[1.3.1] Ferram Aerospace Research: v0.15.9.1 "Liepmann" 4/2/18

[taniwha]

KerbMav: keeping TWR < 2 certainly helps keep the gravity turn more "natural". TWR is a fine art: too high and you have to be really aggressive with your turn, too low and you're wasting delta-v to gravity.

[KerbMav]

Actually I do not have to be very aggressive with my turn in this case, as even with minute steering input the rocket will turn "quite happily" several thousand degrees -> start summersaulting.

My smaller rockets were able to overcome this sometimes and started climbing again when the AP was high enough even inside the atmosphere.

[Soda Popinski]

Great mod Ferram4!

I've been trying to determine the best ascent procedure. In vanilla, I tried to keep my speed at approximately terminal velocity. I've gathered from this thread that terminal velocity is greatly increased with rockets (Ferram mentioned ~400 m/s at KSC). So I would think, time to increase my thrust to weight ratio. But then he also says keeping it at 1.2 to 1.4 is better (is this just for stability?). But I also thought I read to keep acceleration at 20 m/s^2. Since surface gravity pulls at 9.81 m/s^2, wouldn't 20 m/s^2 require at least a thrust to weight ratio of 3?

Or is my understanding of physics wrong. I thought a TWR of 1.0 would be a thrust of 9.81 m/s^2 = no movement, 2.0 would be accelerating at 9.81 m/s^2, and 3.0 would be 19.62 m/s^2 acceleration.

I've only been playing with FAM for less than a day, but I'm able to get to orbit with a gradual gravity turn starting around 2km, but am wondering if I'm wasting a lot of delta-V by not using a higher TWR. So far I've been accelerating at my usual pace based on the old terminal velocities. Don't know if it matters, but I'm using KIDS.

[Surefoot]

Since surface gravity pulls at 9.81 m/s^2, wouldn't 20 m/s^2 require at least a thrust to weight ratio of 3?

Your weight is your mass pulled by the local gravity. Thus a TWR of 1 just counters the local gravity: for Kerbin it's indeed just less than 10 m/s^2. That means TWR of 1 = 10 m/s^2 around Kerbin, and thus TWR of 2 = around 20 m/s^2 (9.81 x 2 yeah yeah..).

I thought a TWR of 1.0 would be a thrust of 9.81 m/s^2 = no movement, 2.0 would be accelerating at 9.81 m/s^2,

That's where you are wrong: if you have a thrust of 9.81 m/s^2 (and keep hovering, basically), that means you ARE accelerating at 9.81 m/s^2 already. This is also your current acceleration while you are sitting on your chair. To be subject to zero acceleration, you'd have to be in free fall (and in a vacuum), that's basically what objects in orbit are doing.

I've only been playing with FAM for less than a day, but I'm able to get to orbit with a gradual gravity turn starting around 2km, but am wondering if I'm wasting a lot of delta-V by not using a higher TWR. So far I've been accelerating at my usual pace based on the old terminal velocities. Don't know if it matters, but I'm using KIDS.

From my experiences, keeping a TWR around 1.4 or 1.5 gets me the best fuel savings, of course it's mostly dependant on your ascent profile, much more than with stock.

[AdmiralTigerclaw]

I actually find that since FAR makes Mech Jeb unable to handle the Terminal Velocity Limit, that using the 'limit accelleration' command works wonders.

In my case, I did quite a bit of work finding the best ascent guidance profile for a large (3.5 m) rocket.

1: Limit acceleration to 19 m/s.

2: Target Altitude of 120 km

3: Ascent path settings.

- Gravity Turn Start: 0.150 km (150 meters, or just after fully clearing the 'tower'.)

- Gravity Turn End: 78-79 km

- Turn shape ~37.5 %

Plus, with the rocket I've built the first side-stages I jettison at about 14 km leave me with a lifting-body configuration on my rocket. Took a little work on the aerodynamics of those stages to keep them safely away from the rocket on release, but it works. The only I issue I have right now is payload related. I swap loads out because I'm building my spacedock, so it tends to be hit and miss on how top-heavy the load becomes.

[Traches]

That's where you are wrong: if you have a thrust of 9.81 m/s^2 (and keep hovering, basically), that means you ARE accelerating at 9.81 m/s^2 already. This is also your current acceleration while you are sitting on your chair. To be subject to zero acceleration, you'd have to be in free fall (and in a vacuum), that's basically what objects in orbit are doing.

I think you're nitpicking semantics here, but I have to disagree. When you're sitting in your chair, you're not accelerating WRT the earth because your chair is applying an upward force that is precisely equal and opposite to downward force caused by gravity.

Acceleration is a change in velocity, not a force. Forces cause acceleration, when they are unbalanced.

[Soda Popinski]

I think you're nitpicking semantics here, but I have to disagree. When you're sitting in your chair, you're not accelerating WRT the earth because your chair is applying an upward force that is precisely equal and opposite to downward force caused by gravity.

Acceleration is a change in velocity, not a force. Forces cause acceleration, when they are unbalanced.

I was going to say, I think he got Normal Force mixed up with acceleration. So, am I correct that an acceleration of 20 m/s^2 would require a Thrust to Weight Ratio of approximately 3?

Thought experiment: We have a ship with 3 LV-909 with a mass of 5.0989t on a launch clamp. The rocket weighs 5.0989t x 9.806 m/s^2 = 50 kN. Prior to launch, the clamp exerts 50 kN of upward Normal Force. If ONE of the LV-909 rocket engines is activated (say infinite fuel cheat, so mass doesn't change) when the clamp is released, the ship now gets the downward force of gravity (50 kN of weight), and upward force of thrust (50 kN for the LV-909). It is not moving, and has a thrust to weight ratio of 1 (50 kN / 50 kN).

Now if we turn on a second rocket engine (forget CoM / CoT for now), we now have 100 kN of thrust against 50 kN of weight, or a TWR of 2.0.

Net force applied to the ship is 100 kN up - 50 kN down = 50 kN.

F=ma

50 kN = 5.0989t * acceleration

acceleration = 9.806 m/s^2

Now if we turn on the 3rd engine, for a total thrust of 150 kN against 50 kN of weight force, we get a TWR of 3.0. Net force is now 150 kN thrust - 50 kN weight = 100 kN

100 kN = 5.0989t * acceleration

acceleration = 19.61 m/s^2

So should I use a TWR of 3 to keep to acceleration of ~20 m/s^2, or should I keep my TWR at 1.2-1.4 for (I'm guessing) stability reasons?

Edited November 22, 2013 by Soda Popinski

[Soda Popinski]

Doing some quick physics calcs (bit rusty since high school / college physics). Distance = velocity * time. Velocity = acceleration * time. I've read start your gravity turn at between 0.150 km and 2000 km. So at an acceleration of 19.61 m/s^2 (TWR of 3.0 calculated above)

d = v * t

v = a * t

d = (a * t) * t

d = a * t^2

150m = 19.61 * t^2

t = 2.77 seconds

So 3 seconds until a gravity turn at 0.150 km. Sounds about right to me. Similarly, at 2000 meters, we get 10.1 seconds (shorter actually, since you burn off fuel mass).

[AdmiralTigerclaw]

I think you're nitpicking semantics here, but I have to disagree. When you're sitting in your chair, you're not accelerating WRT the earth because your chair is applying an upward force that is precisely equal and opposite to downward force caused by gravity.

Acceleration is a change in velocity, not a force. Forces cause acceleration, when they are unbalanced.

On the contrary, when you are sitting in your chair, you ARE undergoing acceleration of 9.8 m/s/s towards the dominant center of gravity at all times. However when you're on the surface, or in a chair, it is immediately counter-balanced by the strong and weak nuclear forces preventing your constituent elements from passing right through the chair. This is why you feel pressure in your behind while you're sitting. You are being accelerated downwards by gravity, however the ground counters that acceleration perfectly. As a result, you are pinned to your chair and not allowed to drift away from it at the slightest shift. It's important to remember that gravity and acceleration are intricately linked. If you remove all frames of references and I strap you to a rocket chair with a 1 G engine, you have no way of telling the difference between 1 G of gravity pulling you against that chair, or the engine firing.

To wit, you have to realize that when starting on a surface, you are starting from a condition where you're already behind. Before you can begin accelerating UPWARDS away from the earth at a positive rate, you have to FIRST negate 9.8 meters per second of gravity-based acceleration. So, your TWR must meet, and then exceed the cost of neutralizing gravity. Especially important when you consider that the moment you leave the ground, the ground's physical counter-force to gravity no longer applies.

TWR of 0 means no thrust. TWR between 0 and .99 means any upwards velocity you have will slowly be lost until you start falling again. TWR of 1 is a hover, and even the tiniest TWR of 1.0000000000000000000000000000000001 is a gain in velocity.

So a TWR of 1 while still in contact with the surface means you're standing there, and you've negated gravity, but without even a starting velocity, you're not moving. However, if holding a TWR of 1, even a stiff breeze could pick you up.

Yes, it may be semantics, but no, it really isn't. The Force of Gravity results in continuous acceleration. It exerts almost evenly across all physical objects in a defined system save for macro events like strong Tidal Forces or spaghettification of a black hole. Which is why you don't 'think' you're accelerating in a Zero-G situation such as orbit. However, you really are. It's that constant acceleration that keeps you in orbit in the first place instead of flying off into deep space as if someone let go of a rope.

[Traches]

I was going to say, I think he got Normal Force mixed up with acceleration. So, am I correct that an acceleration of 20 m/s^2 would require a Thrust to Weight Ratio of approximately 3?

Thought experiment: We have a ship with 3 LV-909 with a mass of 5.0989t on a launch clamp. The rocket weighs 5.0989t x 9.806 m/s^2 = 50 kN. Prior to launch, the clamp exerts 50 kN of upward Normal Force. If ONE of the LV-909 rocket engines is activated (say infinite fuel cheat, so mass doesn't change) when the clamp is released, the ship now gets the downward force of gravity (50 kN of weight), and upward force of thrust (50 kN for the LV-909). It is not moving, and has a thrust to weight ratio of 1 (50 kN / 50 kN).

Now if we turn on a second rocket engine (forget CoM / CoT for now), we now have 100 kN of thrust against 50 kN of weight, or a TWR of 2.0.

Net force applied to the ship is 100 kN up - 50 kN down = 50 kN.

F=ma

50 kN = 5.0989t * acceleration

acceleration = 9.806 m/s^2

Now if we turn on the 3rd engine, for a total thrust of 150 kN against 50 kN of weight force, we get a TWR of 3.0. Net force is now 150 kN thrust - 50 kN weight = 100 kN

100 kN = 5.0989t * acceleration

acceleration = 19.61 m/s^2

So should I use a TWR of 3 to keep to acceleration of ~20 m/s^2, or should I keep my TWR at 1.2-1.4 for (I'm guessing) stability reasons?

Your reasoning looks good to me. A craft with a TWR of precisely 1 will hover at full throttle. Another way to look at it is G's of acceleration-- your TWR will be equal to your G meter, minus drag losses.

Practically speaking, I usually shoot for a launch TWR between 1.7 and 2 or so, but I also usually end up with a steeper ascent than I really want. I usually either have to do an aggressive gravity turn pretty quickly after launch, or just cut engines for a little while around 10km to let the nose come down a bit.

[Soda Popinski]

Your reasoning looks good to me. A craft with a TWR of precisely 1 will hover at full throttle. Another way to look at it is G's of acceleration-- your TWR will be equal to your G meter, minus drag losses.

Practically speaking, I usually shoot for a launch TWR between 1.7 and 2 or so, but I also usually end up with a steeper ascent than I really want. I usually either have to do an aggressive gravity turn pretty quickly after launch, or just cut engines for a little while around 10km to let the nose come down a bit.

Sounds good. I'll stick with TWR of about 2 as long as I can keep it under control.

[egreSS]

Has anyone been able to get any of the stock B9 UAV's to work without serious changes to their aesthetics? Specifically the Centauri exhibits some frighteningly paranormal flight characteristics.

[AdmiralTigerclaw]

Has anyone been able to get any of the stock B9 UAV's to work without serious changes to their aesthetics? Specifically the Centauri exhibits some frighteningly paranormal flight characteristics.

Such as?

Remember that flight envelopes are a thing in real life. Some things can fly just fine, but exhibit odd, unexpected, or dangerous flight characteristics if you leave the 'envelope'.

Example SR-71. Flight envelope cautions against angles of attack above 10 degrees above 20,000 ft or risk engine flameout, and envelope absolute forbids inverting it in a roll. While it's G envelope strictly says no G forces greater than 2 positive and .5 negative.

I even have an aircraft of my own which has a very FINE G envelope, but flies cruises, and lands like a dream... provided you're gentle on it like a lady.

[Frederf]

I tried the control surface on top of Mk1 pod to fly the reentry. L/D increased but not by much. I might try it again with larger deflections. If you want to control longitudinal range some kind of speed brake is probably better than lift.

[Surefoot]

Acceleration is a change in velocity, not a force.

One is exactly the consequence of the other. A force caused by gravity applies to your mass (this force is called a "weight"). Gravity is an acceleration. By standing there on your chair on Earth your are submitted to constant acceleration (of around 9.8 m/s^2), if you were in a falling lift then in that reference frame you'd be submitted to no acceleration (thus, floating inside the lift). But the lift itself would be still submitted to that acceleration and then going downwards.

To have no force applied to you (or no acceleration) you would have to be in a reference frame that is itself in free fall (or equivalent), relative to the local bodies that create significant gravity. That's why "zero G" is called "microgravity" actually

Side note: aerodynamics effects such as lift are also forces, to make an object fly in atmosphere you have to get these forces to counter balance the effects of gravity.

Edited November 23, 2013 by Surefoot

[AdmiralTigerclaw]

Heads Up slick.

Gravity is the Force. Being one of the Fundamental FORCEs and all that jazz.

(Gravitation, Electromagnetism, Strong Nuclear, Weak Nuclear)

[Virindi]

Has anyone been able to get any of the stock B9 UAV's to work without serious changes to their aesthetics? Specifically the Centauri exhibits some frighteningly paranormal flight characteristics.

There are several problems you might be having with the Centauri. The stock model is possible to fly if you enable RCS and SAS, keep your speed under 100m/s, and do not deviate more than a few degrees from prograde at any time.

Something that is easy to solve is the overabundance of roll authority, which causes SAS to 'freak out'. Set the wings to pitch control only and let the thrust vectoring deal with roll alone. This is the single best change you can make.

Since it lacks a vertical stabilizer, it attempts to make up for it with RCS. But since it has such a high TWR, it is easy to go so fast that the RCS and wheels are overwhelmed, and you tumble in yaw. Unless you 'cheat' and add a vertical stabilizer or significantly alter the design, sideslip is always going to be annoying. But, it is manageable.

Finally, moving the wings back a bit and adding weight to the front can give you more stability without significantly altering the design.

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[egreSS]

There are several problems you might be having with the Centauri. The stock model is possible to fly if you enable RCS and SAS, keep your speed under 100m/s, and do not deviate more than a few degrees from prograde at any time.

Something that is easy to solve is the overabundance of roll authority, which causes SAS to 'freak out'. Set the wings to pitch control only and let the thrust vectoring deal with roll alone. This is the single best change you can make.

Since it lacks a vertical stabilizer, it attempts to make up for it with RCS. But since it has such a high TWR, it is easy to go so fast that the RCS and wheels are overwhelmed, and you tumble in yaw. Unless you 'cheat' and add a vertical stabilizer or significantly alter the design, sideslip is always going to be annoying. But, it is manageable.

Finally, moving the wings back a bit and adding weight to the front can give you more stability without significantly altering the design.

Yeah, awesome. That is much simpler a solution - sideslip is still an issue but it's 5 times more manageable now.

[Virindi]

By the way, if you want to design a successful tailless aircraft with less of a sideslip problem, I believe the key is to watch chord vs. span. A wide, flat object is much more inclined to yaw than a long thin one, and FAR appears to correctly model this. I recently built a tailless spaceplane, and sideslip is almost no problem at all. You barely notice it. Compare:

Vehicle: B9 Centauri (modified)

Scaled Chord: 4.1m

Scaled Span: 4.5m

Span/Chord: 1.1

Vehicle: X-36A Spaceplane

Scaled Chord: 3.0m

Scaled Span: 1.9m

Span/Chord: 0.63

By keeping chord larger than span, you should naturally induce the craft to point prograde in yaw. If the span/chord ratio is greater than 1, you should be pushed outward.

Incidentally, the logical conclusion from this is that for vehicles WITH a vertical stabilizer, the size of stabilizer needed is proportional to the width to length ratio. Since large vertical stabilizers on spaceplanes are bad for several reasons (wasted mass, mass offset towards the top of the vehicle, etc), this would indicate that an important quality of spaceplanes is that they are long and thin.

Disclaimer: I am not an aerospace engineer

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Edited November 23, 2013 by Virindi

[Bobhendly]

What Is that Red ball in the picture?

[Traches]

One is exactly the consequence of the other. A force caused by gravity applies to your mass (this force is called a "weight"). Gravity is an acceleration. By standing there on your chair on Earth your are submitted to constant acceleration (of around 9.8 m/s^2), if you were in a falling lift then in that reference frame you'd be submitted to no acceleration (thus, floating inside the lift). But the lift itself would be still submitted to that acceleration and then going downwards.

To have no force applied to you (or no acceleration) you would have to be in a reference frame that is itself in free fall (or equivalent), relative to the local bodies that create significant gravity. That's why "zero G" is called "microgravity" actually

Side note: aerodynamics effects such as lift are also forces, to make an object fly in atmosphere you have to get these forces to counter balance the effects of gravity.

I've had a few, so I apologize if I'm less than eloquent or tactful.

You're right when you say that one is the consequence of the other; forrce and acceleration are intimately related because F=m*a. Gravity is an interesting force, because it's precisely proportional to mass allowing you to divide that term out of the equation for an object in freefall with no drag.

Acceleration is a change in velocity. If your velocity isn't changint with respect to your chosen reference frame, you're n ot accelerating. Period, dot, full stop.

Since you're explaining to me what "weight" is, let me point out that the SI unit for it is newtons: a unit of force defined as the that reqauired to accelerate 1 kilogram at the rate of one meter per second per second. At sea level, a one kilogram mass will experience a force due to gravity of 9.8 newtons. If that mass is sitting on the ground, not moving, the ground is also supplying a precisely equal and opposite force. The forces are balanced, and the mass is not accelerating. Forces are applied, but there is no acceleration happneing. Force and acceleration are not interchangab.e terms. Gravity is a force, not an acceleration. Acceleration is merely an effect, caused by an unbalanced force. I can't make this any clearer.

Edit: unless you want to bring general relativity into th picture and get squirrely with reference frames. In that case, you can win. I'm not smart enough or that right now.

Edited November 23, 2013 by Traches

[Traches]

What Is that Red ball in the picture?

It's his dry CoM. There's a wonderful, magnificent, delicious, compelling and uncannily attractive mod called "RCS fuel balancer" that will tell you not only where your center of mass is with both full and empty tanks of various types, but will tell you how much torque you'll experience about your CoM due to the force from your engines or RCS thrusters.

[Surefoot]

@Traches: well, YOU are nitpicking here, i was short for sake of brevity, since we are quite off topic... And yes i am sorry but i was bringing reference frames in, since it's relevant here (local gravity IS an acceleration, like it or not.. that you have an opposing force that prevents you from falling is another thing entirely, and is related to WEIGHT not to gravity directly !!)

Besides if you are nitpicking then:

forrce and acceleration are intimately related because F=m*a. Gravity is an interesting force

(emphasis mine)

This is wrong: gravity is NOT a force (*) but an acceleration (**). "Weight" is the resulting force of applying gravity to mass (and is totally subject to reference frame, sorry about that). That's where you had your concepts mixed up, which explains the rest... Funnily enough, you go all the way to give a formula, which applies to gravity as well.

(*) that would be Newtons, look at how rocket engines have thrust given in Newtons by the way

(**) in m/s^2

Edited November 23, 2013 by Surefoot

[egreSS]

That's RCS balancer "dry weight/CoM"

btw - you should post that x-36 craft file, it's smooth.

@Surefoot/@Traches Don't get started talking about reference frames and forces because then we'll eventually get on centrifugal force and never hear the end of it.

Edited November 23, 2013 by egreSS

[Surefoot]

@Surefoot/@Traches Don't get started talking about reference frames and forces because then we'll eventually get on centrifugal force and never hear the end of it.

ROFL

Well without even bringing reference frames in, he's got his concepts mixed up (between "weight" and "gravity", as it seems), should bring his views to the science forum instead

PS: dont think that by sitting on a chair you are countering gravity. You are still very much subject to it. That your weight doesnt make you go downwards because of the opposing force of the chair is another thing entirely. But trust me, everything in your body *wants* to go downwards and is pulled by a mysterious force... which is the result of the product of your mass and local gravity.