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Unconventional design. The rocket looks very large and they mention 120 million newtons takeoff thrust so the weight is about 10 million kg, 3 times Saturn V s

Unconventional design. The rocket looks very large and they mention 120 million newtons takeoff thrust so the weight is about 10 million kg, 3 times Saturn V size. The first stage lands back on launch stool, the tanker stage is put on top of it and it launches again. This implies a relatively fast turnaround. Both the second stage and the tanker seem to have complete heat shields and stub wings and ability to re-enter either Mars or Earth. So all parts 1) big 2) landable and 3) short turnaround time This would imply they will launch them a lot, so they're really ramping up the traffic to Mars, not just a few missions? I don't understand the 100 000 km/h which is 30 km/s speed for the Mars cruise. Would be really hard to achieve with chemical propulsion. With 4 km/s exhaust velocity and delta vee of say 20 km/s you get a required mass ratio of e^(20/4) which is 150. And they lug the upper stage engines all the way, and there's heatshield and wings. Impossible. Also the 200 kW electrical power advertised is puny for electrical propulsion. With an ISP of 1000 (exhaust 10 km/s) you get an 20 Newtons of thrust [correction from 200N], which is nothing for their ship size. If they only have ten tons of mass, then it takes 200 days to accelerate to full speed. Typically you might need 10 to 20 km/s speed for Mars. Their plan does not compute for me. EDIT: So it can work if they only go for 10 km/s.

pinewurst 10y ago
Speed to Mars though is really relative speed along whatever orbit (Hohmann transfer or the moral equivalent) that you choose. Only in SF stories do we have fully powered propulsion to Mars. I am very curious what the selected family of orbits is, and even more importantly how long & how much life support for the large passenger number.
- twoodfin 10y ago
  Yeah, transit time & payload mass seem to be the major missing numbers in a video that wasn't shy about including them.
  - codeulike 10y ago
    Average 115 days, payload to Mars 450 tonnes. 100 people he reckons.
  - jonknee 10y ago
    There's a slide with all the numbers and even a graph: https://youtu.be/A1YxNYiyALg?t=1h52s https://youtu.be/A1YxNYiyALg?t=1h52s
- exDM69 10y ago
  > I am very curious what the selected family of orbits is ... There's not much leeway, it's going to be something close to a Hohmann transfer, with the aphelion ahead of and above Mars to get to a hyperbolic capture orbit with the periareion deep in the atmosphere for (almost) direct re-entry. This kind of orbits can be solved for arbitrary departure date and time of flight using a Lambert's problem solver and numerical optimization. Not a particularly difficult problem by modern standards. Pure Hohmann transfer to another planet will result in inevitable collision due to relative velocity, but it's useful for lower bound estimates. Another alternative for recurring Mars intercept is is the "Aldrin cycler" (after Buzz Aldrin) which is a free return trajectory but that is difficult when the goal is to land and take off again. There are lots of possibilities for lower energy transfers but most of them are out of question when life support is required for humans on board.
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Retric 10y ago
I don't think they would stick to pure ION drive or pure chemical rocks. Mixing them seems like a great option depending on how cheap they can get fuel to orbit. The could also refuel in mars orbit. A pure ion drive for all the equipment even if this means it takes 4x as long to get there, then a 'small' capsule back and forth for people.
- Gravityloss 10y ago
  The amount of power is a few orders of magnitude too low to propel these kind of masses electrically. Basically you need at least 10 kW per Newton.
  - Retric 10y ago
    If it only saves 2 weeks that's still useful. Don't forget if your using solar power you have much less power the closer you get to mars. So, you effectively get a free surplus at the start of your journey. The other option, is to have a pure transit vehicle from earth orbit to mars orbit and then a 3rd vehicle to land with. This, would work really well if we wanted to really colonize mars. But, if your only sending say 10 people the R&D is not worth the fuel savings. PS: Let's say it's a 20,000 lbs craft or ~1/10th of the shuttle and your getting ~8.8 N or 1 lbs of thrust for 120 days. That's 9.8 m/s / 20,000 * 60 * 60 * 24 * 120 = 5k m/s. If you already need the panels while in mars orbit, that seems like a no brainier. Finally, If that's 200kw in mars orbit then the numbers seem even more reasonable as that's 460 kw near earth and the extra 260kw ~= 26 newtons which could add ~5km/s to a 60,000 lb spacecraft.
    - Gravityloss 10y ago
      The craft's mass seems to be 15x that. http://imgur.com/a/vwG1S http://imgur.com/a/vwG1S Maybe for course corrections where you need to fine tune the orbit a couple of mm/s here and there? For unmanned missions it does make good sense as the mass doesn't grow so strongly if the trip time gets longer.
      - 10y ago
Gravityloss 10y ago
Ok, now it's starting to make a little bit more sense to me. They massively oversize the second stage and launch it almost empty, so it can just do the 5 km/s or whatever is needed for orbit. Then they launch multiple tankers to refuel it. They don't need lots of engines on the second stage because it's launched light. So it's pure chemical propulsion. But still with only 4 km/s exhaust velocity, no matter how you slice it, the mass ratios required from a single stage get impossible, ie even if the rocket tanks and engines didn't have any mass, payload sets an upper limit to the mass ratio. EDIT: So looks like it has 10 km/s delta vee. Not 30.
vkou 10y ago
The 30 km/s is speed relative to the Sun, at a half-way point between Earth and Mars. Most of it, you would get for free, since the ship is launched from the Earth, which is already moving at ~30 km/s. The delta-v cost is much less then that.

pinewurst 10y ago

Speed to Mars though is really relative speed along whatever orbit (Hohmann transfer or the moral equivalent) that you choose. Only in SF stories do we have fully powered propulsion to Mars. I am very curious what the selected family of orbits is, and even more importantly how long & how much life support for the large passenger number.

twoodfin 10y ago
Yeah, transit time & payload mass seem to be the major missing numbers in a video that wasn't shy about including them.
- codeulike 10y ago
  Average 115 days, payload to Mars 450 tonnes. 100 people he reckons.
- jonknee 10y ago
  There's a slide with all the numbers and even a graph: https://youtu.be/A1YxNYiyALg?t=1h52s https://youtu.be/A1YxNYiyALg?t=1h52s
exDM69 10y ago
> I am very curious what the selected family of orbits is ... There's not much leeway, it's going to be something close to a Hohmann transfer, with the aphelion ahead of and above Mars to get to a hyperbolic capture orbit with the periareion deep in the atmosphere for (almost) direct re-entry. This kind of orbits can be solved for arbitrary departure date and time of flight using a Lambert's problem solver and numerical optimization. Not a particularly difficult problem by modern standards. Pure Hohmann transfer to another planet will result in inevitable collision due to relative velocity, but it's useful for lower bound estimates. Another alternative for recurring Mars intercept is is the "Aldrin cycler" (after Buzz Aldrin) which is a free return trajectory but that is difficult when the goal is to land and take off again. There are lots of possibilities for lower energy transfers but most of them are out of question when life support is required for humans on board.

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Retric 10y ago

I don't think they would stick to pure ION drive or pure chemical rocks. Mixing them seems like a great option depending on how cheap they can get fuel to orbit. The could also refuel in mars orbit. A pure ion drive for all the equipment even if this means it takes 4x as long to get there, then a 'small' capsule back and forth for people.

Gravityloss 10y ago
The amount of power is a few orders of magnitude too low to propel these kind of masses electrically. Basically you need at least 10 kW per Newton.
- Retric 10y ago
  If it only saves 2 weeks that's still useful. Don't forget if your using solar power you have much less power the closer you get to mars. So, you effectively get a free surplus at the start of your journey. The other option, is to have a pure transit vehicle from earth orbit to mars orbit and then a 3rd vehicle to land with. This, would work really well if we wanted to really colonize mars. But, if your only sending say 10 people the R&D is not worth the fuel savings. PS: Let's say it's a 20,000 lbs craft or ~1/10th of the shuttle and your getting ~8.8 N or 1 lbs of thrust for 120 days. That's 9.8 m/s / 20,000 * 60 * 60 * 24 * 120 = 5k m/s. If you already need the panels while in mars orbit, that seems like a no brainier. Finally, If that's 200kw in mars orbit then the numbers seem even more reasonable as that's 460 kw near earth and the extra 260kw ~= 26 newtons which could add ~5km/s to a 60,000 lb spacecraft.
  - Gravityloss 10y ago
    The craft's mass seems to be 15x that. http://imgur.com/a/vwG1S http://imgur.com/a/vwG1S Maybe for course corrections where you need to fine tune the orbit a couple of mm/s here and there? For unmanned missions it does make good sense as the mass doesn't grow so strongly if the trip time gets longer.
    - 10y ago

Gravityloss 10y ago

Ok, now it's starting to make a little bit more sense to me. They massively oversize the second stage and launch it almost empty, so it can just do the 5 km/s or whatever is needed for orbit. Then they launch multiple tankers to refuel it. They don't need lots of engines on the second stage because it's launched light. So it's pure chemical propulsion. But still with only 4 km/s exhaust velocity, no matter how you slice it, the mass ratios required from a single stage get impossible, ie even if the rocket tanks and engines didn't have any mass, payload sets an upper limit to the mass ratio. EDIT: So looks like it has 10 km/s delta vee. Not 30.

vkou 10y ago

The 30 km/s is speed relative to the Sun, at a half-way point between Earth and Mars. Most of it, you would get for free, since the ship is launched from the Earth, which is already moving at ~30 km/s. The delta-v cost is much less then that.