Actually, it's extremely easy to argue against this. Smart bombs routinely achieve meter-level accuracy using only fins, and have for decades. It's well proven technology.
Is it known that the fins are for guidance and not just stability?
The altitudes where winds are the strongest (above 25k ft) are exactly where terminal speed is still significant (but subsonic), hence grid fins will have maximum effect.
Quote from: LouScheffer on 01/08/2016 03:34 pmQuote from: abaddon on 01/08/2016 03:10 pm It is true that the grid fins have more control authority at a higher speed, but we don't know that they are capable of a pinpoint landing without any engine burn. In other words, I am asserting the engine burn is not just to bring vertical velocity to zero, it is also to target the final landing point. I think this would be difficult to argue against. Actually, it's extremely easy to argue against this. Smart bombs routinely achieve meter-level accuracy using only fins, and have for decades. It's well proven technology. Is it known that the fins are for guidance and not just stability?
Quote from: abaddon on 01/08/2016 03:10 pm It is true that the grid fins have more control authority at a higher speed, but we don't know that they are capable of a pinpoint landing without any engine burn. In other words, I am asserting the engine burn is not just to bring vertical velocity to zero, it is also to target the final landing point. I think this would be difficult to argue against. Actually, it's extremely easy to argue against this. Smart bombs routinely achieve meter-level accuracy using only fins, and have for decades. It's well proven technology.
It is true that the grid fins have more control authority at a higher speed, but we don't know that they are capable of a pinpoint landing without any engine burn. In other words, I am asserting the engine burn is not just to bring vertical velocity to zero, it is also to target the final landing point. I think this would be difficult to argue against.
Quote from: LouScheffer on 01/08/2016 03:34 pmQuote from: abaddon on 01/08/2016 03:10 pm It is true that the grid fins have more control authority at a higher speed, but we don't know that they are capable of a pinpoint landing without any engine burn. In other words, I am asserting the engine burn is not just to bring vertical velocity to zero, it is also to target the final landing point. I think this would be difficult to argue against. Actually, it's extremely easy to argue against this. Smart bombs routinely achieve meter-level accuracy using only fins, and have for decades. It's well proven technology.Smart bombs have far more control authority and glide capability than a F9R stage.
Warning: Wall of text incoming! tl;dr: Looks like SpaceX could save fuel/weight by landing more aggressively. Could they? Will they? ......So, first off can anyone point out any obvious math errors? (i know "total impulse" for breaking + Gravity losses is probably the wrong terminology) Beyond the feasibility of my scenarios, I would like to know if I didn't even calc the fuel use to within an order of magnitude. But more interestingly, how useful is a fuel savings of about 2000kg? or even just 600kg? Is there any reason to expect that SpaceX will attempt to land more aggressively and really push the limits? I don't want to say that I was disappointed by the landing, but it wasn't exactly as "brown pants" of a maneuver as I was expecting. But maybe my perception betrays just how difficult of landing it already was? Anyway, after doing all these calcs I figured that with how much use I get out this forum, that I should try to give back and hope someone else finds this interesting as well. It is now way too late for me to still be awake please forgive typos.
Nobody realized that saving 2000 kg of propellant in the landing burn decreases the mass during reentry burn (and boostback burn, if done).Therefore saving could be easily double that, because propellant for reentry burn (and boostback burn) can be less.
...or enable a less lofted trajectory.
Quote from: cambrianera on 01/08/2016 04:35 pmNobody realized that saving 2000 kg of propellant in the landing burn decreases the mass during reentry burn (and boostback burn, if done).Therefore saving could be easily double that, because propellant for reentry burn (and boostback burn) can be less.Not really, because the dominant factor is the 26,000 kg dry mass on the first stage, so carrying 2,000 kg less propellant through boostback and re-entry reduces vehicle mass by less than 10%, and thus gives you less than 10% additional propellant savings for a given deltaV.
Quote from: PreferToLurk on 01/08/2016 08:16 pm...or enable a less lofted trajectory. More loft is more air time, which means less velocity back to landing site required, which means less boostback required.
Time for a smaller analogy. Let's say you have a car that gets 40 mpg. This car has a 10 gallon tank, so for each tank you can go 400 miles. Put another way, to go one mile your car burns about 3.2 ounces of gas. Let's say you think you might save gas by coasting in downhill sections of roadway rather than keeping your foot on the gas. Let's say your calculations show that by doing this you will save 0.5% of your car's tank of fuel, which is 6.4 ounces of gas. This enables you to drive approximately 2 additional miles. Given the car normally goes 400 miles on a tank anyway, is this extra 2 miles going to be worth it?
Landings should be boring, safe, routine, with plenty of margin.
To re-iterate a little more about the projected capability for FT. 1. F9 v1.1 has "demonstrated" an expendable capability of ~ 4,850kg to GTO-1800. Two sets of data compound to that assertion. a. The biggest GTO payload was the 4,707kg TurkmenAlem52E. It was placed in a roughly GTO-1765 orbit (180x36600x25.5)b. Thaicom 6 (a 3,016kg sat) got to a GTO-1500 equivalent orbit (295x90000x22.5). This stretched S2 fuel reserves to almost complete depletion (according to USAF, which evaluated this flight as part of the SpaceX EELV certification procedure). 2. F9 FT as a whole is reported to be around 30% more capable than F1 v1.13. DPL (barging) costs about 15% payload. This means that F9 FT has a theoretical capability of about 6,300kg to GTO-1800Barging moves it to 5,355kg.SES9 is 5330 kg.Its looking very close. Depending on whether the rocket goes to DPL or not, as well as what the end orbit is, we are going to get a lot of info about the current Falcon variant capabilities. If this pans out, and RTLS removes another 15% of performance, F9 FT would be able to RTLS after sending a 4410kg payload to GTO-1800. This number is interesting for some of the following missions (quoting from here):Thaicom 8 3100kg GTO YesABS 2A, Eutelsat 117 West B ~4000kg? GTO Possibly (based on ABS-3A, Eutelsat 115 West B mass)JCSAT-14 ~3400kg? GTO Probably (based on JCSAT-15 mass)BulgariaSat-1 ~3400kg? GTO Probably (based on JCSAT-15 mass, same SSL-1300 bus)JCSAT-16 ~3400kg? GTO Probably (based on JCSAT-15 mass)KoreaSat-5 4465kg GTO PossiblyEs'hail-2 ~3000kg GTO ProbablyIf F9 FT performance upgrade over v1.1 I listed above is correct (some say its more, like 33%), then almost all the missions above would be eligible for RTLS, OR a better orbit than GTO-1800 + DPL.
Quote from: macpacheco on 01/08/2016 12:35 pmI think the legs could burn with rocket exhaust if exposed too soon.If the deploy mechanism would be changed to allow this they could do a partial deploy first. Make it look like an arrowhead. It would provide drag and keep the legs away from the flames. It should cause less stability issues too.
I think the legs could burn with rocket exhaust if exposed too soon.