Griffin's CEV plans?????

[starfhury]

I did mean to start a new thread for this, but I believe my original post on the idea would have gotten lost in some other thread.<br /><br />Here's the question I asked about the stick launched CEV. Can the second stage attached to the CEV achieve LEO and if it can achieve LEO can it not also achieve LLO if launched by itself from the moon? This might explain the additional mass requirement Griffin is looking for. The same rocket engine used on the CEV to reach LEO would see reuse to launch it from the moon. The CEV would attach to a TLI stage to be ferry to and from LEO and LLO. If that were the case, NASA would achieve a smart level of integration, a reduction in the total amount of vehicle required and even a reduction of the need for a SDLV for any CEV support. They are already considering a cargo version of the stick CEV. So a stick CEV to refuel the TLI stage and one to lauch the CEV for link up in orbit. So the question I found needing an urgent answer to is whether a CEV massing 30 to 40 tons can carry enough internal prop to land and relaunch from the moon? Please keep in mind that we'd have the TLI stage in LLO for return to LEO. <div class="Discussion_UserSignature"> </div>

 

[henryhallam]

<font color="yellow"><br />So the question I found needing an urgent answer to is whether a CEV massing 30 to 40 tons can carry enough internal prop to land and relaunch from the moon?<br /></font><br /><br />Well considering the Apollo LM massed under 15 tonnes (full of propellant), in theory the answer is yes. But it would be a bit of a one-trick pony since a lunar lander is quite a specialised vehicle. Also, are you sure that the TLI stage will remain in lunar orbit for return to Earth? I thought the current plan was not to leave anything in lunar orbit, thus allowing long-duration stays at locations other than the lunar equator.

 

[starfhury]

That's the beauty of this approach, the TLI does not have to stay in LLO. It can return to LEO and pick up another CEV before the one on the moon is ready for return. Ideally we'd like to have one in LLO at all times for emergencies, but if you have two or more TLIs it'd be nothing to keep one on station all the time.<br /><br />I also had another thought. My thought is that we bring back the Delta Clipper or a modified version to server CEV duties. We attach it to a five segment SRB and use the Delta Clipper (DC) as CEV second stage to achieve LEO. It attaches and refuel at the TLI is ferried to LLO then lands on the moon. The DC was extensively tested before its program was terminated, but I see it as the test case for a CEV capable of a moon landing and also earth return vehicle. <div class="Discussion_UserSignature"> </div>

 

[henryhallam]

I'm sorry but that really doesn't make a great deal of sense to me. You want to build a single stage that can accomplish TLI, LOI, TEI and then brake into LEO ready for use again? The problem is this:<br /><br />Initial mass for a stage that can do TLI, LOI, TEI (all with CEV along for the ride) = x tonnes<br />Initial mass for a stage that can do TLI, LOI, TEI and brake into LEO = more than 2*x tonnes.<br /><br />So if you want this stage to be reusable then you have to lift MORE THAN TWICE the mass into LEO, and you STILL have to lift more propellant to fill it up again for the next trip! (about 80% of its mass). Furthermore everything on it must be rated for long-term operation with multiple restarts and no servicing.<br /><br />For the same amount of mass lifted to LEO from Earth, you can deliver well over twice the payload to the moon with an expendable approach - even in the long run, the "economics" of the resuable system STILL give barely over half the mass delivered to the moon as you would get by repeatedly launching expendable stages.

 

[starfhury]

If you are only looking at launching maximum mass to the moon, then yes a one way trip with an expendable stage is the way to go. We are talking about astronauts though. I don't think you'd have too many volunteers for a one week one way trip to the moon. That said, you are going to require at minimum three delta V changes. TLI, LOI, and TEI. EOI is optional because you can aerobrake to save on prop on return to earth. I'm not sure how you came up with the figures, but in order to balance the equation, TLI + LOI would be nearly equal to TEI + EOI, the latter needing more prop because of earths greater gravitation pull. I don't know what the factor is. But the only place we'd be able to save on prop would be to aerobrake into EOI instead of using prop. <br /><br /> <div class="Discussion_UserSignature"> </div>

 

[henryhallam]

Very roughly speaking TLI=EOI = 3100 to 3200 m/s<br />LOI = TEI = 900 to 950 m/s.<br /><br />If you want to fiddle around with the numbers and see how the mass comes out, download this quick spreadsheet I made:<br /><br />http://henryhallam.cjb.net/~henry/dV.xls<br /> <br />Remember to leave enough in the "mass after" cell at the end to cover the engines, tanks and other systems.

 

[starfhury]

Thanks for the spreadsheet. After adding up the numbers though, it turns out that you'd require roughly 62% of the mass of TLI, LOI, TEI added in so you can do TLI, LOI, TEI, and EOI. Rounded off it can be construed as twice the mass which is not good. But then, if you took a fifty mile round trip in a car, you'd have the same situation. Again, I'd say since we are talking about returning astronauts and a limited amount of payload to and from the moon, the sacrifice in ultimate payload to the moon might be worth the trade offs. We'd have a permanent in space transporter which might eventually be refueled by other means and not just from earth. Even private industry might be contracted to launch prop containers. I'd serve to created a need for a new market and possible other innovative solutions to solve other problems. <div class="Discussion_UserSignature"> </div>

 

[henryhallam]

<font color="yellow"> After adding up the numbers though, it turns out that you'd require roughly 62% of the mass of TLI, LOI, TEI added in so you can do TLI, LOI, TEI, and EOI.<br /></font><br /><br />I don't see this?<br />Put 100 tonnes in as initial mass in cell B5. You see that this lets you get 31 tonnes to the moon and back for re-entry (TLI, LOI, TEI) - i.e. look in cell D6. Now imagine you want to get that same 31 tonnes all the way back to LEO. Increase cell B5 until you get 31 tonnes in cell E6. You will need to increase B5 (initial mass) to 205 tonnes. <br /><br /> This is more than twice as much, not just 62% more.<br />The situation gets worse with lower Isp, and better with greater Isp but you aren't going to do an awful lot better than 440 seconds without a nuclear rocket.

 

[starfhury]

I see your point. It also points out why we should consider other options like a nuclear TLI. Since that won't happen in this life time, it's going to be very interesting to see how the CEV pans out and what we get for the billions spent. <div class="Discussion_UserSignature"> </div>

 

[nacnud]

You might be right about there not being a nuclear stage but I think a solar electric stage is a possibility. Having a launcher capable of putting 120MT into LEO in one shot makes it more likely that a one peace solar electric stage will be planned for eventually.<br /><br />Just as a reference the ISS solar truss weights 136.7 tonnes and generates 110kw of power.<br />

 

[gunsandrockets]

"Just as a reference the ISS solar truss weights 136.7 tonnes and generates 110kw of power. "<br /><br />Ugh. That's very disappointing. No wonder nuclear reactor power is so important for ion rockets. I believe the canceled JIMO nuclear electric rocket was going to have 100kw of power and yet the entire spacecraft complete with xenon rocket propellent only massed 20 tonnes.<br />