Reactionless drive ships orbital flight in 3rd ed physics?

[weby]

In 3rd edition space there was a thing where low thrust ships with wings could fly to orbit on planets with atmosphere if they had a certain % of their mass as thrust.

It was something like 8% for thin, 5% normal and 3% heavy density atmosphere.

What is the math/physics on that or was it just arbitrary?

And if the thrust % is based on reality, would it not require some fancy aerodynamics to be able to not have the atmospheric drag slow you down at an altitude where you can fly with your wings?

[Fred Brackin]

Originally Posted by weby

In 3rd edition space there was a thing where low thrust ships with wings could fly to orbit on planets with atmosphere if they had a certain % of their mass as thrust.

?

I never saw any mathemtical support for this idea. It first showed up in Space 1e I think but i don't know how the idea got there. It never made any sense to me.

I'm not sure anything with a thrust-to-weight ratio that low could even make a runway take off. The earliest jetliners (i.e. 707) had a thrust-to-weight of 0.15 to 1. A 747 at full throttle was more like 0.25 to 1. At 0.05 to 1 you'd need at least 3x as much runway and you might not be able to get to take-off speed due to rolling resistance or drag.

Further problems would come up with airframe shape and re-entry. You'd need a shape like the Space Shuttle for re-entry but that would give you poor performance at low speed in thick air. That's why the Shuttle's landing speed was so high. Getting up to that speed for take-off with a super low thrust-to weight ratio would take a long, long runway. Even longer than the special landing strip at KSC.

Now, if you had enough _contragravity_ lift you could get by with low thrust-to-weight but it'd still take a long, long time to reach orbital speed.

[weby]

Originally Posted by Fred Brackin

I never saw any mathemtical support for this idea. It first showed up in Space 1e I think but i don't know how the idea got there. It never made any sense to me.

Me neither, thus the question to ask if there was something I had missed.

I'm not sure anything with a thrust-to-weight ratio that low could even make a runway take off. The earliest jetliners (i.e. 707) had a thrust-to-weight of 0.15 to 1. A 747 at full throttle was more like 0.25 to 1. At 0.05 to 1 you'd need at least 3x as much runway and you might not be able to get to take-off speed due to rolling resistance or drag.

Further problems would come up with airframe shape and re-entry. You'd need a shape like the Space Shuttle for re-entry but that would give you poor performance at low speed in thick air. That's why the Shuttle's landing speed was so high. Getting up to that speed for take-off with a super low thrust-to weight ratio would take a long, long runway. Even longer than the special landing strip at KSC.

And the "maximum lift from air" that you need to take off and to raise hight enough to keep the air resistance low so you can go fast.. contradicts also "minimum drag" you need as well for the fast thing..

Now, if you had enough _contragravity_ lift you could get by with low thrust-to-weight but it'd still take a long, long time to reach orbital speed.

Not really that long overall, few hours in those single digit % of G for earth sized planets.

[Anthony]

Originally Posted by weby

In 3rd edition space there was a thing where low thrust ships with wings could fly to orbit on planets with atmosphere if they had a certain % of their mass as thrust.

It was something like 8% for thin, 5% normal and 3% heavy density atmosphere.

What is the math/physics on that or was it just arbitrary?

Sounds arbitrary. Flying to orbit requires at least (surface gravity) / (hypersonic L/D ratio), plus a hull that can actually survive doing that (hard to do without force fields or magic materials). Nothing about that is variable with atmospheric density.

[Pursuivant]

Originally Posted by weby

In 3rd edition space there was a thing where low thrust ships with wings could fly to orbit on planets with atmosphere if they had a certain % of their mass as thrust.

I checked GURPS 3E Space 3rd Edition & I didn't see anything like what you're describing.

You might be thinking of ships built using modular ship design rules having to use a certain percentage of fuel/reaction mass to get into orbit. Streamlined/winged designs which generate lift would reduce the amount of fuel required, but not by a fixed percentage.

You might be thinking of some of the Transhuman Space books, where there were different formulas for fuel consumption, time, etc. required for operations in different atmospheres or Low, High, or Geosynchronous orbits.

[weby]

Originally Posted by Pursuivant

I checked GURPS 3E Space 3rd Edition & I didn't see anything like what you're describing.

You might be thinking of ships built using modular ship design rules having to use a certain percentage of fuel/reaction mass to get into orbit. Streamlined/winged designs which generate lift would reduce the amount of fuel required, but not by a fixed percentage.

You might be thinking of some of the Transhuman Space books, where there were different formulas for fuel consumption, time, etc. required for operations in different atmospheres or Low, High, or Geosynchronous orbits.

It is from Gurps space 2nd edition and many of the books based on that. The subject line might be misleading so changed in this one. Gurps space 3rd is very late 3rd efition books so most of gurps 3rd edition books with spaceshps used the older space.

I remembered the % values wrong it is 1% of actual mass in Dense
atmosphere, 2% in Standard atmosphere, and 5% in Thin atmosphere if winged. (sidebar on p 83 of Gurps space second edition)

[David Johnston2]

Originally Posted by weby

In 3rd edition space there was a thing where low thrust ships with wings could fly to orbit on planets with atmosphere if they had a certain % of their mass as thrust.

It was something like 8% for thin, 5% normal and 3% heavy density atmosphere.
?

Don't think so. Lifting bodies can fly into space provided only that they have enough reactionless thrust to exceed the planetary gravity.

[Ulzgoroth]

Originally Posted by David Johnston2

Don't think so. Lifting bodies can fly into space provided only that they have enough reactionless thrust to exceed the planetary gravity.

That's a crazy high requirement.

[doctorevilbrain]

Why reactionless?

[dcarson]

Originally Posted by doctorevilbrain

Why reactionless?

Because it needs to fly for a lot longer than any fuel could last.