As a note, it occured to me where Rocketplane Kistler must have gotten then idea for the shape of the K-1.;-)
Quote from: Robotbeat on 06/18/2015 05:57 pmMars surface to earth is much less than 9km/s. More like 7-7.5km/s.But still the largest delta v requirement. At some point for growing vehicle dry weight other delta v requirements become larger. A 90mt vehicle weight may even be beyond that point. But leaving Mars surface it would be prudent to have significantly more capability than just the minimum. The item here is that use as 2nd stage to LEO, use as EDS and the use as direct return all have close to the same propellant requirements. The vehicle dry weight minus the payload weight drives which one is the driver for the amount the MCT must hold.My values gives the information that it is doable and the reasoning behind some of the speculation on what the MCT would do. For Earth departure you just keep filling the tanks until you get enough prop. Note here is that you need also prop for the landing phase on Mars of ~1km/s or more based on the vehicle shape and weight.Edit: Difference between 2nd stage or EDS and Mars liftoff is payload size 100mt vs 25mt.
Mars surface to earth is much less than 9km/s. More like 7-7.5km/s.
MCT will only be reused 15-30 times (once each for Mars entry and Earth entry), so for the crew version, PICA-X may be fine, perhaps with TPS replacement every decade or so. Metallic TPS probably doesn't make sense for MCT, except for perhaps a tanker or cargo version that would just travel to LEO but could be reused hundreds or even thousands of times and which isn't as mass-sensitive.PICA-X should be just fine for MCT, though, if it works as good as SpaceX says it does. metallic would be suboptimal except for tanker/cargo duty.
Quote from: oldAtlas_Eguy on 06/18/2015 02:39 pmFrom Statements made by SpaceX representatives:- 100mt payload delivery to Mars- 1/4 payload SSTO return to Earth from mars surface- prop density 1m^3 for 1mt (LOX and CH4)- 15m diameter vehicle (this was hited at not actually specified by SpaceX- Raptor engines 380-385 vacuum ISP 500klbfA vehicle like this results:- Vehicle structure+engines+ shield =40mt- Max propellant load 900mt- propulsion section (engines and tanks) cylindircal or nearly cylindrical section at base 15m diameter and 6m tall- bi-conal payload section (first section 15m to 10m diameter 10m tall) (second section 10m to 0m 10m tall) ~1800m^3 volume-MCT can be its own 2nd stage on the BFR (BFR is basically just the 1st stage) would have ~7.5km/s delta v capability with a 100mt payload+40mt vehicle dry weight +900mt propellant load-An MCT tanker variant would be a Cargo MCT without any cargo which could deliver ~150mt of propellant to LEO would have 6km/s delta v capabilityIn order to get to Mars 6-9 tankers docking in LEO-MEO are required Edit Added: BTW An MCT cargo used as the 2nd stage going just to LEO would be capable of delivering 180mt of payload. Note the 1st stage needs to be capable of ~3km/s delta v with a fully loaded MCT + 180mt of payload on top ~1120mt MCT+payload GLOW15m diameter and other such details have not been mentioned. Please cite your sources and put the source quote in the MCT source thread so we know exactly what was said: http://forum.nasaspaceflight.com/index.php?topic=37839.0
From Statements made by SpaceX representatives:- 100mt payload delivery to Mars- 1/4 payload SSTO return to Earth from mars surface- prop density 1m^3 for 1mt (LOX and CH4)- 15m diameter vehicle (this was hited at not actually specified by SpaceX- Raptor engines 380-385 vacuum ISP 500klbfA vehicle like this results:- Vehicle structure+engines+ shield =40mt- Max propellant load 900mt- propulsion section (engines and tanks) cylindircal or nearly cylindrical section at base 15m diameter and 6m tall- bi-conal payload section (first section 15m to 10m diameter 10m tall) (second section 10m to 0m 10m tall) ~1800m^3 volume-MCT can be its own 2nd stage on the BFR (BFR is basically just the 1st stage) would have ~7.5km/s delta v capability with a 100mt payload+40mt vehicle dry weight +900mt propellant load-An MCT tanker variant would be a Cargo MCT without any cargo which could deliver ~150mt of propellant to LEO would have 6km/s delta v capabilityIn order to get to Mars 6-9 tankers docking in LEO-MEO are required Edit Added: BTW An MCT cargo used as the 2nd stage going just to LEO would be capable of delivering 180mt of payload. Note the 1st stage needs to be capable of ~3km/s delta v with a fully loaded MCT + 180mt of payload on top ~1120mt MCT+payload GLOW
Quote from: Robotbeat on 06/18/2015 06:34 pmMCT will only be reused 15-30 times (once each for Mars entry and Earth entry), so for the crew version, PICA-X may be fine, perhaps with TPS replacement every decade or so. Metallic TPS probably doesn't make sense for MCT, except for perhaps a tanker or cargo version that would just travel to LEO but could be reused hundreds or even thousands of times and which isn't as mass-sensitive.PICA-X should be just fine for MCT, though, if it works as good as SpaceX says it does. metallic would be suboptimal except for tanker/cargo duty.So how would the PICA-X be arranged? Just on the nose like F9USR and K-1? Or on the nose and along one half of the cylinder like most biconic concepts?And how long would/could the PICA-X be made to last? Assuming two EDL's per round trip, and no more than 1 trip every 2 years, would mean 5 trips per decade max per MCT, and 10 EDL's? I think Dragon's PICA-X is supposedly good for 10 EDL's, but that's from LEO reentry speeds. The idea is to not have to replace the whole TPS system too often. If it only needed replaced once a decade, that wouldn't seem to be too cumbersome.A tanker MCT could do several LEO EDL's per year though. Then again, from LEO, metallic TPS should be adequate without the overheating issues of interplanetary return?But do they want two different TPS configurations? I would think they might rather go with a common platform. Would replacing a PICA-X TPS on a cargo/tanker MCT be too cumbersome to do every 10 missions? (which could be within one year theoretically) Can PICA-X tiles be made to last more than 10 LEO reentries? Or would that be overly heavy?Maybe a cargo/tanker MCT could have the PICA-X just on the nose like F9US-R and K-1, as it would be coming back only from LEO speeds like them, and would be uncrewed, so fairly high g-loading for that reentry profile wouldn't be a problem for humans. That's be a pretty ballistic reentry without much lift vs. a belly entry. But SpaceX and Rocketplace Kistler seemed to think it would work without crushing their beer cans.
I really don't understand why SpaceX would not limit the BFR to 12 million lbs thrust to take advantage of the Kennedy, then so what if they can only get 75 tons to Mars on MCT instead of 100. Once this BFR/MCT gets going, it will put SLS out of business since it will cost less and the entire Kennedy facility with 4 high bay doors and room for 6 BFR/MCT rockets at a time. That would save SpaceX a ton of money, it seems to me, building the infrastructure for such a large rocket. What would Kennedy be limited to 14m in diameter and 12 million lbs thrust?
A small forward facing heat-shield won't present enough surface area to slow down on Mars, you would probably impact the surface at high speed. A smaller ballistic coefficient works on Earth because we have a much deeper atmosphere to slow down in.
- propulsion section (engines and tanks) cylindircal or nearly cylindrical section at base 15m diameter and 6m tall- bi-conal payload section (first section 15m to 10m diameter 10m tall) (second section 10m to 0m 10m tall) ~1800m^3 volume
Peak heat is also peak deceleration, small forward face delays deceleration in time and in depth into the atmosphere so the time remaining to do any other deceleration is reduced even if your able to increese your cross-sectional area and create more drag. It's the same principle as opening a parachute on Mars, if your too late you impact the surface so I favor presenting maximum surface area as soon as possible.
And even under my lower velocity entry assumption something like ceramic tiles may be needed on the nose and control flaps, stagnation point temperatures can still be quite high.
I don't favor any kind of ablative because I believe we can eventually do more then 2 EDL per synod by sending cargo containers outside of a vehicle and loading them into the lander in Low Mars orbit. Then landing unloading refueling and returning to orbit with enough propellent to land again. At a generous 1 week turn around one landing can bring down ~100 cargo loads per synod, so an indefinite lifespan TPS would be needed for that. While only applicable to cargo this would be a huge improvement in efficiency over the baseline of 1 cargo in 1 lander in 1 synod.
Unfortunately, NASA's funding is not purely for exploration, science, tech, etc, it is also a regional development program for Dixie and a couple other places. So you can have the coolest spaceship ever imagined for super cheap and amazing capability, and it still won't get but a small fraction of funding from NASA. Additionally, NASA is so much more than human spaceflight and launch services. $4 billion annually is the max possible I can imagine anything like BFR/MCT ever getting from NASA (in the next half century), with maybe $1-2 billion being far more likely.
Then there's the 1st stage with expen$ive 10-15 meter tooling
Quote from: oldAtlas_Eguy on 06/18/2015 02:39 pm- propulsion section (engines and tanks) cylindircal or nearly cylindrical section at base 15m diameter and 6m tall- bi-conal payload section (first section 15m to 10m diameter 10m tall) (second section 10m to 0m 10m tall) ~1800m^3 volumeYour volume estimate is badly off, I calculate your segments are from bottom to top 1060 m ^3 for the cylinder, 1244 m ^3 for the first frustum, 261 m^3 for the top cone (which would in reality need to be blunted to some degree).Total 2565 m^3, a 60% increase over the vehicle size I'm proposing.Keep in mind that a F9 first stage has a volume of 480 m^3 and has dry masses of 25 mT and holds just shy of 400 mT of propellents, this should really show how absurd a vehicle with 5 times the volume holding more then twice the propellent and with all the heat shields necessary to do high speed EDL could possibly have a mass only 15 mT more then a F9 first stage.
It's a pressure vessel (and internal equipment needs internal supports) so weight scales with volume, not just area.