L3 internal frame construction method

Now with a 75/5120 case in hand and an M1297 reload around the corner, its time to get serious about an L3 cert machine. An initial notion is to use Sonotube with a transition from 8" to 10", Ply/foam fins, and a foam/glass nosecone. There have been several builds using internal wood framing in the booster section that look interesting.

Has anyone built a complete internal wood frame top to bottom for strength and the exterior tubing just to cover it up? Seems like there would be an advantage to an internal structure to attach things to. Or is this method way too heavy?

Internal frame is done on numberous commercial rockets IIRC. Something to think about though - what load does it have to take? Under boost, the rocket weighs what? Going off under how many G's of acceleration (plus 1 for earth's gravity)? So what sort of compressive force is this? Now suppose it hits wind shear? How much side load at max-Q does it have to take? What are the aerodynamic loads?

A tube is a very simple and pretty robust structural member.

You're looking at L3. That's an M or larger. Probable thrust, say, 300 pounds to over a thousand? Mass fraction of motor hardware and propellant, probably around 30% give or take a lot? So, the structure would then need to take roughly 270 to a thousand pounds of compressive force applied at the retainer if rear retainer. Or a mix of compressive and tension if applied at the head-end of the motor which is better structurally for the sorts of motors we use as the compressive force is greatly reduced by being split up and most structures are stronger under tension than compression. That's with no margin of error. What sort of margin do you want? 100%? So double the values.

Note, 50% margins are considered adequate for many parts of commercial rockets; however, they are much better engineered, tested, certified, and operated by crews of professionals. At least, that is my understanding

So, picture most of a ton of weight sitting on your structure, with the structure supported at the motor thrust plate wherever that is. Now push on it some from various angles, yank on the fins, etc. Will it take it? If so, please, go ahead and build it! It'll be a really cool project!

How big a motor do you want to eventually put in the project? I warn you, once you fly an M, you'll want to fly an N...

It's also a really cool project for a second rocket after getting the L3. I did my big complex project for my L3... I'd say it delayed the L3 cert flight by a year over what it could have been. More, if design time were counted! Many L3 projects are built as simply, as basically, and as bullet-proof as possible. It's purpose is to get the cert in as close to a guaranteed fashion as possible in rocketry. Afterwards, it is good to go for lots of cool flights while building other projects.

Good luck whatever you decide!

Gerald

PS - I never mentioned recovery loads.

I asked 6 P.E.'s how much load would the centering rings of a rocket with 100lbs of thrust would need to be able to withstand. They all 6 said 100lbs. They were all wrong. The motor, only imparts enough force onto the rocket equal to the mass of the rocket plus any resistance to flight, the remaining force is turned into an acceleration. During thrust phase, arodynamic drag increases this seen resistance against motor force. So, My 27# m flight generates an estimated 240lbs of drag at full speed on an M1882. This means at one moment the rocket saw this as a 267lb resistance against a 500lb thrust. (it went pretty fast.) (for instance the numbers are not something i looked up.)

With that said, you can assume loads from aerodynamic and recovery forces are easily greater than any force the airframe needs to handle from the big 75/5120 beast. The more mass you add makes everything harder because recovery is where mass hurts you... Larger chutes, more G-forces and impact loads during rocket separation.

If a goal is to fly a rocket with a space frame, its been done, and successfully. I think a guy built one and covered it with cardboard for a successfull L3... I dont know much about that flight, so dont hold me to it. So, I won't knock your idea. Just try to remember Lots of sheep in the hobby and a lot of people who tout supreme thinking. Try to think about more than just the added impulse, that is the easy part, think about the weight and what that "extra strength" is really doing.

Its a bad cycle, Make it stronger which makes it heavier, so you need to make it stronger and it makes it heavier, so you need to make it stronger and it makes it heavier, so you need to make it stronger and it makes it heavier. (and thats how you get an 80lb M flight.)

The very hardest part of a L3 is getting good tap members or mentoring hands down....

there is a rocket called the Kloudbuster. It is made from 8" sonotube, it uses 1/4" ply and is built just like an estes fatboy. People modify these from thier stock 54mm mount to 3" and 4" motor configurations. While sonotube is a crappy cardboard, it is very strong, there is no need for interior support other than centring rings, which dimensionaly stabilizes the tube where all the loads are seen anyway. Most of these scratch built kits, users mount the recovery harness directly to the forward closure of the motor, or an U-bolt to the forward centering ring. (or both.) Make sure you remove the plastic lining from inside the tube

Here is my current build. not much more has been done except the tip to tip fiberglass for the fins.
https://stuntedgenius.blogspot.com/p/blog-page.html

I am working on #5. None of my level 3 birds have had any internal frames. I use fiberglass and carbon fiber to reinforce the bulkheads.

I have seen folks use wood, aluminum (threaded rods, angles, and plate), and steel (threaded rods). Many have had very successful flights and very heavy birds. Some chose high thrust motors that they pre-purchased and built the rocket to that the motor. I bought my motor once I had the weight of my rocket.

You will have to make a decision. Discuss it with your TAP.

All of the thrust force of the motor is exerted on the rocket.
Some (1G) is to counter gravity and the rest is drag...

Unless you made a weird design, all of the thrust is transmitted through the rocket via the thrust ring pressing on the aft bulkhead.
Internal CR's see NOTHING.

FredA said:

All of the thrust force of the motor is exerted on the rocket.
Some (1G) is to counter gravity and the rest is drag...

Unless you made a weird design, all of the thrust is transmitted through the rocket via the thrust ring pressing on the aft bulkhead.
Internal CR's see NOTHING.

Click to expand...

This I agree with, but if you connect the centering rings with wood slats or threaded rods, it will be an additive resistance.

Mind you I have never done this.

Thanks for the great commments so far.

The thought experiment is to understand if it is possible to build an RBR (Really Big Rocket) using Sonotube that does not need to be glassed. Would a lightweight internal frame to provide rigidity and through the wall fins against both the booster tube and framework to help distribute the thrust forces be enough?

ClayD said:

I asked 6 P.E.'s how much load would the centering rings of a rocket with 100lbs of thrust would need to be able to withstand. They all 6 said 100lbs. They were all wrong. The motor, only imparts enough force onto the rocket equal to the mass of the rocket plus any resistance to flight, the remaining force is turned into an acceleration. During thrust phase, arodynamic drag increases this seen resistance against motor force. So, My 27# m flight generates an estimated 240lbs of drag at full speed on an M1882. This means at one moment the rocket saw this as a 267lb resistance against a 500lb thrust. (it went pretty fast.) (for instance the numbers are not something i looked up.)

Click to expand...

Do you mean 100 pounds of drag in that first sentence? That could be why you think the PEs gave the wrong answer.

Other than landing, the force from the motor on the centering rings will never exceed the peak thrust. It will exceed the peak drag, though.

Add in one person with a little extremely rusty physics background.

F=MA

Let's say: T instantaneous thrust. D instantaneous aero drag. V instantaneous vehicle mass. M instantaneous motor mass. a is acceleration.

1: T-D = (V+M)a
or rephrased,
2: a = (T-D)/(V+M)

What is the force seen at the thrust transfer point between the motor and the vehicle? That force equals the drag force plus the force due to acceleration of the vehicle mass, but NOT the force due to accelerating the motor mass.

Let's say: f is the force transferred from the motor to the airframe.

3: f = D + Va
substituting via equation 2 for a yields:
4: f = D + V[(T-D)/(V+M)]

Note that at startup, D is zero. In that instant, the equation simplifies to:
5: f = VT/(V+M)
or shuffling,
6: f = T[V/(V+M)]

Equation 6 tells us that the force at startup (when the rocket isn't moving so no air drag) seen at the thrust transfer point between the motor and the airframe is the motor thrust times what I'll call here the vehicle mass fraction. Intuitively that is what one would expect: the thrust of the motor goes into equally accelerating the motor itself and the vehicle or airframe.

Now, let's plug in the example: 27lb instantaneous rocket, which is V+M. 240lbf instantaneous drag. 500lbf instantaneous thrust. Just because we need some number, let's say the motor instantaneously weighs 15lb therefore the vehicle weighs 12lb.

Note, why do I keep saying instantaneous? Each of these quantities is a function of time. The motor burns propellant, so M is really M(t). Drag varies with velocity and altitude at a minimum, so D is really D(t). Thrust follows a thrust profile, so T is really T(t). Instantaneous is picking some value of t so that these functions collapse to single values.

Using equation 4, f = D + V[(T-D)/(V+M)]
substituting in the values yields:
7: f = 240lbf + 12lb[(500lbf-240lbf)/(12lb+15lb)]
8: f = 240lbf + 12lb[260lbf/27lb]
9: f = 240lbf + 12lb[9.63lbf/lb]

(pound force per pound is the acceleration in G's)

10: f = 240lbf + 116lbf = 356lbf

I took the liberty of rounding out the numbers. Hopefully I didn't screw up the screwball units. I so much prefer MKS units!

Gerald

Thrustcurves tell it all.
The thrust of the motor is counteracted by gravity and drag....the sum of which pushes on the thrust plate (aft bulkhead).
No math needed - look at the curve for whatever motor you select, you'll see the force.

Since drag is a SKIN effect (by that, I mean outer surface only) you want to connect the BT and FINS to the thrust plate.
Internal "junk" usually add nothing but weight...make the connection at the thrustplate and be done.

CarVac said:

Do you mean 100 pounds of drag in that first sentence? That could be why you think the PEs gave the wrong answer.

Other than landing, the force from the motor on the centering rings will never exceed the peak thrust. It will exceed the peak drag, though.

Click to expand...

No i meant thrust. This was a question I asked when building my Level 1. For a rocket where the motor tube is held by centering rings, and the centering rings are attached to the air-frame, i was initially concerned with the moment between the motor tube and air-frame, and the shear loads on the centering rings. Have you ever seen the motor come through the rocket leaving it on the pad... i have . So you ask yourself why?

P.E. is kind of an oxymoron in the real world, mostly its Over.Engineered. sure doesn't mean performance engineered

For your statement,

other than landing, the force from the motor on the centering rings will never exceed the peak thrust.

Click to expand...

, there's not really any way you can claim that. Especially without data or explanation to support it. I have many flights where G's from impact loads recorded during recovery(separation ) events which far exceed the G loading from thrust phase. Even some events where falling objects falling in different directions reach the end of their tether. So its not just landing. As far as i have data for.

FredA said:

Thrustcurves tell it all.
The thrust of the motor is counteracted by gravity and drag....the sum of which pushes on the thrust plate (aft bulkhead).
No math needed - look at the curve for whatever motor you select, you'll see the force.

Click to expand...

I have never seen the mass of the rocket, or the velocity of the rocket on a thrust curve to get near the ballpark of actual loads on the rocket.
so when your summing gravity and drag, without mass or velocity, your always going to be.. incomplete?

Well, that's why PEs are charged with public safety and you're just a hobbiest flying toy rockets.

FWIW, when I was a PE at NASA, we used factors of safety of 1.2/1.4/2.0 against yield stress and 1.4/2.0/2.6 against ultimate stresses for protoflight/tested/analyzed structural components.

JordanT said:

Well, that's why PEs are charged with public safety and you're just a hobbiest flying toy rockets.

FWIW, when I was a PE at NASA, we used factors of safety of 1.2/1.4/2.0 against yield stress and 1.4/2.0/2.6 against ultimate stresses for protoflight/tested/analyzed structural components.

Click to expand...

Just a hobbiest flying toy rockets...

you make it sound like one is superior over the other,, I could show you first handed one is not.

P.E. is like anything there is good and there is bad, none of which necessarily equate to "better". Fwiw, is exactly nothing, as I have no value for anything NASA has ever done. You might point to just a hobbyist, and i could point to an ultimate fail in bureaucracy. They cant even sustain a space program. People are measured by their character, and single things they do professionally or personally, means very little to the ultimate character they are seen as.

Touched a nerve? Yeah, me too. Except I insulted your (our) hobby, you insulted my entire profession.

To conflate the beurocratic failures of NASA with their technical achievements and conclude that nothing of value has been done on the engineering side is the mark of someone without an understanding of the facts.

The error you made above about the forces on the thrust ring above is taught in high school physics, BTW. Moment in a quasi-fixed-fixed annular ring under a load you'll find in Roark chapter 10, case 8. Well, you'll see the fixed and pinned versions; semi-final requires a numerical solution. That actually *is* a challenging problem. Even moreso if you consider the dynamics. I don't know that the vibration frequency spectrum is characterized for any hpr motors, but if you find it, you can check it again the fundamental frequencies in Blevins, chapter 11 for the annular plate and chapter 12 for an unstiffened cylindrical airframe.

If you want to actually design an internal frame, Bruhn, Analysis and Design of Flight Vehicle Structures is the Bible. Stiffened frames, diagonal tension field stiffeners (the kind that buckle elasically but do not fail), shear transfer to and from skins, other stiffener types and lightening holes, and a bunch of other tremendously useful stuff is in there.

FWIW, you'll find that when I play with our toys, I often skirt the wrong side of minimal design. We learn more from our failures more than our successes. Part of it is knowing when it's a safety issue and when it's just seeing how close to the edge you can skate. I haven't been around here much lately, but if you search the archives you'll find some pretty spectacular failures, and a couple of cool successes (ex:internal cardboard frame rocket with cardstock skin, search for More Cheese).

Good luck with your L3, and stop pissing on engineers.

Mr G said:

Thanks for the great commments so far.

The thought experiment is to understand if it is possible to build an RBR (Really Big Rocket) using Sonotube that does not need to be glassed. Would a lightweight internal frame to provide rigidity and through the wall fins against both the booster tube and framework to help distribute the thrust forces be enough?

Click to expand...

I did a hybrid version of this on my stretched 10" diameter Polecat Goblin. The fin can centering rings are tied together by hardwood struts which also provide additional support to the fins. The coupler sits on the forward centering ring; the altimeter bay sits on the coupler, so the body tubes really aren't required to carry much load except for the nose cone. It didn't add much weight; GTOW was around 56 lbs.

I have never seen the mass of the rocket, or the velocity of the rocket on a thrust curve to get near the ballpark of actual loads on the rocket.
so when your summing gravity and drag, without mass or velocity, your always going to be.. incomplete?

Where else does the thrust go?
100% +/- 0 of the thrust counters drag and gravity.

JordanT said:

Touched a nerve? Yeah, me too. Except I insulted your (our) hobby, you insulted my entire profession.

To conflate the beurocratic failures of NASA with their technical achievements and conclude that nothing of value has been done on the engineering side is the mark of someone without an understanding of the facts.

The error you made above about the forces on the thrust ring above is taught in high school physics, BTW. Moment in a quasi-fixed-fixed annular ring under a load you'll find in Roark chapter 10, case 8. Well, you'll see the fixed and pinned versions; semi-final requires a numerical solution. That actually *is* a challenging problem. Even moreso if you consider the dynamics. I don't know that the vibration frequency spectrum is characterized for any hpr motors, but if you find it, you can check it again the fundamental frequencies in Blevins, chapter 11 for the annular plate and chapter 12 for an unstiffened cylindrical airframe.

If you want to actually design an internal frame, Bruhn, Analysis and Design of Flight Vehicle Structures is the Bible. Stiffened frames, diagonal tension field stiffeners (the kind that buckle elasically but do not fail), shear transfer to and from skins, other stiffener types and lightening holes, and a bunch of other tremendously useful stuff is in there.

FWIW, you'll find that when I play with our toys, I often skirt the wrong side of minimal design. We learn more from our failures more than our successes. Part of it is knowing when it's a safety issue and when it's just seeing how close to the edge you can skate. I haven't been around here much lately, but if you search the archives you'll find some pretty spectacular failures, and a couple of cool successes (ex:internal cardboard frame rocket with cardstock skin, search for More Cheese).

Good luck with your L3, and stop pissing on engineers.

Click to expand...

My level 3 was completed over a year ago, Thank you, it was a fun project. There is a big difference between scientist and an engineer, one can think and the other can't. I absolutely positively hate engineering as a profession i would rather have my hand in poop all day. Probably because rather than do the work, most of the lazy guys are too scared to do real analysis, inflating the costs of anyting by OVER DESIGNING.

what exact error did i make on a thrust ring?

FredA said:

I have never seen the mass of the rocket, or the velocity of the rocket on a thrust curve to get near the ballpark of actual loads on the rocket.
so when your summing gravity and drag, without mass or velocity, your always going to be.. incomplete?

Where else does the thrust go?
100% +/- 0 of the thrust counters drag and gravity.

Click to expand...

acceloration/ momentum / velocity ?. If the rocket resisted 100% +/- of the motors energy, it would never leave the pad.

If you ever take altimeter data like barometric, accelorometer, and directional data of the flight. Plus the atmospheric data to model the atmosphere, you can produce a flight corrected thrust curve and plot the exact ammount of energy the motor expelled.

100% +/- 0 of the thrust counters drag and gravity

Click to expand...

this is like getting your numerator and denominator backwards, drag and gravity counters 100% of the thrust.(not the other way around) This is why we have a coast phase, thrust that exceeds drag and mass is turned into momentum. Thus drag and gravity working at varying rates throughout the flight, eventually bring it back to earth. If you have the same motor , say a certified 75/5120 motor, all flights are going to be +/- 2% thrust wise, but aerodynamics and mass will change drastically depending on the vehicle. So, not all flights will see the same loads or even the same types of loads.

ClayD said:

No i meant thrust. This was a question I asked when building my Level 1. For a rocket where the motor tube is held by centering rings, and the centering rings are attached to the air-frame, i was initially concerned with the moment between the motor tube and air-frame, and the shear loads on the centering rings. Have you ever seen the motor come through the rocket leaving it on the pad... i have . So you ask yourself why?

Click to expand...

I have seen a motor go through the rocket. That's because the rocket's centering rings or the motor's thrust ring...wasn't able to handle the motor's thrust. Nothing more, nothing less.

P.E. is kind of an oxymoron in the real world, mostly its Over.Engineered. sure doesn't mean performance engineered

Click to expand...

Uhh...what? Clearly there is some breakdown in communication here, because I cannot understand what you wrote there. P.E. means "professional engineer" and all it means is that that person passed some test which I have not bothered to take.

For your statement, , there's not really any way you can claim that. Especially without data or explanation to support it. I have many flights where G's from impact loads recorded during recovery(separation ) events which far exceed the G loading from thrust phase. Even some events where falling objects falling in different directions reach the end of their tether. So its not just landing. As far as i have data for.

Click to expand...

I admit that if you anchor recovery hardware to the motor, the ejection shock can also possibly exceed the thrust. I now revise my statement to "In forward flight, the force from the motor on the centering rings will never exceed the peak thrust as long as the motor is rigidly attached."

ClayD said:

I have never seen the mass of the rocket, or the velocity of the rocket on a thrust curve to get near the ballpark of actual loads on the rocket.
so when your summing gravity and drag, without mass or velocity, your always going to be.. incomplete?

Click to expand...

The difference between gravity+drag and thrust...is the force that causes acceleration. It's not "incomplete" or a discrepancy. It results in a change in velocity. If the forces are balanced, the rocket does not accelerate or decelerate.

Ted Cochran said:

I did a hybrid version of this on my stretched 10" diameter Polecat Goblin. The fin can centering rings are tied together by hardwood struts which also provide additional support to the fins. The coupler sits on the forward centering ring; the altimeter bay sits on the coupler, so the body tubes really aren't required to carry much load except for the nose cone. It didn't add much weight; GTOW was around 56 lbs.

View attachment 247764View attachment 247752

Click to expand...

I built the Nike Smoke and it came to 48 lbs.

CarVac said:

Uhh...what? Clearly there is some breakdown in communication here, because I cannot understand what you wrote there. P.E. means "professional engineer" and all it means is that that person passed some test which I have not bothered to take.

Click to expand...

First you dont even have to be an engineer to sit for the P.E., I know a few of those guys.
Curious, you haven't bothered, or don't meet the min requirements for working in the required fields so you can't. One of the companies I do business with, EXPECTS all their designers to sit for the P.E. so that professional liability is spread throughout. Not just 1 or 2 guys on the hook for everyone else.

Most P.E.'s are in the real world of business (not a failed bureaucracy piping away tax dollars), and in that world, people dont design for performance of each and every aspect. Thus, most situations that are "like " other situations, don't get analyzed for their actual performance, Rather, they are treated like a model example that is "near" the situations. At this point, you cant say exactly what is correct, but you can put some safety factor on top of that, so your in the clear. But what happens most, is if something is not normal or easily modeled, the system doesn't work well, and all design efficiency is thrown away for time and safety. Making it expensive for everyone else, and maybe not necessarily safer as I have personally seen. I call this the ignorance factor.... It was an esoteric dig, and I get that.

CarVac said:

The difference between gravity+drag and thrust...is the force that causes acceleration. It's not "incomplete" or a discrepancy. It results in a change in velocity. If the forces are balanced, the rocket does not accelerate or decelerate.

Click to expand...

look closely what I said....

"so when your summing gravity and drag, without mass or velocity, your always going to be.. incomplete?"
Gravity is a constant acceleration, and drag is a varying force due to aerodynamic properties, environment and velocity.
So without mass, and velocity, you cannot calculate any resistive forces on the rocket.

I didnt say anything about thrust.

ClayD said:

First you dont even have to be an engineer to sit for the P.E., I know a few of those guys.
Curious, you haven't bothered, or don't meet the min requirements for working in the required fields so you can't. One of the companies I do business with, EXPECTS all their designers to sit for the P.E. so that professional liability is spread throughout. Not just 1 or 2 guys on the hook for everyone else.

Most P.E.'s are in the real world of business (not a failed bureaucracy piping away tax dollars), and in that world, people dont design for performance of each and every aspect. Thus, most situations that are "like " other situations, don't get analyzed for their actual performance, Rather, they are treated like a model example that is "near" the situations. At this point, you cant say exactly what is correct, but you can put some safety factor on top of that, so your in the clear. But what happens most, is if something is not normal or easily modeled, the system doesn't work well, and all design efficiency is thrown away for time and safety. Making it expensive for everyone else, and maybe not necessarily safer as I have personally seen. It was an esoteric dig, and I get that.

Click to expand...

I do work as a manufacturing engineer and do all sorts of things, but I don't do any designing or customer facing analysis of the actual products my company makes, so I don't need a P.E. I do design fixtures and internal processes that affect yields and productivity, but not performance. Maybe I will get a PE eventually, but I haven't had a need to.

Secondly, I was referring to the grammatical issues with your statement. As an engineer I find that 95% of disagreements come from bad communication, and I ask that you keep things easy to read and understand for the rest of us to prevent needless arguments.

ClayD said:

look closely what I said....

"so when your summing gravity and drag, without mass or velocity, your always going to be.. incomplete?"
Gravity is a constant acceleration, and drag is a varying force due to aerodynamic properties, environment and velocity.
So without mass, and velocity, you cannot calculate any resistive forces on the rocket.

I didnt say anything about thrust.

Click to expand...

Then I don't see your original point. Are you saying that the longitudinal forces the rocket experiences exceeds gravity and drag?

Lift can certainly be greater than drag, and that is what rips rockets apart in most shreds, but that is not longitudinal and has little to do with the shear forces on the centering ring joints that started the whole discussion.

So without mass, and velocity, you cannot calculate any resistive forces on the rocket.

You can go though the math dance...but in the end you will see that all the force of the motor is "resisted" by drag and gravity.
Less mass, you accelerate faster and build more drag....
It must zero out.

The motor puts out a force over time (thrustcurve) and the rocket resists it....all of it...

Now, after the Wildman Black Saturday Sale, I am/should be committed with an M1297 on the way. So, the future marvel of amateur engineering (that a helpful TAP will review for minimum safety requirements) will have a motive force of 460 lbs against MTOW at takeoff before all those other forces start pushing the other way.



Ted, thanks for the image and description of your Goblin internals - that's very close to what I was imagining. And coming from the NAR Prez (I just renewed my NAR membership - thank you for your service to rocketry), I'm sure there is some backing for your techniques.

ClayD/FredA, you both appear quite passionate about the engineering/science regarding the forces to be considered. It will take some time to sift through the objective data and glean some tidbits of your knowledge. Thanks for the input! In the aviation sector, where my career continues, we use a minimum ultimate strength factor of 1.5 for all load bearing structures so that's what I was planning on for calculating loads on this structure.

Looking forward to more advise as the project develops.

Has anyone built a complete internal wood frame top to bottom for strength and the exterior tubing just to cover it up?

Click to expand...

Almost. Built a complete wood frame (balsa) and covered it with flat pieces of posterboard. Rocket was powered by a D motor. This kept it from warping and allowed building the motor mount structure inside a frame.

ClayD said:

I asked 6 P.E.'s how much load would the centering rings of a rocket with 100lbs of thrust would need to be able to withstand. They all 6 said 100lbs. They were all wrong. The motor, only imparts enough force onto the rocket equal to the mass of the rocket plus any resistance to flight, the remaining force is turned into an acceleration.

Click to expand...

You're both wrong. The motor must lift and accelerate itself. However, if the remaining force is 100 lbs., it will press with 100 lbs. into the rocket, no matter what the breakdown of where that goes: drag, gravity, and accelerative.

I don't get the impression over-building is seen as a bad thing in HP rocketry. The heavier it is, the bigger motor it can take and be seen again. If you don't over-build, you're liable to end up having to carry your L1 rocket on the roof of your car, it's so big.