[ExI] extropy-chat Digest, Vol 87, Issue 17

[Keith Henson]

On Fri, Dec 10, 2010 at 1:12 PM,  Adrian Tymes <atymes at gmail.com> wrote:
>
> On Fri, Dec 10, 2010 at 7:50 AM, Eugen Leitl <eugen at leitl.org> wrote:
>> On Thu, Dec 09, 2010 at 08:44:51PM -0800, Adrian Tymes wrote:
>>> Could you use tungsten as a heat exchanger?
>>>
>>> Have an onboard energy source, optimized for heat generation instead
>>> of thrust. ?(Maybe even fission/radioactives, so long as they don't get
>>
>> http://en.wikipedia.org/wiki/Nuclear_thermal_rocket
>>
>> see "Risks".
>>
>> They're not that good http://en.wikipedia.org/wiki/Specific_impulse
>
> The first article gives a specific impulse of 850 seconds for a simple
> solid-core design.  The latter gives specific impulses of 250-450 for
> modern chemical (solid or liquid fuel) rockets.  Yes, there are better
> examples listed, but none that give 1G+ thrust.  (Fuel efficiency is
> very desirable, but high thrust is required, for launching.  So we can
> ignore any engines that do not give 1G+ thrust for that purpose, no
> matter their other benefits, even though they may be superior after
> orbit is achieved.)

If anyone really wants to understand what goes on with rocket design,
it can be done with a spread sheet and graphing software.

The key concepts are mission velocity and rocket exhaust velocity.  To
get into LEO takes around 9km/sec delta V.  Orbital is only 8 km/sec
but you burn somewhat more because of the time gravity is accelerating
you downwards and from air drag.

In simple terms, going to twice the exhaust velocity (which is what
you have to do with LH2/LOX), the mass of the fuel has to be about 85%
of the takeoff mass.  A reusable rocket is about 15% structure, which
leaves no payload using LH2/LOX.

Laser heated hydrogen at 3000 deg K has an exhaust velocity of 9.8
km/sec.  That means the structure plus payload can be 1/3rd of the
takeoff mass.  I.e., a 300 ton vehicle could reach orbit with 50 tons
of structure and 50 tons of payload.

To get in excess of one g takes around 6 GW.  The only way this makes
economic sense is if the lasers are run 90% of the time.

Keith