Why is a LOX/Kero SSTO not rather easy?

I am not a particular fan of SSTO, but it appears to me that SSTO has
long been within our grasp.

The best figures that I could get for the Titan II 1st stage
(based on Rusty Barton's reply to my earlier post) are a
GLOW 0f 258000 lbs, and a dry weight of 10900, giving it a mass ratio of
23.7. Now comparing it to a LOX/kero vehicle powered by something like
the Russian NK-33, I see the following plus and minus factors affecting
its mass ratio (I am not including a payload in these calculations):

On the minus side, the Titan II 1st stage is not a complete vehicle. It:

(a) lacks a nose cone
(b) lacks most avionics
(c) lacks a cargo bay
(d) has 6% denser fuel so a slightly smaller fuel tank

On the plus side:
(a) the NK-33 is 420 lbs lighter than the Titan II engine
yet it is sufficient for a vehicle nearly 10% heavier
(b) the structure does not have to support the 32 ton 2nd stage
and so can be significantly lighter.

My guess is that those factors mostly cancel out. I also assume that
making a vehicle reusable adds about 40% to its dry weight:
Wings add 7%
Landing gear add 3%
TPS add 15%
Other add 15%

So if we crank those factors in:
Dry weight = 10900*1.4 = 15300 lbs
Glow = 258000 + (15300 - 10900) = 262400
MR = GLOW/(GLOW-Dry weight) = 262400/15300 = 17.15

Now for the NK-33 we have an average Isp of 331, and given a required
dV of 9200 m/s (300 m/s less than a LH2/LOX rocket due to less air
resistance, lower back pressure losses, and earlier peak acceleration)
we get a required MR of 17.01, which is slightly less than what we
can achieve. So we can make orbit with a single stage using very old
technology.

Of course, this is without payload, but given the fact that the Titan II
1st stage was not optimized for weight (you would not normally optimize a
1st stage) and we have lighter materials today, such as aluminum-lithium
and carbon fiber, I would think we would have the necessary margin
for a significant payload.

We also might fly with a wet wing and eliminate the kerosene tank
altogether. And of course if we scaled it up by a factor of 3
we would gain a substantial economy of scale.

So it appears to me that we have had reusable SSTO capability for
dense fuel vehicles for a long time.

-- Larry

Hi Larry!

Well, I'd like to add a few nitpicks. I removed a lot of the text where
I do not disagree.

Larry Gales wrote:
[...]
My guess is that those factors mostly cancel out. I also assume that
making a vehicle reusable adds about 40% to its dry weight:
Wings add 7%
Landing gear add 3%
TPS add 15%
Other add 15%

I do not think wings could be done for only 7% of the weight, especially
if one takes into account the added structure needed for a winged
vehicle. However I am partial to wingless vehicle and I did the following
calculations: The spaceship has in the earth a terminal velocity of
50...100 m/s (example: cw=1, 10 m diameter, 50t weight = 60m/s)
Say you want 200m/s fuel including reserves, so you would need 6-7%
mass depending on Isp - and the engines and structure needed are already
there.

Question: which items do you include among "other"? I see only
restfuel and a bit more avionics, but this should not amount to 15%.
However, I might easily miss something...

So if we crank those factors in:
Dry weight = 10900*1.4 = 15300 lbs
Glow = 258000 + (15300 - 10900) = 262400
MR = GLOW/(GLOW-Dry weight) = 262400/15300 = 17.15

Now for the NK-33 we have an average Isp of 331, and given a required
dV of 9200 m/s (300 m/s less than a LH2/LOX rocket due to less air
resistance, lower back pressure losses, and earlier peak acceleration)
we get a required MR of 17.01, which is slightly less than what we
can achieve. So we can make orbit with a single stage using very old
technology.

I notice most modern rockets are more "fat" than Titan. However, I am
not sure how much weight could be saved if the tanks have more diameter
and less length, and there is some delta-V loss due to additional air
resistance. Additional advantage to the fat version: I want a lot of
drag for breaking during descend.

Of course, this is without payload, but given the fact that the Titan
II 1st stage was not optimized for weight (you would not normally
optimize a 1st stage) and we have lighter materials today, such as
aluminum-lithium and carbon fiber, I would think we would have the
necessary margin for a significant payload.

You can optimize a RLV a bit more than a ELV: increased construction
costs for the airframe would be spread over a high number of flights.
However, initially you would *have* to keep down development costs,
so this might not be possible for the first generation.

We also might fly with a wet wing and eliminate the kerosene tank
altogether. And of course if we scaled it up by a factor of 3
we would gain a substantial economy of scale.

As befo I think you underestimate the weight of a wing.

So it appears to me that we have had reusable SSTO capability for
dense fuel vehicles for a long time.

Well, I agree. I just do not think a winged vehicle would be the way to
go.

Robert Kitzmueller