Air-breathing hypersonic engines

--- In spacesettlers@yahoogroups.com, Ian Woollard
wrote:
> It's not nearly as simple as that though- the thrust to weight
ratio of
> scramjets is a lot less than a rocket (rockets often manage 100:1
or
> better, scramjets would be lucky to get 20:1,

Is this because of physical laws or is scramjet technology immature
compared to rockets? Can more efficiency be squeezed from scramjets?

> and are less fuel efficient

Why?

and heavier due to having hypersonic air intakes)

Doesn't the lack of oxidizer compensate?

, and
> carrying the oxidiser with you isn't as bad as it sounds- the
oxidiser
> is almost free (pennies per kilogram for LOX for example), and the
tanks
> aren't much more expensive.

But the rocket equation limits the payload as a function of the ISP
and the mass of the fuel and oxidizer.

My understanding of the scramjet's appeal was that by not having to
carry the oxidizer onboard, the rocket equation would allow a larger
proportion as payload. Is this incorrect?

I never thought that the appeal was about saving the cost of the
oxidizer.

TangoMan