This might not be quite as big of an issue as is normally argued if they can launch from VAFB. It might be possible to transport a wider diameter stage from Hawthorne to VAFB by boat/barge. Though, they might lose the ability to test it in TX first.
Quote from: Jimmy Murdok on 04/11/2015 09:03 pmIn a practical way you don't obtain substantial improvement with methane over querosene, the slight improvement on ISP (3,8%) is more than lost in the higher volume (22%). Falcon 9 is designed for road transport, if you switch to methane you have to increase the size of the tanks and you lose this capability, so it doesn't make sense.This might not be quite as big of an issue as is normally argued if they can launch from VAFB. It might be possible to transport a wider diameter stage from Hawthorne to VAFB by boat/barge. Though, they might lose the ability to test it in TX first.
In a practical way you don't obtain substantial improvement with methane over querosene, the slight improvement on ISP (3,8%) is more than lost in the higher volume (22%). Falcon 9 is designed for road transport, if you switch to methane you have to increase the size of the tanks and you lose this capability, so it doesn't make sense.
What is the advantage? Kerosene is already great, very dense and easy to storage. If you don't need to collect from Mars is better than methane.
Quote from: dror on 04/11/2015 08:05 pmQuote from: spacenut on 04/11/2015 06:53 pmDoes anyone think they will make a metholox Merlin engine? If so, what kind of capacity would the Falcon heavy have converting all to metholox?My bold What I would like to know is the expected isp of a gas generator methalox engine compared to a kerolox gg engine, and that would be a good beginning to answer your question. If the difference is not substantial , I would stick to the speculation (adopted from the mini BFR thread) that with\without crossfeed, the next step is a reusable single raptor upper stage for FH, followed by replacing both F9R and FH cores with a 9 raptor reusable booster.In a practical way you don't obtain substantial improvement with methane over querosene, the slight improvement on ISP (3,8%) is more than lost in the higher volume (22%). Falcon 9 is designed for road transport, if you switch to methane you have to increase the size of the tanks and you lose this capability, so it doesn't make sense.Methane improves the reutilization of the engines through a cleaner combustion with les maintenance. The second good point of methane over querosene is the capacity of ISRU production in Mars. What it would work would be a querosene closed cycle Merlin Engine to improve the ISP, with methane you would end up with a too long stick. Methane has never been used for rockets because querosene and hydrogen work better. This might change with reutilization once you don't care about road transportation (BFR).
Quote from: spacenut on 04/11/2015 06:53 pmDoes anyone think they will make a metholox Merlin engine? If so, what kind of capacity would the Falcon heavy have converting all to metholox?My bold What I would like to know is the expected isp of a gas generator methalox engine compared to a kerolox gg engine, and that would be a good beginning to answer your question. If the difference is not substantial , I would stick to the speculation (adopted from the mini BFR thread) that with\without crossfeed, the next step is a reusable single raptor upper stage for FH, followed by replacing both F9R and FH cores with a 9 raptor reusable booster.
Does anyone think they will make a metholox Merlin engine? If so, what kind of capacity would the Falcon heavy have converting all to metholox?
Well, the question was about a methalox Merlin (gas generator) , not about a kerolox mini Raptor (staged combustion) . I reckon that was assuming the first option is easier to develop. Thanks for your answer -"the slight improvement on ISP (3,8%) is more than lost in the higher volume (22%)."As I get it, since the rocket equation is exponential for isp, the 3,6% isp increase can't compare with the 22% volume reduction. Some math has to be made in order to know if the payload increases or decreases with these changes and the same total volume of the F9.
Falcon 9 is designed for road transport, if you switch to methane you have to increase the size of the tanks and you lose this capability, so it doesn't make sense.
But in the particular case of Falcon rockets there is no discussion as the tank size is the big constraint that can not grow.
Quote from: dror on 04/11/2015 10:16 pm But in the particular case of Falcon rockets there is no discussion as the tank size is the big constraint that can not grow. But even if it could grow I don't see any advantage.Tank diameter is not an absolute constraint. It is a constraint that SpaceX has placed upon itself to reduce costs, if it ever truly gets in the way of an important goal it will be discarded. A true need to get FH a much higher energy upper stage might be enough reason if that market opportunity ever materializes.But that would be years down the road, and SpaceX will likely never encounter the business need.
But in the particular case of Falcon rockets there is no discussion as the tank size is the big constraint that can not grow. But even if it could grow I don't see any advantage.
Quote from: dror on 04/11/2015 10:16 pmWell, the question was about a methalox Merlin (gas generator) , not about a kerolox mini Raptor (staged combustion) . I reckon that was assuming the first option is easier to develop. Thanks for your answer -"the slight improvement on ISP (3,8%) is more than lost in the higher volume (22%)."As I get it, since the rocket equation is exponential for isp, the 3,6% isp increase can't compare with the 22% volume reduction. Some math has to be made in order to know if the payload increases or decreases with these changes and the same total volume of the F9.If you keep the same tanks you have 22% less fuel, no way it compensates throught ISP. Otherwise Musk would have gone for methane from the beginning.In case you make bigger tanks (22%) then you increase the tank weight in small percentage and is here where you loose the small advantage in ISP. Same engine for Kerosene, Methane, Hydrogen give ISP of: 355,368,456s. For Hydrogen you always need bigger tanks with isolation, so for same weight of fuel you have much more weight, but the ISP (+24%) here is making big big difference. In kerosene VS methane the difference is really small and you probably end up in something symilar. But in the particular case of Falcon rockets there is no discussion as the tank size is the big constraint that can not grow. But even if it could grow I don't see any advantage.
Quote from: Jimmy Murdok on 04/11/2015 10:38 pmQuote from: dror on 04/11/2015 10:16 pmWell, the question was about a methalox Merlin (gas generator) , not about a kerolox mini Raptor (staged combustion) . I reckon that was assuming the first option is easier to develop. Thanks for your answer -"the slight improvement on ISP (3,8%) is more than lost in the higher volume (22%)."As I get it, since the rocket equation is exponential for isp, the 3,6% isp increase can't compare with the 22% volume reduction. Some math has to be made in order to know if the payload increases or decreases with these changes and the same total volume of the F9.If you keep the same tanks you have 22% less fuel, no way it compensates throught ISP. Otherwise Musk would have gone for methane from the beginning.In case you make bigger tanks (22%) then you increase the tank weight in small percentage and is here where you loose the small advantage in ISP. Same engine for Kerosene, Methane, Hydrogen give ISP of: 355,368,456s. For Hydrogen you always need bigger tanks with isolation, so for same weight of fuel you have much more weight, but the ISP (+24%) here is making big big difference. In kerosene VS methane the difference is really small and you probably end up in something symilar. But in the particular case of Falcon rockets there is no discussion as the tank size is the big constraint that can not grow. But even if it could grow I don't see any advantage.But this is no case for SpaceX. They are going from RP-1 Gas Generator to Methane Full Flow. Big difference. And actual RSC Energyia engineer has calculated that a methane Falcon 9 with the same dimensions as v1.1, but engine performance as Raptor, would get 25tonnes to LEO and 8tonnes to GTO. That covers 100% of current market. And that's without the 10% tank lengething and propellant densification that SpaceX is implementing on the enhanced Falcon 9, nor 2050 aluminum tanks and other "cheap" enhancements. Probably could hit 30/10 with that.If they do a mini Raptor upper stage, they'll probably move the cores later. They'll have the performance margin for upper stage reuse and validate everything for MCT for a lot less money than a whole new development.
But this is no case for SpaceX. They are going from RP-1 Gas Generator to Methane Full Flow. Big difference. And actual RSC Energyia engineer has calculated that a methane Falcon 9 with the same dimensions as v1.1, but engine performance as Raptor, would get 25tonnes to LEO and 8tonnes to GTO. That covers 100% of current market. And that's without the 10% tank lengething and propellant densification that SpaceX is implementing on the enhanced Falcon 9, nor 2050 aluminum tanks and other "cheap" enhancements. Probably could hit 30/10 with that.If they do a mini Raptor upper stage, they'll probably move the cores later. They'll have the performance margin for upper stage reuse and validate everything for MCT for a lot less money than a whole new development.
If Raptor is 500k lb thrust and Merlin vacuum is 200k lb thrust, that is a lot more thrust. Would the upper be widened to hold more fuel, or stretched? Unless the Raptor vacuum can be throttled down.
Do you think Spacex will do a min-raptor upper stage or just use the BO engine to replace the current upper stage?
Quote from: HIP2BSQRE on 04/12/2015 12:48 am Do you think Spacex will do a min-raptor upper stage or just use the BO engine to replace the current upper stage? SpaceX is not going to use a non-SpaceX engine in any of its launch vehicles - period. Whatever they use will be built in-house.
Quote from: spacenut on 04/11/2015 07:39 pmIf Raptor is 500k lb thrust and Merlin vacuum is 200k lb thrust, that is a lot more thrust. Would the upper be widened to hold more fuel, or stretched? Unless the Raptor vacuum can be throttled down. If you assume 550 klbf for the vac version (~250 tf), that would need a minimum ~20t payload for 6g burnout @ 50% throttle. Fine for LEO with FHR, but not for GTO / escape unless they go with FHE (maybe recover the boosters?) Good for a prop tanker for MCT, though. Cheers, Martin
I think some of your figures might be a little off. The estimate I've seen several times for the ISP of a vacuum Raptor is 380, and the best estimate I've seen for the ISP of the present upper stage Falcon is 345. So this would be more like a 10% ISP improvement rather than 3.8%. And with the exponential nature of the rocket equation, this is quite significant.
Quote from: GORDAP on 04/12/2015 11:50 amI think some of your figures might be a little off. The estimate I've seen several times for the ISP of a vacuum Raptor is 380, and the best estimate I've seen for the ISP of the present upper stage Falcon is 345. So this would be more like a 10% ISP improvement rather than 3.8%. And with the exponential nature of the rocket equation, this is quite significant.This ISP are between different fuels for same X engine. The big increase in ISP between merlin and raptor is mainly because of the full flow engine not because of the fuel. I think it would make a lot of sense to have a full flow engine for the upper stage and improve the ISP. What I don't see is the switch to methane. A mini raptor methane upper stage would fit and maybe in 5 years we will see it in case they have a mini-raptor, but FMPOV a kerosene mini raptor would fit better. The switch between kerosene and methane "is not a big deal", so I don't see that crazy that once they have a mini raptor they adapt it to kerosene. But thats lot of speculation.