xs.Oh... Well, that's going to change a few numbers. o.0 Link to comment Share on other sites More sharing options...
Xivios Posted September 4, 2011 Share Posted September 4, 2011 The calculator consistently outputs orbital velocities that are too low; I've noticed this as well. I believe HarvesteR has also upped the size of the planet to 660km in .10Xs.No, it's changed from 1200Km to 1200.12Km diameter, a fairly small change - 60 meters added to the previous radius of 600,000m. Link to comment Share on other sites More sharing options...
foamyesque Posted September 4, 2011 Share Posted September 4, 2011 Wurps. :-[ Link to comment Share on other sites More sharing options...
kerbalkicker Posted September 5, 2011 Author Share Posted September 5, 2011 For Hohmann transfers, how far before you reach your goal altitude do you do your second burn? Link to comment Share on other sites More sharing options...
Xivios Posted September 5, 2011 Share Posted September 5, 2011 In an ideal world: one in which your engine can instantly accelerate you to your desired velocity, you do so at the very top of the transfer (or bottom, depending on which way you're going.) Otherwise, erm, since it all varies depending on ship, altitude, starting altitude, I would say...wing it best you can? Link to comment Share on other sites More sharing options...
kerbalkicker Posted September 5, 2011 Author Share Posted September 5, 2011 Otherwise, erm, since it all varies depending on ship, altitude, starting altitude, I would say...wing it best you can?Naturally it varies. I was looking for guidelines other people use, i.e. 'with a light ship I start my 2nd burn 500 m from my target altitude' or something of that nature. Link to comment Share on other sites More sharing options...
foamyesque Posted September 5, 2011 Share Posted September 5, 2011 Naturally it varies. I was looking for guidelines other people use, i.e. 'with a light ship I start my 2nd burn 500 m from my target altitude' or something of that nature.I do it essentially at arrival except on very large transfers or using a low-thrust engine. Once you're in space, the standard liquid engine provides hojillions more thrust than you'll be likely to use, so you can approximate the ideal Hohmann burns pretty closely. Link to comment Share on other sites More sharing options...
kerbalkicker Posted September 6, 2011 Author Share Posted September 6, 2011 I do it essentially at arrival except on very large transfers or using a low-thrust engine. Once you're in space, the standard liquid engine provides hojillions more thrust than you'll be likely to use, so you can approximate the ideal Hohmann burns pretty closely.ok, thanks. The last time I tried that my rocket didn't stop and just plunged... down... to a fiery death :/ Link to comment Share on other sites More sharing options...
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