Fuel tank rules of thumb

In article ,
Iain McClatchie wrote:
...Once you've achieved low earth orbit, if you wanted
significant delta-V from there, say, to get to an outer planet,
wouldn't you use a small, low pressure engine with a huge
expansion ratio, pressure-fed from tanks?

Maybe, and maybe not. There are still performance advantages to doing
*relatively* short burns. Being in orbit reduces the pressure to do
things quickly, but doesn't eliminate it. You can think in terms of 0.1G
rather than 3G, but you don't want to take it too much farther down or
various problems surface.

Also, if you want a large delta-V, meaning big tanks, you can still see
a performance gain from pump feed. Designers accept remarkable mass
penalties out of superstitious fear of pumps.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

In article ,
Gordon D. Pusch wrote:
...OTOH, long burns lead to high gravity losses, and you lose some of the
benefits of the "Oberth Effect"...
Use perigee kicks. Minimizes gravity losses from low thrust systems.

...At the price of having to make a large number of restarts, any one of which
may fail ... plus multiple passages through the van Allens...

Plus the fact that it just plain takes a godawful long time if your thrust
is really low. Once your orbit is highly elliptical, you pass through
perigee very quickly -- meaning that your available burn time is short --
and those brief perigees are days or even weeks apart.

More fundamentally, though, a multiple-perigee-kicks strategy has limits,
because *it requires perigees*. In other words, it can only get you up to
slightly over escape velocity, because once you reach escape velocity, you
*don't come back* for another perigee. This sucks if you want to achieve
a planetary trajectory, because now the hyperbolic excess (extra velocity
beyond escape velocity) has to be acquired after departure.

I have studied this at some length, looking at using electrothermal
thrusters to go from GTO to planetary trajectories. The low-thrust
penalties are really pretty bad. Acquiring the necessary hyperbolic
excess using *cold gas jets* at the final perigee can actually be better
than using an Isp=800s electrothermal rocket after escape.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

Henry Spencer wrote:

In article ,
Gordon D. Pusch wrote:
...OTOH, long burns lead to high gravity losses, and you lose some of
the benefits of the "Oberth Effect"...
Use perigee kicks. Minimizes gravity losses from low thrust systems.

...At the price of having to make a large number of restarts, any one of
which may fail ... plus multiple passages through the van Allens...

Plus the fact that it just plain takes a godawful long time if your thrust
is really low. Once your orbit is highly elliptical, you pass through
perigee very quickly -- meaning that your available burn time is short --
and those brief perigees are days or even weeks apart.

More fundamentally, though, a multiple-perigee-kicks strategy has limits,
because *it requires perigees*. In other words, it can only get you up to
slightly over escape velocity, because once you reach escape velocity, you
*don't come back* for another perigee. This sucks if you want to achieve
a planetary trajectory, because now the hyperbolic excess (extra velocity
beyond escape velocity) has to be acquired after departure.

I have studied this at some length, looking at using electrothermal
thrusters to go from GTO to planetary trajectories. The low-thrust
penalties are really pretty bad. Acquiring the necessary hyperbolic
excess using *cold gas jets* at the final perigee can actually be better
than using an Isp=800s electrothermal rocket after escape.


I've always thought the best way to go to the planets would be a
combination low-thrust with final high-thrust kick at the end. Basically,
to use a low-thrust extremely high ISP ion engine to slowly climb out of
the earth's gravity well. You do this with all the massive stuff your
taking, for example human exploration of Mars. Then use a couple of passes
by the Moon (one outbound, and one inbound) to change the your orbit from
an extremely high circular orbit to a highly elliptical, possibly
hyperbolic orbit with a perigee of a couple of hundred miles. Then at Earth
perigee do your final High thrust Low ISP chemical burn.

You get the best of both world, extremely high ISP to get most of your
velocity with a continuous burn from the Earth to the Moon. Then use the
Moon to change your orbit, giving one last pass by the Earth. Falling back
deep into the gravity well to do a final high thrust low ISP burn where it
does the most good.

If it's a manned mission, they could leave later, just as the massive
payload is approaching the moon for the second time. The men could fly
around the moon at the right time, then they would have 5 days or so to
rendezvous with the equipment/fuel payload before the final Earth perigee
burn.

This concept supports Holman transfers to Mars for equipment
and much faster transfers for humans (bigger final burn).

Additionally, the large ion engine could be aerobraked back into low earth
orbit to be refueled and repaired to start lifting the next massive
resupply payload.

I've never thought of it as a low thrust penalty, but rather a deep gravity
well bonus.

Craig Fink

Craig Fink wrote:

Henry Spencer wrote:

In article ,
Gordon D. Pusch wrote:
...OTOH, long burns lead to high gravity losses, and you lose some of
the benefits of the "Oberth Effect"...
Use perigee kicks. Minimizes gravity losses from low thrust systems.

...At the price of having to make a large number of restarts, any one of
which may fail ... plus multiple passages through the van Allens...

Plus the fact that it just plain takes a godawful long time if your
thrust
is really low. Once your orbit is highly elliptical, you pass through
perigee very quickly -- meaning that your available burn time is short --
and those brief perigees are days or even weeks apart.

More fundamentally, though, a multiple-perigee-kicks strategy has limits,
because *it requires perigees*. In other words, it can only get you up
to slightly over escape velocity, because once you reach escape velocity,
you
*don't come back* for another perigee. This sucks if you want to achieve
a planetary trajectory, because now the hyperbolic excess (extra velocity
beyond escape velocity) has to be acquired after departure.

I have studied this at some length, looking at using electrothermal
thrusters to go from GTO to planetary trajectories. The low-thrust
penalties are really pretty bad. Acquiring the necessary hyperbolic
excess using *cold gas jets* at the final perigee can actually be better
than using an Isp=800s electrothermal rocket after escape.


I've always thought the best way to go to the planets would be a
combination low-thrust with final high-thrust kick at the end. Basically,
to use a low-thrust extremely high ISP ion engine to slowly climb out of
the earth's gravity well. You do this with all the massive stuff your
taking, for example human exploration of Mars. Then use a couple of passes
by the Moon (one outbound, and one inbound) to change the your orbit from
an extremely high circular orbit to a highly elliptical, possibly
hyperbolic orbit with a perigee of a couple of hundred miles. Then at
Earth perigee do your final High thrust Low ISP chemical burn.

You get the best of both world, extremely high ISP to get most of your
velocity with a continuous burn from the Earth to the Moon. Then use the
Moon to change your orbit, giving one last pass by the Earth. Falling back
deep into the gravity well to do a final high thrust low ISP burn where it
does the most good.

If it's a manned mission, they could leave later, just as the massive
payload is approaching the moon for the second time. The men could fly
around the moon at the right time, then they would have 5 days or so to
rendezvous with the equipment/fuel payload before the final Earth perigee
burn.

This concept supports Holman transfers to Mars for equipment
and much faster transfers for humans (bigger final burn).

Additionally, the large ion engine could be aerobraked back into low earth
orbit to be refueled and repaired to start lifting the next massive
resupply payload.

I've never thought of it as a low thrust penalty, but rather a deep
gravity well bonus.

Craig Fink


For pure payload, Lunar-Earth burn-Lunar would make the most sense. The
first Lunar pass to change the orbit to highly eliptical. Burn at perigee
of Earth, with a Lunar assist on the way out. It doesn't give the
oportunity for people to join the expedition before the escape burn, but
would be more efficient. Ion engines are just so much more efficent in
terms of required mass to orbit and climbing out of the gravity well.

Craig Fink