New Mars Forums

And nothing is as expensive as a dead crew,  especially one killed by bad management decisions.  THAT is why NASA is so fatally risk-averse now.  Spacex is still focused on designing and building this BFR/BFS vehicle.  Ultimately,  they will have to address this rough field landing issue,  which so far they so very obviously have not.

It is this rough-field risk that impels me to insist that BFS's to Mars (or the moon) should be landed on hard-paved,  large,  very level landing pads,  as early in the process as possible.  The first ones must make rough-field landings,  but that risk can be reduced by a proper pathfinder probe to the site.  I have already described such in another thread.

GW

I've never seen a thing like that proposed before.  I honestly don't know how it might really perform.  It in effect gives the flow separation a definite point to happen,  rather than the typical oscillating-around unsteadily.  Whether that would really work,  I dunno.

GW

I think the idea is fundamentally sound,  but there's a lot of devils in the details.  They have to do with making the aerospike structure,  which is buried deep within all the rocket gas,  survivable at heat transfer recovery temperatures that are very nearly chamber temperature,  all the way to the tip.

I think this can be done,  probably with fuel regenerative cooling.  But the added-weight and complexity-reliability issues may overpower the benefit you would otherwise gain.  I honestly don't know.  But a version of this was the propulsion design on X-30,  so it has to be at least partially beneficial.

As for conventional nozzles,  a sea level nozzle works just fine in vacuum,  it just doesn't get quite as high a thrust and specific impulse as a vacuum design with a larger exit bell.  It even gets more thrust than it did at sea level,  via the positive backpressure vs expanded pressure term on the exit area.  The converse is not true:  a vacuum design usually won't work at all at sea level.  This is way beyond the negative thrust term for backpressure versus expanded pressure on the exit area;  most such failures really are full-blown backpressure-induced flow separation. 

GW

Now this may or may not apply to whatever it is you had in mind,  I dunno.  But be careful,  you can't do this as some sort of nacelle outside the main body of the spacecraft.  Above about Mach 5 to 6,  there's no way to survive the shock-impingement aeroheating where the nacelle bow shock strikes the main body. 

GW

We need something that reduces reactor mass like Timberwind, but ensures the engine is restartable like NERVA. Timberwind had Isp=1000s, the 1991 study to update NERVA had Isp=925s. That difference was due to temperature. If a new reactor only produce 925s, good enough. We certainly don't have to go back to the 1972 or 1974 version of NERVA, or even the 1991 version. We should be able to apply technology from Timberwind to reduce reactor mass while ensuring it's restartable.
.

For the NTR I think that the heavier the atomic weight that is liquid will give the biggest level of force when energized by the thermal heat of the reactor but I think we are a bit off topic with the NTR discusion.

That just means we are duration of use limited for distance such as lunar play ground only....we need another fuel for longer durations and distances from earth.

So they built the thing in multiple experimental forms and it worked!  2000 sec Isp at engine T/W ~ 10.  Not bad at all!  Plus,  the start-up solid core rig is an electric power generator between gas core "burns".  Very nice!  Many thanks,  Quaoar!

GW

update/edit:  no they never actually built one!  I found a report on line done for the nuke group in Idaho in 1992 documenting this.  The design was more advanced than the one on the US side,  but the critical experiments were never funded or ever done. 

That on-line report is difficult to download in a readable form.  I have a pdf copy of it,  but I can copy nothing from it,  nor characterize what it actually is or where it came from.  Some sort of government security must still be applied to this.  Either that,  or somebody is trying to sell it.   Not sure which as of this date.

The RD-600-series nuke engines were never actually built and tested,  that much is clear. 

GW

Use of the vehicle skin as a radiator has limitations. You have to orientate your radiator so that the sun isn't shining on it. A skin radiator would mean that the vehicle would have to be pointing at, or away from, the sun for effective heat dissipation. A separate set of panels can be arranged edge on to it. My point was that these are going to be big.

Why not look at a modification of the compact nuclear steam-turbine-electric plants they use in US Navy submarines?  They generate multi-megawatts of power,  with a demonstrated safety record unsurpassed by any other entity.  For a nuclear pulse propulsion ship,  the weight of the shielding is not an issue.  Higher ship size and mass is just larger Isp and delta-vee. 

GW

SAFE-400 can be scaled up to provide 400kWe from 1.6MWt, but as designed and tested 4 reactors would also provide 400kWe.  The cores aren't much bigger than small propane tanks, but weigh quite a bit.  Figure about 1,000kg to 1,200kg per complete unit (core, some shielding, and radiators).  You'd want some redundancy.

A 1MWe LENR, while perhaps not as compact as SAFE-400, would have a lower mass because it requires no shielding and fewer radiators.  Even if you have to refuel the reactors every 6 to 12 months, the fuel amounts to a few kg's of material at most.  If EMDrive only works as well as it works in the labs, then reasonable transit times are achievable with 1MWe, there's on radioactive power sources or debris from low yield nuclear explosions to contend with, and a craft that could carry a dozen or so people is the sort of thing that two or three SLS flights could reasonably put into orbit.